The Cell in Development and Inheritance

The two accounts just given represent two extremes, the first agreeing essentially with Ascaris, the second with the copepods or insects. When we compare them with others, we encounter a truly bewildering confusion. Strasburger and Mottier ('97) themselves soon abandoned their acceptance of the reducing division, returning to the conclusion that in both sexes {Liliuin, Podopliylhnn) both divi- sions involve a longitudinal splitting of the chromosomes (Figs. 133, 134). In the first division the longitudinally split spireme segments into twelve double rods, which bend at the middle to form double V's, with closely approximated halves. Becoming attached to the spindle by the apex, the limbs of each separate to form a o -figure. At telophase the daughter-V's shorten, thicken, and join together to form a daughter-spireme consisting of a single contorted thread. This splits lengthwise tJirougJioiit its whole extent^ and then segments into double chromosomes, the halves of which separate at the second division (Fig. 135, L-M). The latter, therefore, like the first, involves no reducing division. This result agrees in substance with the slightly earlier work of Dixon ('96) and of Miss Sargant (96, '97), whose account of the origin of the 0 -figure of the first division differs, however, in some interesting details. It is also in harmony with the general results of Farmer and Moore ('95), of Gregoire ('99), and of Guignard ('98), who, however, describes the first division nearly in accordance with Strasburger's account of 1895, as stated above. On the other hand, Ishikawa (pollen-mother-cells of Allium, 'gy) and especially Belajeff (pollen-mother-cells of Iris, 98) conclude that the second division is a true transverse or reducing division.

^ Ishikawa described the first division as being nearly similar to the ring-forma- tion in copepods, the four elements of the ring being often so condensed as -nearly to resemble an actual tetrad. In the early ana- phases the daughter-V's break at the apex ; and, although in the later anaphases the limbs reunite, Ishikawa is inclined to regard the trans- verse division as being a preparation for the second mitosis. Bela- jeff's earlier work ('94) on Liliuin gave an indecisive result, though one on the whole favourable to a reducing division. In his latest paper, however ('98, i), Belajeff takes more positive ground, stating that after the examination of a large number of forms he has found

1 Schaffner ('97, 2) reaches exactly the reverse result in Lilium philaJelphicum, i.e. the first division is transverse, the second longitudinal. in the pollen-mother-cells of Iris a nuich more favourable object of investi<;ation than Liliu))i, I'^i til/aria, and the other forms on which most of the work thus far has been done, and one in which the sec- ond division takes place with "admirable clearness"; he also gives interesting^ additional details ot the first division in this and other forms. In the first division the spireme splits lengthwise, and then breaks into chromosomes, which assume the shape of a V, Y, or X (Fig. 135, X-Q). The two limbs of these bodies do not, as might be

Fig;. 134. — Tlie seconrl maturation-division in flowering plants. [B. SXRASBURGER and Mullii.K; the others from Moi iiKR.] A. Nucleus of secondary spermatocyte {Podophyllum). D. Prophase of second division {Ltliutn, male) with longitudinally divided chromatin-threads. E. Corresponding stage in the female. E. Metaphase of second division {Podophyllnm, male). G. Initial anaphase {Lilmvi, female). CD. illustrate Mottier's earlier conclusions. ('. Second division {Lilium, male), with chromosomes bent together so as to simulate a sjilit. A Slightly later stage [Eritillaria, male), showing stage supposed to result from breaking apart of the limbs of the U at point of flexure.

supposed, represent sister-chromosomes (resulting from the longitu- dinal division of the spireme) attached by one end or at the middle, since each X, Y, or V is double, consisting of two similar superim- posed halves. Belajeff, therefore, regards these figures as longitu- dinally divided bivalent chromosomes, having the value of tetrads, each limb being a longitudinally split single chromosome. The double V's, Y's, and X's take up a position with the apex (or one end of the X) attached to the spindle, and the longitudinal division in the equatorial plane. The halves then progressively diverge from the

point of attachment, thus giving rise to 0 -shaped, ^>- -shaped, or XX -shaped figures, all of which in the end assume the 0 -shape. This part of the process is in the main similar to that described by Strasburger and Mottier, and the daughter-V's diverge in the same way as these authors describe. The second division, however, differs radically from their account, since no splitting of the spireme-thread occurs. The chromosomes reappear in the V-, Y-, and X-forms, but are ufidivided, and only half as thick as in the first division. Passing to the equator of the spindle, the V- and Y-forms break apart at the apex, while the X-forms separate into the two branches of the X, the daughter-chromosomes having the form of rods slightly bent at the outer end to form a J-figure (Fig. 135, R-T). This division is, accordingly, a transverse or reducing one, which " corresponds com- pletely to the reduction-division in the animal organism" ('98,2, p. 33.) Atkinson ('99) reaches the same general result in Tnlliuni, stating very positively that no longitudinal division occurs in the second mitosis, and believing that the daughter-V's of the first (hete- rotypical) mitosis retain their individuality throughout the ensuing pause, and break apart at the apex (reducing division) in the second mitosis. This observer finds further that in Arisc^ma the heterotvpi- cal rings of the first mitosis condense into true tetrads, by one longi- tudinal and one transverse division, but believes that in this case it is th.Qjirst division that effects the reduction, as in the insects.

Such confusion in the results of the most competent observers of reduction in the flowering plants is itself a sufficient commentary on the very great difficulty and uncertainty of the subject; and it would be obviously premature to draw any positive conclusions until further research shall have cleared up the matter.^ 1 Strasburger's new book, entitled Uber Redtiktionsiheiliing, Spindelbildung, Cfutroso- men nnd Cilienhildner im PJlanzenreich (Jena, 1900), is received while this work is in press, too late for analysis in the text. In this treatise the author gives an exhaustive review of the entire subject, contributing also many new and important observations on Liliuni, Iris, Podophylliwi, Tradescantia, Allium, larix, and several other forms. The general result of these renewed researches leads Strasburger to return, in tlie main, to his conclu- sions of 1895, ^^'ith which agree, as stated above, the results of (luignard and Cregoire; and. in a careful critique of Belajeff's work, he shows how the results of this observer may be reconciled with his own. The essence of Strasburger's interpretation is as follows. In the prophases of the first division the chromosomes first undergo a longitudinal division, shorten to f9rm double rods, and then again split lengthwise in a plane at right angles to the lirst. The following stages vary even in the same species {Uliiini) \ and here lies the explanation of much of the divergence between the accounts of different observers, (i) In the typical case, the chromosomes are placed radially, with one end next the spindle; ami, during the metaphase, they open apart along the first division-jilane, from the spindle outwards, to form h- -shaped figures. These figures meanwhile open apart from the free end inwards along the second division-plane. Thus arise the characteristic o -shaped figures, the daughter-V\s having separated along the first (equatorial) division-plane, while the two hmbs ut each V have resulted, not through bending, but from a secoml (axial) split (I'ig. 135, A-//'). The

Fig. 135. — Diagrams illustrating different accounts of reduction in the flowering plants. A-D. Vegetative mitoses (heterotypical form) in Picea. [BelajEFF.] E-I. Illustrate Strasburger's earlier account (95) and the later one of Guignard, of the first maturation-division. E. Doubly split rod. F. Metaphase, in profile. G. The same en face, showing the heterotype ring. H. I. Opening out and breaking apart of the ring. J-M. Later account of Strasburger and Mottier {cf. Figs. 133, 134). J. Longitudinally split, V-shaped chromosome of first division. K. Opening out of the ring. L. Prophase of second division, showing longitudinally split segmented spireme. M. Initial anaphase of second division.

N-Q. First division. [BELAJEFF.] A'. Longitudinally split chromosomes, viewed in the equa- torial plane. O. The same viewed in the axis of the spindle. P. Separation of the daughter- chromosomes. Q. Anaphase, all the chromosomes assuming the V-form. /?- 7: Second division in /Wj. [Belajeff.] R. Equatorial plate, limbs of X's and V's break- ing apart (reducing division). 6'. Slightly later stage, with daughter-chromosomes still united at one end. T. Anaphase.

Resume. In reduction without tetrad-formation the spireme seg- ments into half the somatic number of chromosomes, which split lengthwise and open out to form rings for the first (heterotypical) mitosis. According to one set of observers, including Flernming, Meves, McGregor, Kingsbury, Moore, Klinckowstrom, Van der Stricht', Francotte, Grifhn, Belajeff, Farmer, Dixon, Strasburger, Sarganti Mottier, Ishikawa, and Atkinson, the ring arises by a single longi- tudinal division. According to another group, including Carnoy, Le Brun, Guignard, and Gregoire, the ring arises through a double longitudinal division, one representing the axial and the other the equatorial plane of the <> -figure. The second group of observers regard both maturation-divisions as longitudinal. Among the first group, Flemming, Meves, McGregor, Kingsbury, Moore, Farmer, Dixon, Strasburger, Sargant, and Mottier likewise believe both divi- sions to be longitudinal, the daughter-V's or their products again splitting lengthwise for the second division ; while Klinckowstrom, Van der Stricht, Francotte, Griffin, Belajeff, Ishikawa, and Atkinson beHeve one of them to be transverse, the daughter-V's breaking apart at the apex, and thus giving the reducing division of Weismann.^

We may here briefly consider some interesting observations which show that in some cases the nuclear substance may be unequally distributed to the germ-nuclei. Henking (''90) discovered that in the second spermatocyte-division of Pyrrocho- r is one of the *' chromosomes ^' passes undivided into one of the daughter-cells (spermatids) which receives twelve chromatin-elements while its sister receives but eleven. (The number of chromosomes in the spermatogonia, and of rings in the first spermatocyte-division is twenty-four). This anomalous process is conrirmed with interesting additional details by Paulmier ('99) in Anasa, and obviously rt-lated phenomena are described by Montgomery ('99, i ) in Pentatonia., and by McClung ("99) in XipJiidin7n.

breaking apart of the V's at the apex, as described by Belajeff, is, therefore, not a transverse division, but merely the completion of the second longitudinal division. (2) In a second and exceptional type, the chromosomes are placed tnngcniially to the spindle, and the halves separate from the middle, again producing <> -shaped tigures. These, however, arc not of the same nature as those arising in the first case, since they are formed by a bending out of each daughter-chromosome at the middle to form the V, and not by the secoml longi- tudinal split. The effect of the latter is in this case to render each daughter-V in itself double, precisely as in the salamander. The difference between the two types results merely from the difference of position of the chromosome with respect to the spindle, and the final result is the same in both, i.e. two longitudinal divisions and no reducing one.

This highly important work brings very strong evidence against the occurrence of trans- verse or reducing divisions in the higher plants, and seems to explain satisfactorily most of the differences of interpretation given by other observers. It will be interesting:: to see whether a similar interpretation is possible in the case of mollusks, anneliiis. and arthropods, where the early stages, in many cases, so strikingly resemble those occurring in the plants.

In Pcntatoma the number of chromosomes in the spermatocyte is fourteen. During the final anaphases of the hist division, one of the fourteen daughter-chronio- somes\ssumcs a ilitTerent stainin.i;-cai)acity from the others, and becomes a " chro- matin-nucleohis " which fra.iimcnts into several smaller bodies during the ensuing rcsting-stage. During each of the succeeding spermatocyte-divisions appear seven chronrosomes and a single small chromatin-nucleolus, and both of these kinds t)f bodies are halved at each division, so that each spermatid receives seven chromo- somes and a single chromatin-nucleolus.' In Xipliidittin a body called by McClung the -accessory chromosome," and believed by him to correspond to the "chromatin- nucleolus '* of VVz/A/Av//./, appears in the early prophases of the last spermatogoniuni- division while the remaining chromatin still forms a reticulum. In the equatorial plate this lies outside the ring of chromosomes, but divides like the latter. The same bodv appears in the ensuing resting-stage, and during both of the spermatocyte- divisions.' In these it lies, as before, outside the chromosome-ring, and ditiers markedly from the other chromosomes, but divides like the latter, each of the halves passing into one of the spermatids, where it appears to form an important part of the sperm-nucleus.

Despite the peculiarities described above, tlie chromatin, as a whole, seems to be equally distrii)uted in both Pcntatoma and Xiphidimn. In Anasa, however, Paul- niier's studies (98, '99), made in my laboratory, give a result agreeing with that of Henking. and suggest some very interesting further questions. The spermatogonia- nuclei contain two nucleolus-like bodies, and give rise to twenty-two chromosomes, of which two are smaller than the others (Fig. 126). In the first spermatocyte-divi- sion appear eleven tetrads. Ten of these arise from rings like those of uryllotalpa. etc. The eleventh, which is much smaller than the others, seems to arise from a single nucleolus-like body of the spermatocyte-nucleus. and by a process diftering coirsiderably from the others. All of these bodies are halved to form dyads at the first division. In the second spermatocyte-division (Fig. 127) the larger dyads divide to form single chromosomes in the usual manner. The stnall dyad, Jwwevcr, fails to divide, passitii^ over bodily into one of the spermatids. In tliis case, there- fore, half of the spermatids receive ten single chromosomes, while the remainder receive in addition a small dyad.

A comparison of the foregoing results indicates that the small tetrad (dyad) corre- sponds to the e.xtra chromosome observed by Henking in Pyrrochoris, and perhaps also to the "accessory chromosome"' oi Xiphidiuni. Whether it corresponds to the '•chromatin-nucleolus'' oi Pentatonia is not yet clear. The most remarkable of these strange phenomena is the formation of the small tetrad, which seems to be a non-essential element, since it does not contribute to all the spermatozoa. Paulmier is inclined to ascribe to it a vestigial significance, regarding it as a "degenerating" chromosome which has lost its functional value, though still undergoing in some measure its original morphological transformation : in this connection it should be pointed out that the spermatocyte-nucleolus, from which it seems to be derived, is represented in the spermatogonia by two such nucleoli, just as the single small tetrad is represented by two small chromosomes in the spermatogonia-mitoses. The real meaning of the phenomenon is, however, wholly conjectural.

There are many peculiarities in the early history of the germ- nuclei, both in plants and animals, that have a special interest in con- ^ On this latter point Montgomery's observations do not seem quite decisive. nection with the reduction-problem ; and some of these have raised some remarkable questions regarding the origin of reduction. A large number of observers are now agreed that during the growth- period preceding the maturation-division (p. 216), in both sexes, the nucleus of the mother-cell (spermatogonium, oogonium), both in plants and in animals, passes through some of the changes prepara- tory to reduction at a very early period. Thus, in the (ty:^^ the pri- mary chromatin-rods are often present in the very young ovarian eggs, and from their first appearance are already split longitudinally.' Hacker ('92, 2) made the interesting discovery that in some of the copepods {CantJiocamptiis, Cyclops) these double rods could be traced

Fig. 136. — Longitudinal section through the ovary of the copepod Canthocamptus. [Hacker,] og. The youngest germ-cells or oogonia (dividing at og.'^) ; a. upper part of the growth-zone; oc. oocyte, or growing ovarian egg; ov. fully formed egg, with double ciiromatin-rods. back continuously to a double spireme-thread, following immediately upon the division of the last generation of oogonia, and that at no period is a true retictibnn formed in tJie gerniiual vesicle (Fig. 136). In the following year Ruckert('93, 2) made a precisely similar discm-- ery in the case of selachians. After division of the last generation of oosconia the daufrhtcr-chromosomes do not give rise to a reticu- lum, but spHt lengthwise, and persist in this condition throughout the entire growth-period of the ^^g. Riickert therefore concluded that the germinal vesicle of the selachians is to be regarded as a " daughter-spireme of the oogonium {Vr-ei) grown to enormous dimensions, the chromosomes of which are doubled and arranged in

1 Hacker, Vom Rath, Riickert, in copepods; Ruckert in selachians; Rom and Fick in Amphibia; HoU in the chick; Ruckert in the rabbit. pairs." ^ In this case their number seems to be at first the somatic number (thirty-six), which is afterward halved by conjugation of the elements tw(^ and two ( Riickert ), as in /.//w/t/V//^- (Calkins). It is, however, certain that in many cases (insects, copepods) the double rods first appear in the reduced number, and the observations of Vom Rath ('93) and Hacker ('95, 3 ) K*^'^ some reason to believe that the reduced number mav in some forms be present in the earlier proi^eni- tors of the germ-cells, the former author having found but half the normal number in some of the embryonic cells of the salamander, while Hacker ('95, 3) finds that in Cyclops brcviconiis the reduced number of chromosomes (twelve) appears in the primordial germ-cells which arc differentiated in the blastula-stage (Fig. 74). He adds the inter- esting discovery that in this form the somatic nuclei of the cleavage- staL^es show the same number, and hence concludes that all the chromosomes of these stages are bivalent. As development proceeds, the germ-cells retain this character, while the somatic cells acquire the usual number (twenty-four) — a process which, if the conception of bivalent chromosomes be valid, must consist in the division of each bivalent rod into its two elements. We have here a wholly new light on the historical origin of reduction ; for the pseudo-reduction of the germ-nuclei seems to be in this case a persistence of the embryonic condition, and we may therefore hope for a future explanation of the process by which it has in other cases been deferred until the penul- timate cell-generation, as is certainly the fact in Ascaris?

This leads to the consideration of some very interesting recent dis- coveries regarding the relation of reduction to the alternation of gen- erations in the higher plants. As already stated (p. 263), Strasburger, Guignard, and other observers have found that in the angiosperms the two maturation-divisions are in both sexes followed by one or more divisions in which the reduced number persists. The cells thus formed are generally recognized as belonging to the ve.stiges of the sexual generation (prothallium) of the higher cryptogams, the pollen- grains (or their analogues in the female) corresponding to the asexual spores of the archegoniatc cryptogams. We should, therefore, expect to find reduction in the latter forms occurring in the two correspond- ing divisions, by which the "tetrad " of spores is formed (as was first pointed out by Hartog, '91). Botanists were thus led to the surmise, first expressed by Overton in 1892, that the reduced number would be found to occur in the ]-)rothallium-cclls derived from those spores.

- It may be recalled that in Ascai'is Boveri proved that the ]irimordial germ-cells have the full number of chromosomes, and Hertwig clearly showed that this number is retained up to the last division of the spermatogonia. Ishikawa ('97) finds that in Alliiivi the reduced number (eight) appears in the mitosis of the " UrpoUenzellen " preceding the pollen-mother-cells. This is, however, contradicted by Mottier ('97, 2). This surmise quickly became a certainty. Overton himself dis- covered ('93 ) that the cells of the endosperm in the i^ymnosperm Ccratozamia divide with the reduced number, namely eight ; and Dixon observed the same fact in Pinus at the same time. In the following year Strasburger brought the matter to a definite conclusion in the case of a fern {Osmitnda), showing that a// the cells of the prothallinni, from the original spore-mother-cell ouzuanls to the for- mation of the germ-cells, have one-half the number of chromosomes found in the asexual generation, namely twelve instead of twentv- four ; in other words, the reduction takes place in the formation of the spore from which the sexual generation arises, many cell-genera- tions before the germ-cells are formed, indeed before the formation of the body from which these cells arise. Similar facts were determined by Farmer in Pallavicinia, one of the Hepaticae, where all of the nuclei of the asexual generation (sporogonium) show eight chromo- somes during division, those of the sexual generation (thallus) four. It now^ seems highly probable that this will be found a general rule.

The striking point in these, as in Hacker's observations, is that the numerical reduction takes place so long before the fertilization for which it is the obvious preparation. Speculating on the meaning of this remarkable fact, Strasburger advances the hypothesis that the reduced number is the ancestral number inherited from the ancestral type. The normal, i.e. somatic, number arose through conjugation by which the chromosomes of two germ-cells were brought together. Strasburger does not hesitate to apply the same conception to ani- mals, and suggests that the four cells arising by the division of the oogonium {o-gg plus three polar bodies) represent the remains of a separate generation, now a mere remnant included in the body in somewhat the same manner that the rudimentary prothallium of angi- osperms is included in the embryo-sac. This may seem a highly improbable conclusion, but it must not be forgotten that so able a zoologist as Whitman expressed a nearly related thought, as long ago as 1878 : '' I interpret the formation of polar globules as a relic of the primitive mode of asexual reproduction.'^ ^ Strasburger's \iew is exactly the reverse of this in identifying the polar bodies as the remains of a sexual generation; and as Hacker has pointed out ('98, p. 102), it is difficult to reconcile with the fact that true reduction appears to occur already in the unicellular organisms (p. ijy). The hypothesis is nevertheless highly suggestive and one which suggests a quite new point of view for the study not only of maturation but also of the whole problem of sexuality.

We may now return to the consideration of some details. In a considerable number of forms, though not in all, the early prophase is characterized, especially in the male, by a more or less complete concentration of the chromatin-substance at one side of the nucleus. This sta<;e, to which Moore has given the name synapsis {Y\g. 120, A\ sometimes occurs when the spireme thread is already split {Ascaris, Lilium), sometimes before the division is visible (insects). In cither case till- cluv}natin-sig)iu)its ijfii?-i^c from the synapsis stage longitndi- nallv divided and in the redneed nnniber, a fact which gives ground for the conclusion that the synapsis is in some way concerned with the rearrangement of the chromatin-substance involved in the numer- ical reduction. During the synapsis the nucleolus remains cjuite distinct from the chromatin, and in many cases it afterward persists beside the tetrads, in the formation of which it takes no j^art, to be cast out into the cytoplasm (Fig. 124) or to degenerate /// situ during the hrst maturation-division.

A suggestive phenomena, described by several observers,^ is the casting out of a large part of the nuclear reticulum of the germinal ./. I'irst polar spindle with tetrads, in Hetetocope. [HaCKER]. B. Second polar spindle in Triton. [Caknoy and LeBkun.] C. First polar spindle of ^jrjr/^. [FiJRST.] vesicle at the time the polar bodies are formed (Figs. 97, 128). In these cases {Asterias, PolyeJio^nis, Tlialassenia, Nereis) only a small fraction of the chromatin-substance is preserved to form the chromo- somes, the remainder degenerating in the cytoplasm.-

As a final point we must briefly consider the varying accounts of the achromatic maturation-figures in the female already briefly referred to at page 85. In many forms {^e.g. in turbellarians, nemertines, anne- lids, mollusks, echinoderms) the polar am]:)hiasters are of quite ty])ical form, with large asters and distinct centrosomes nearly similar to those of the cleavage-figures. In others, however (nematodes, arthropods, tunicates, vertebrates), the polar spindles differ markedly from those of the cleavage-figures, being described by many authors as entirely devoid of asters and even in some cases of centrosomes (Fig. 137).

2 Cf. the enormous reduction of the chromatin-substance in the elasmobranch egg, p. 338. There can be no doubt that these polar spindles differ from the usual type, and that they approach those recently described in the mitosis of the higher plants, but it is doubtful whether the apparent absence of asters and centrosomes is normal. In Ascaris, the first polar spindle arising by a direct transformation of the germinal vesicle (Fig. i ij) has a barrel-shape, with no trace of asters. At the poles of the spindle, however, are one or two deeply staining granules (Fig. 137), w^hich have been identified as centrosomes by Hacker (94) and Erlanger ('97, 4), but by Fijrst ('98) are regarded as central granules, the whole spindle being conceived as an enlarged centrosome.* For the reasons stated at page 314, I believe the former to be the correct interpretation. 2 Spindles without centrosomes have been described in the eggs of tunicates (Julin, Hill, Crampton), in Ajup/iioxits CSohoiin), in some species of copepods (Hacker), and in some vertebrates {Dic- myctyhis, Jordan ; mouse, Sobotta). In Aviphioxus (Sobotta) and Triton (Carnoy and LeBrun) complete asters are not formed, but fibrillas apparently corresponding to astral rays and converging to the spindle-poles are found outside the limits of the spindle (Fig. 137). In the guinea-pig, according to Montgomery ('98), centrosomes and asters are present in the first polar spindle, but absent in the second. The evidence is on the whole rather strong that the achromatic figure in these cases approaches in form that seen in the higher plants ; but it is an open question whether the appearances described may not be a result of imperfect fixation.

Although the one-celled and other lower forms have not yet been sufficiently investigated, we have already good ground for the conclu- sion that a process analogous to the reduction of higher types regularly recurs in them. In the conjugation of Infusoria, as already described (p. 223), the original nucleus divides several times before union, and only one of the resulting nuclei becomes the conjugating germ-nucleus, while the others perish, Hke the polar bodies. The numerical corre- spondence between the rejected nuclei or " corpuscules de rebut " has already been pointed out (p. 227). Hertwig could not count the chro- mosomes with absolute certainty, yet he states ('89) that in Piircivuv- cinm caudatum, during the final division, the number of spindle-fibres and of the corresponding chromatic elements is but 4-6, while in the

2 Sala ('94) and Fiirst have shown that occasionally the polar spindles of Ascaris are provided with large typical asters, and thus resemble those of annelids or mollusks. Sala believed this to be an effect of lowered temperature, but Fiirst 's observations are unfavour- able to this conclusion. earlier divisions the number is aj^proxiniately double this (8-9). This observation makes it nearly certain that a numerical reduction of chromosomes occurs in the Protozoa in a manner similar to that of the higher forms ; but the reduction here aj)])ears to be deferred until

Fig. 138. — Conjugation and formation of the polar bodies in Actinophrys. [SCHAUDINN.] A. Union of the gametes; first polar spindle. /?, Fusion of the cell-bodies; a single polar body near the periphery of each. C. Fusion of the nuclei. the final division. In the gregarines Wolters ('91) has observed the formation of an actual polar body as a small cell segmented off from each of the two conjugating animals soon after their union ; but the number of chromo- somes was not deter- mined. Schaudinn ('96, 2) has observed a like process in Acti- iwpJuys, each of the

off a single polar body, after which the germ- nuclei fuse (Fig. 138). It is ])ossible, as R. Hertwig ('98) points out, that in both these forms a second ])olar body may have been overlooked, owing per- haps to its rapid dis- Fig. 139. — Formation of polar bodies and conjugation in HltCgration. \\\. ActlllO- A. Two gametes ("secondary cysts"), resulting from the p tt • /' o \ fU division of a "primary cyst"; second maturation-spindle in rierLWlg ^ 9*^ A '-^^