The Cell in Development and Inheritance

1 The Hertwigs attribute this to a diminished irritability on the part of the egg-substance. Normally requiring the stimulus of only a single spermatozoon for the formation of the vitel- line membrane, it here demands the more intense stimulus of two, three, or more before the membrane is formed. That the membrane is not present before fertilization is admitted by Hertwig on the ground stated at page 132. - On the other hand, Morgan states ('95, 5, p. 270) that one or more spermatozoa will enter nucleated or enucleated egg-fragments whether obtained before or after fertilization.

only after formation of tJic polar bodies ; for when in sea-urchins the spermatozoa enter immature eggs, as they freely do, they penetrate but a short distance, and no further change occurs. (^) TJie Ovum. — The entrance of the spermatozoon produces an extraordinary effect on the ^g^, which extends to every part of its organization. The rapid formation of the vitelHne membrane, already described, proves that the stimulus extends almost instantly through- out the whole ovum.^ At the same time the physical consistency of the cytoplasm may greatly alter, as for instance in echinoderm eggs, w^here, as Morgan has observed, the cytoplasm assumes immedi- ately after fertilization a peculiar viscid character which it afterward loses. In many cases the egg con- tracts, performs amoeboid movements, or shows wave-like changes of form. Again, the egg-cytoplasm may show active streaming movements, as in the formation of the entrance-cone in echinoderms, or in the flow of periph- eral protoplasm toward the region of entrance to form the germinal disc, as in many pelagic fish-eggs. An interesting phenomenon is the formation, behind the advancing sperm-nucleus, of a peculiar funnel- shaped mass of deeply staining

material extending outward to the during fertilization, [whitman.] periphery. This has been carefully A^- P^'f ^^^^ies- /..polar rings; ptii^jiiti;'. X xiio xi«.o J ^.ig^yage-nucleus near the centre. described by Foot ('94) in the earth- worm, where it is very large and conspicuous, and 1 have smcc ob- served it also in the sea-urchin (Fig. 94). The most profound change in the ovum is, however, the migration of the germinal vesicle to the periphery and the formation of the polar bodies. In many cases either or both these processes may occur before contact with the spermatozoon (echinoderms, some vertebrates). In others, however, the ^gg awaits the entrance of the spermatozoon (annelids, gasteropods, etc.), which gives it the necessary stnmilus. This is well illustrated by the o.^^ of Nereis. In the newly dis- charged ^zg the germinal vesicle occupies a central position, the yolk, consisting of deutoplasm-spheres and oil-globules, is uniformly distributed, and at the periphery of the egg is a zone of clear peri- vitelline protoplasm (Fig. 60). Soon after entrance of the sperma-

1 I have often observed that the formation of the membrane, in To.xopumsUs proceeds Hke a wave from the entrance-point around the periphery, but this is often irregular. tozoon the germinal vesicle moves toward the periphery, its membrane fades away, and a radially directed mitotic figure appears, bv means of which the first polar body is formed (Fig. 97). Meanwhile the protoplasm flows toward the iipj)er })oIe, the peri-vitelline zone disap- pears, and the <i^'^i, now shows a sharply marked polar differentiation. A remarkable phenomenon, described by Whitman in the leech ('78), and later by Foot in the earthworm ('94), is the formation of "polar rings," a j^rocess which follows the entrance of the sj^ermatozoon and accompanies the formation of the polar bodies. These are two ring-sha])ed cytoplasmic masses which form at the periphery of the ^g^ near either pole and advance thence toward the poles, the upper one surrounding the point at which the polar bodies are formed (Fig. 102). Their meaning is unknown, but Foot ('96) has made the interesting discovery that they are probably of the same nature as the yolk-nuclei (p. 156).

After the entrance of the spermatozoon, both germ-nuclei move through the egg-cytoplasm and finally meet one another. The paths traversed by them vary widely in different forms. In general two classes are to be distinguished, according as the polar bodies are formed before or after entrance of the spermatozoon. In the former case (echinoderms) the germ-nuclei unite at once. In the latter case the sperm-nucleus advances a certain distance into the q.%^ and then pauses while the germinal vesicle moves toward the periphery, and gives rise to the polar bodies {Ascaris, annelids, etc.). This signifi- cant fact proves that the attractive force between the two nuclei is only exerted after the formation of the polar bodies, and hence that the entrance-path of the sperm-nucleus is not determined by such attraction. A second important point, first pointed out by Roux, is that the path of the sperm-nucleus is curved, its "entrance-path" into the &^^ forming a considerable angle, with its "copulation-path " toward the egg-nucleus.

These facts are well illustrated in the sea-urchin Q.gg (Fig. 103), where the egg-nucleus occupies an eccentric position near the point at which the polar bodies are formed (before fertilization). Entering 1 The terms fcvialc pro-uttc/eus, male pro-niicleiis (Van I>eneden), are often applied to the germ-nuclei before their union. These should, I think, be rejected in favour of Hertwig's terms egg-nucleus and sperm-nucleus, on two grounds: (i) The germ-nuclei are true nuclei in every sense, differing from the somatic nuclei only in the reduced number of chromosomes. As the latter character has recently been shown to be true also of the somatic nuclei in the sexual generation of plants (p. 275), it cannot be made the ground for a special designation of the germ-nuclei. (2) The germ-nuclei are not male and female in any proper sense (p- 243).

the egg at any point, the sperm-nucleus first moves rapidly inward along an entrance-path that shows no constant relation to the position of the egg-nucleus and is approximately but never exactly radial, i.e. toward a point near the centre of the ^gg. After penetrating a Fig. 103. — Diagrams showing the paths of the germ-nuclei in four different eggs of the sea- urchin, Toxopneustes. From camera drawings of the transparent living eggs. In all the figures the original position of the egg-nucleus (reticulated) is shown at 9 ; the point at which the spermatozoon enters at E (entrance-cone). Arrows indicate the paths traversed by the nuclei. At tlie meeting-point (yl/) the egg-nucleus is dotted. The cleavage-nucleus in its final position is ruled in parallel lines, and through it is drawn the axis of the resulting cleavage- figure. The axis of the &gg is indicated by an arrow, the point of which is turned away from the micromere-pole. Plane of first cleavage, passing near the entrance-point, shown by the curved dotted line.

certain distance its direction changes slightly to that of the copula- tion-path, which, again, is directed not precisely toward the egg- nucleus, but toward a meeting-point where it comes in contact with the egg-nucleus. The latter does not begin to move until the entrance-path of the sperm-nucleus changes to the copulation-path. It then begins to move slowly in a somewhat curved path toward the meeting-point, often showing slight amoeboid changes of form as it forces its way through the cytoplasm. From the meeting-point the apposed nuclei move slowly toward the point of final fusion, which in this case is near, but never precisely at, the centre of the (t^^^.

These facts indicate that the paths of the germ-nuclei are deter- mined bv at least two different factors, one of which is an attraction or other dvnamical relation between the nuclei and the cytoplasm, the other an attraction between the nuclei. The former determines the entrance-path of the sperm-nucleus, while both factors probably operate in the determination of the copulation-path along which it travels to meet the egg-nucleus. The real nature of neither factor is known.

Hertwig first called attention to the fact — which is easy to observe in the living sea-urchin egg — that the egg-nucleus does not begin to move until the sperm- nucleus has penetrated some distance into the egg and the sperm-aster has attained a considerable size; and Conklin (94) has suggested that the nuclei are passively drawn together by the formation, attachment, and contraction of the astral rays. While this view has some fiicts in its favour, it is, 1 believe, untenable, for many reasons, among which may be mentioned the fact that neither tlie actual paths of the pro-nuclei nor the arrangement of the rays support the hypothesis ; nor does it account for the conjugation of nuclei when no astral rays are developed (as in Protozoa or in plants). I have often observed in cases of dispermy in the sea-urchin, that both sperm-nuclei move at an equal pace toward the egg-nucleus ; but if one of them meets the egg-nucleus first, the movement of the other is immediately retarded, and only conjugates with the egg-nucleus, if at all, after a considerable interval ; and in polv.spermy the egg-nucleus rarely conjugates with more than two sperm-nuclei. Probably, therefore, the nuclei are drawn together by an actual attraction which is neutralized bv union, and their movements are not improbably of a chemotactic char- acter. Conklin (99) has recently suggested that the nuclei are drawn together by the agency of protoplasmic currents in the egg-substance.

The earlier observers of fertilization, such as Auerbach, Stras- burger, and Hertwig, described the germ-nuclei as undergoing a com- plete fusion to form the first embryonic nucleus, termed by Hertwig the cleavage- or segmentation-nucleus. As early as 1881, however, Mark clearly showed that in the slug Li max this is not the case, the two nuclei merely becoming apposed without actual fusion. Two years later appeared Van Beneden's epoch-making work on Ascaris, in which it was shown not only that the nuclei do not fuse, but that they give rise to two independent groups of chromosomes which separately enter the equatorial plate and whose descendants pass separately into the daughter-nuclei. Later observations have given the strono-est reason to believe that, as far as the chromatin is con-

cerned, a true fusion of the nuclei never takes place during fertili- zation, and that the paternal and maternal chromatin may remain separate and distinct in the later stages of development — possibly throughout life (p. 299). In this regard two general classes may be distinguished. In one, exemplified by some echinoderms, by Aviplti- oxus, Phallnsia, and some other animals, the two nuclei meet each other when in the reticular form, and apparently fuse in such a manner that the chromatin of the resulting nucleus shows no visible distinc- tion between the paternal and maternal moieties. In the other class, which includes most accurately known cases, and is typically repre- sented by Ascaris (Fig. 90) and other nematodes, by Cyclops (Fig. 98), and by Pterotrachea (Fig. 93), the two nuclei do not fuse, but only place themselves side by side, and in this position give rise each to its own group of chromosomes. On general grounds we may confi- dently maintain that the distinction between the two classes is only apparent, and probably is due to corresponding differences in the rate of development of the nuclei, or in the time that elapses before their union. ^ If this time be very short, as in echinoderms, the nuclei unite before the chromosomes are formed. If it be more prolonged, as in Ascaris, the chromosome-formation takes place before unicm.

With a few exceptions, which are of such a character as not to militate against the rule, tJie number of cJiromosomcs arisitij^ f'om tJic germ-nuclei is always the same in botJi, and is one-half the number characteristic of the tissue-cells of the species. By their union, there- fore, the germ-nitclei give rise to an equatorial^ plate eonfanung the typical niLuiber of chromosomes. This remarkable discovery was first made by Van Beneden in the case of Ascaris, where the number of chromosomes derived from each sex is either one or two. It has since been extended to a very large number of animals and plants, a partial list of which follows.

1 Indeed, Boveri has found that in Ascaris both modes occur, though the fusion of the A Partial List showing the Number of Chromosomes Char- acteristic OF THE Germ-nuclei and Somatic Nuclei in Various Plants and Animals ^ ^ This table is compiled from papers both on fertilization and maturation, brackets are inferred. The above data are drawn from sources so diverse and show so remarkable a uniformity as to establish the general law with a very high degree of probability. The few known exceptions are almost certainly apparent only and are due to the occurrence of plurivalent chromosomes. This is certainly the case with Asarris {cf. j). ^j). It is probably the case with the gasteropod Ariou, where, as described by Platner, the egg-nucleus gives rise to numerous chromosomes, the sperm-nucleus to two only ; the latter are, however, plurivalent, for Garnault showed that they break up into smaller chromatin-bodies, and that the germ-nuclei are exactly alike at the time of union. W'c may here briefly refer to remarkable recent observations by Riickert and others, which seem to show that not only the paternal and mater- nal chromatin, but also the chromosomes, may retain their individu- ality throughout development.^ Van Keneden, the pioneer observer

in this direction, was unable to follow the paternal and maternal chromatin beyond the first cleavage-nucleus, though he surmised that they remained distinct in later stages as well ; but Rabl and Hoveri broujjht forward evidence that the chromosomes did not lose their identity, even in the resting nucleus. Riickert ('95, 3) and Hacker ('95, I ) have recently shown that in Cyclops \.\\<i paternal and mater- nal chromatin-groups not only remain distinctly separated during the anaphase, but give rise to double nuclei in the two-cell stage (Fig. 146). I^^ach half again gives rise to a separate grou]) of chromosomes at the second cleavage, and this is repeated at least as far as the blas- tula stage. Herla and Zoja have shown furthermore that if in Ascaris the egg of variety bivalois, having two chromosomes, be fertilized with the spermatozoon of variety univalens having one chromosome, the three chromosomes reappear at each cleavage, at least as far as the twelve-cell stage (Fig. 145); and according to Zoja, the paternal chromosome is distinguishable from the two maternal at each step by its smaller size. We have thus what must be reckoned as more than a possibility, that every cell in the body of the child may receive from each parent not only half of its chromatin-substance, but one-half of its chromosomes, as distinct and individual descendants of those of the parents.

In examining more critically the history of the centrosomes we may conveniently take Boveri's hypothesis of fertilization as a point of departure, since it has long formed the focus of discussion of the entire subject. Before the hypothesis is more closely scrutinized we may first eliminate two other views, both of which are irreconcilable with it, though neither has stood the test of later research. The first of these, doubtfully suggested by Van Beneden {''^J) and definitely maintained by Wheeler ('97) in the case of Myzostonia, is that the cleavage-centrosomes have no definite relation to the spermatozoon, but are derived from the egg — a conclusion that has the a priori support of the fact that in parthenogenesis the centrosomes are cer tainly of maternal origin.

Van Beneden's early statement may be passed by, since it was no more than a surmise. Wheeler, after a careful research, found that no sperm-aster accompanied the sperm-nucleus — a fact correlated with the absence of a middle-piece in the spermatozoon, — and reached the conclusion that after, formation of the polar bodies, the egg-centro- somes persisted to become directly converted into the cleavage-centro- somes (Fig. 104). That the absence of a distinct middle-piece is not a valid argument is shown by the insect-spermatozoon, where the region

of the middle-piece is likewise not marked off from the tail, yet as we have seen (p. 165) the centrosome passes into this part of the sperma- tozoon. Kostanecki's later examination of the fertilization of the Fig. 104. — Fertilization of the &gg of the parasitic annelid, Myzostoma. [WUKKLHk.j A. Soon after entrance of the spermatozoon ; the sperm-nucleus at cT ; at 9 the germinal vesicle; at c the double centrosome. B. First polar body formins; at 9 ; ;/. tlic cast-out nucle- olus or germinal spot. C. The polar bodies formed {p.b.)\ germ-nuclei of equal size; at <: the centrosomes. D. Approach of the germ-nuclei ; the amphiaster formed,

same animal ('98), while inconclusive on the main point, loaves little doubt that Wheeler's evidence was equally so ; for he has on the one hand shown that the sperm-nucleus is often accompanied by a sperm- aster containing a pair of centrosomes, on the other hand that these, Hke the egg-centrosonies, wholly disappear from view at a later period, the cleavage-centrosomes having only a conjectural origin. The second of the views in question is that the cleavage-centro- somes are derived from both germ-cells ; and this in turn has in its favour the a priori evidence that in the Infusoria conjugation takes place between two mitotic figures (p. 224). It appears in two forms, of which the first, though undoubtedly erroneous, has had so interest- ing a historv as to deserve a brief review. It was predicted by Rabl in i8;S9 that if the centrosome be a permanent cell-organ, the con- jugation of germ-cells and germ-nuclei would be found to involve also a conjugation of centrosomes. Unusual interest was therefore aroused when Fol, in 1891, under the somewhat dramatic title of the " Quadrille of Centres," described precisely such a conjugation of centrosomes as Rabl had predicted. The results of this veteran observer were very positively and specifically set forth, and were of so logical and consistent a character as to command instant accept- ance on the part of many authorities. In the eggs of the sea-urchin the sperm-centrosome and egg-centrosome w^ere asserted to divide each into two, the daughter-centrosomes then conjugating two and two, paternal with maternal, to form the cleavage-centrosomes. The same result was announced by Guignard ('91) in the lily, by Conklin ('93) in the gasteropod Crcpidnla, less definitely by Blanc (93) in the trout, and still later by Van der Stricht (95) in AinpJiioxus. None of these results have stood the test of later w^ork. Fol's result was opposed to the earlier conclusions of Boveri and Hertwig, and a careful reexamination of the fertilization of the echinoderm ^^^g, indepen- dently made in 1894-95 by Boveri {Ec/iinus), by myself ( Toxopncustes\ and Mathews {Arbacia, Astcrias\ and slightly later by Hill (95) and Reinke ('95) in Splicerechiiius, demonstrated its erroneous character.

Various attempts have been made to explain Fol's results as based on double-fertilized eggs, on imperfect method, on a misinterpretation of the double centrosomes of the cleavage-spindle, yet they still remain an inexplicable anomaly of scientific literature. Serious doubt has also been thrown on Conklin's conclusions by subsequent research. Kostanecki and Wierzejski ('96) made a very thorough study, by means of serial sections, of the fertilization of the gasteropod Pliysa, and reached exactly the same result as that obtained in the echinoderms. Here, also, the egg-centres degenerate, their place being taken by a new pair, arising in intimate relation with the middle-piece of the spermatozoon, about which forms a sperm- amphiaster (Fig. 89). Conklin, after renewed research, himself admitted that no quadrille occurs in Cnpidula, though he still believes that a union of paternal and maternal attraction-spheres takes place.

Guignard's results, too, have entirely failed of confirmation by later observers (p. 221), and in his own latest contribution to the subject ('99) the centrosomes are conspicuous by their absence in both the text and the figures. In like manner Van der Stricht's conclusions have been shown by Sobotta ('97) to be without substantial founda- tion, while Blanc's account, opposed to the earlier work of I^ohm, is too incomplete to carry any weight. The entire case for the " qua- drille " has thus fallen to the ground. In its second form the sup]:)o.sed double origin of the centrosomes rests upon a single research upon Ascaris by Carnoy and Le Brun ('97, 2), who assert that the cleavage- centrosomes arise de novo and separately, one inside of each of tlie germ-nuclei, to migrate thence out into the cytoplasm. At the close of mitosis they wholly disappear, to be replaced by a new pair, like- wise of intranuclear origin. Since this result is totally opposed to those of Van Beneden, Boveri, Erlanger, and Kostanecki and Sied- lecki on the same object, and is contradicted in the most positive man- ner by Fiirst,! it may be received with some scepticism. The work of Kostanecki and Siedlecki ('96) demonstrates the division of the sperm- centrosome in Ascaris as described by Boveri; and while it still remains possible that the daughter-centrosomes may for a very brief period disappear (as in some of the mollusks described beyond), no ground is given for such a conclusion as Carnoy has drawn. No one familiar with the object can repress the suspicion that Carnoy and Le Brun have confused the centrosomes with the nucleoli ; but only renewed research can determine the point.

The ground is now clear for a closer study of Boveri's hypothesis in the light of more recent research. It should first be pointed out that that hypothesis is based upon and forms a part of the more gen- eral theory of the autonomy of the centrosome ; and it the latter theory cannot be sustained, the a priori side of Boveri's hypothesis assumes a different aspect. In point of fact the general outcome of recent research on fertilization has been on the whole unfavourable to the view that the cleavage-centrosomes must necessarily be intlividu- ally identical with permanent preexisting centrosomes — indeed, it is in this very field that some of the most convincing evidence against the persistence of the centrosome has been produced. The mode of origin of the cleavage-centrosomes is nevertheless a question of high interest on account of the unmistakable genetic relations existing between the centrosome of the spermatid and spermatozoon and those of the sperm-amphiaster within the QgZ-

There are two points of capital importance to be determined before a definite decision re2:ardino: the origin of the cleavage-centrosomes can be reached. First, are the centrosomes of the sperm-aster within the ^g^ identical with, or the descendants of, a centrosome or pair of centrosomes in the middle-piece of the spermatozoon ? Second, do they actually persist to form those of the cleavao^e-amphiaster ? In the present state of knowledge we are not in a position to give an affirmative answer to the first of these questions. As has been shown in Chapter III., it is no longer possible to doubt that the middle- piece either contains or is itself a metamorphosed centrosome ; but, as pointed out at page 196, it does not seem possible that the extremely minute centrosome of the sperm-aster can represent the entire cen- trosome of the middle-piece (however we conceive the origin of the latter). At most we can only assume that a part of the latter per- sists as the sperm-centrosome within the ^gg. The exact origin of the latter still remains problematical. A large number of observers are now agreed that the sperm-aster is formed about a focus that is either in or very near the middle-piece ; ^ but no one, I believe, has yet succeeded in showing that the centrosome actually is the meta- morphosed middle-piece, or escapes from it.- The possibility there- fore remains that the centrosome of the sperm-aster is not actually imported as such into the Qgg, but is either only a portion of the original spermatid-centrosome, or, as was first suggested by Miss Foot ('97) and further discussed by Mead (98, 2), is, like the aster, formed anew in the egg-cytoplasm. If the latter alternative be the case, the original form of Boveri's hypothesis would have to be abandoned;

2 I mvself formerly concluded ('95, 2) that the entire middle-piece of echinoderms is the centnjsome — a result apparently confirmed in a most positive manner by Erlanger ('98), as well as by R. Hertwig ('96) and Doflein ('97, 2). I have, however, demonstrated this to be an error, showing that the extremely minute centrosome is (juite distinct from the mi. Idle-piece, the latter being thrown aside and degenerating in the egg-cytoplasm outside of the newly formed sperm-aster (Figs. 12, 94). This fact, of which the phenomena in Toxopufustes leave no doubt (see Wilson, '97, '99), is, I think, fatal to Kostanecki's and Wierzejski's theory of fertilization ('96, pp. 374-375)' according to which the archojilasm of the middle-piece gives rise to the new astral system and is thus the essential fertilizmg sub- stance (the centrosome being merely a mechanical centre for the attachment of the rays) ; but the most careful examination has still failed to show whether the centrosome actually escapes from the middle-piece, nor have other observers had l)ctter success with any animal. Erlanger ('96, 2, '97, 4) believes he has seen the centrosome in the Ascaris spermatozoon as a distinct body lying behind the nucleus, and that it can be traced continuously into the egg and after its division into the two poles of the cleavage-figure. Neither the schematic figures of his preliminary nor the photographic ones of his final paper seem sufficient to establish either the identity or the subsequent history of the granule in question.

though in substance it would still retain an element of truth, as pointed out beyond. We may now examine the question whether the sperm-centrosomcs are actually identical with the cleavage-centrosomes. That such is the case is positively maintained in the case of Ascaris by I^overi Kostanecki, and Erlanger, in PJiysa by Kostanecki and \Vierzejski ('96), in Thalassema by Griffin ('96, '99), and in Chcetoptcrus by Mead ('95, '98). The two last-mentioned observers, who have followed the phenomena with especial care, produce very strong evidence that at no time do the sperm-centrosomes and asters disa])pear, and that the former may be traced in unbroken continuity from the time of their first appearance to the daughter-cells resulting from the first cleavage (Figs. 99, 155). On the other hand, a considerable number of observers, beginning with W^x\.\V\g{PhyllirrJioc\ Ptcrotmchca, '75), have found that as the sperm-nucleus enlarges the sperm-asters di- minish in size, until, in many cases, they nearly or quite disappear ; for example, in /^r^i-///^f^r^;/j-(KHnckowstrom, '97), in the mouse (Sobotta, '95), in PlciiropJiyllidia (MacFarland, '97), Pliysa (Kostanecki and Wierzejski, '96), Arenicola {Q\i\\(\i, '97), 6';//^ (Lillie, '97), Myzostoma (Kostanecki, '98), and Cerebmtulus (Coe, '98).^ Several of these observers (KUnckowstrom, MacFarland, Lillie, Child) have found that not only the asters but also the ccntrosomes totally disappear about the time the germ-nuclei come together, a new pair of cleavage- centrosomes and asters being afterward developed at the poles of the united nuclei. These conclusions, if correct, place in a new Hght the disappearance of the egg-centrosomes ; for this process

1 Coe has pointed out that the eggs of various animals may be arranged in a scries show- ing successive graduations in the disappearance of the sperm-asters. " At the head of the series we must place the eggs of Ascaris and I\[yzostouia (according to Kostanecki) and similar ones in which the sperm-asters make their appearance only a short time befitrc the formation of the cleavage-spindle, and which, consequently, suffer no diminutit)n in si/c. Following these are the eggs of Chixtopterus (Mead) and Ophryotrocha (Korschclt) and of some echinoderms in which the sperm-asters develop very early, but are not described as decreasing in size before the formation of the cleavage-spindle. Then cume the eggs of Toxopnensies (Wilson) and Thalassema (Griffm), where the sperm-asters appear early and develop to a very considerable size, but nevertheless become very much smaller ant! less conspicuous after the germ-nuclei have come together. After these we must place the eggs of Physa (Kostanecki and Wierzejski), for here the sperm-asters, after becoming very large and conspicuous, degenerate to such an extent that only a very few exceeilingly delicate fibres remain. Those of Cerebratulus follow next.

" Here the sperm-asters increase in size until they extend thrt>ughout the whole body of the cell, but at the time of fusion of the germ-nuclei they degenerate completely. The peripheral portions of their fibres, however, may be followed, as stated above of PUuro- phyllidia, Prosthecerccus, etc., where the sperm-asters degenerate soon after their forma- tion, so that for a considerable period the tg^ is without trace of aster-llbres. Vet in all of those cases where the sperm-asters disappear and their centrosomes become lost among the other granules of the cell, we are justified in believing that the sperm-centrosomes nevertheless retain their identity, and later reappear in the cleavage-asters " ('98, p. 455).

wuuld thus seem to be of the same nature as the disappearance of the sperm-centrosomes, and both Hoveri's theory of fertilization and the f>-eneral hypothesis (^f the ]K"rmanence of the centrosomes would receive a serious bhnv. The investii^ators to whom these observations are due have ranged themselves in two gnnips in the interpretation of the phenomena. On the one hand. Lillie and Child do not hesitate to maintain that the centrosomes actually go out of existence as such, to be re-formed like the asters out of the egg-substance ; and that such a new forma- tion of centrosomes is possible seems to be conclusively shown by the experiments of Morgan and Loeb described at pages 2 1 5 and 307. On the other hand, Sobotta, .MacF'arland, Kostanecki, and Coe, relying partly on the analogy of other forms, partly on the occasional pres- ence of the centrosomes during the critical stage, urge that the dis- appearance of the sperm-centrosomes is only apparent, and is due to the disappearance of the asters, which renders difficult or impossible the identification of the centrosomes among the other protoplasmic granules of the c,^^. These authors accordingly still uphold Boveri's

It is difficult to sift the evidence at present, for it has now become very im])ortant to reexamine, in the light of these facts, those cases in which the absolute continuity of the centrosome has been main- tained - for example, in Ascaiis, ChcBtopterus, and TJialassema — in order to determine whether there may not be here also a brief critical period in which the centrosomes disappear. There are, however, some facts which tend to sustain the conclusion that even though the sperm-centrosomes disappear from view, there is some kind of genetic continuity between them and the cleavage-centrosomes. First, both Kostanecki and Wierzejski (96) and Coe ('98) have found that there is some variation in eggs apparently equally well preserved, a few individuals showing the sperm-centrosomes at the poles of the united nuclei at the same period when they are invisible in other individuals. Second, both these observers, Coe most clearly, have shown that the egg-centrosomes di.sappear considerably earlier than the sperm-cen- trosomes, and Coe has traced the sperm-centrosomes continuously to the exact points {tJic poles of tJie united nuclei) at zvhicJi the cleavage- centrosomes afterzvartl appear {V\g. 155). This important observation leads to the suspicion that the apparent disappearance of the centro- somes may be due to a loss of staining-capacity at the critical period, or that even though the formed centrosome disappears its substance reappears in its successor. Here again we come to the view sug- gested at page 1 1 1, that the centrosome may be regarded as the vehicle of a specific chemical substance which is transported to the nuclear poles by its division, and may there persist even though the body of the

centrosome be no longer visible. On such a basis we may perhaps find a reconciliation between these observations and Boveri's theory, and may even bring the fertilization of plants into relation with it (p. 221). Even in case of the nucleus, universally recognized as a permanent cell-organ, it is not the whole structure that persists as such during division, but only the chromatin-substance — in some cases only a small fraction of that substance. The law of genetic continuity therefore would not fail in case of the centrosome, though only a portion of its substance were handed on by division ; and even if we take the most extreme negative position, assuming that the sperm-centrosome is wholly formed anew under the stimulus of the spermatozoon, we should still not escape the causal nexus between it and the centrosome of the spermatid.