The Cell in Development and Inheritance

each; first polar bodv shown in the right gamete, at/. B. Both nuclcUS of Cach gamete one formmir m the left gamete. C. Subsequent fusion of the ^ gametes; nuclei uniting, two polar bodies (probably the second. SUCCeSSion tO form tWO the first having been absorbed) at/. D. The young AcfinospAcF- num escaping from the cyst-wall; the cleavage-nucleus has the first having been absorbed) at/. D. The yonng Actinosphce- t-)q]ot- bodies (^nnrlei^ Which the germ-nuclei unite (Fig. 139). Whether a reduction in the number of chromosomes occurs in these cases was not determined •

dkt^' f'^^'n ^J^^' ""'^"' ^°"' chromatophores. B. Chromatophores reduced to two. nuclei distinct, a Fusion of the nuclei. D. First cleavage of tiie zygote. /.-. Resulting 2-cell stage /-. becond cleavage. G. Resulting stage, each cell bi-nucleate. //. Separation of tl,e cells- one 01 the nuclei in each enlarging to form the permanent nucleus, the other (probably reprd senting a polar body) degenerating. ' 1 Achnosphceriufu forms one of the most extreme known cases of in-iireeding; for the gametes are sister-cells which immediately reunite after forming the polar bodies. The general facts are as follows : The mother animal, containing verv numerous nuclei, l>ecomes encysted, and a very large number of the nuclei degenerate. TJie bodv then segments into

Adclea (one of the Coccidire) is a \cry interesting case, for accord- ing to Sicdlecki ('99) polar bodies or their analogues are formed in both sexes. The gametes are here of very unequal size. Upon their union the smaller male cell divides twice to form apparently equiva- lent spermatozoids, of which, however, only one enters the ovum, while three degenerate as polar bodies. These two divisions are of different tvpe ; the first resembles true mitosis, while the second is of sim]-)ler character and is belie\ed bv Siedlecki to effect a reduction in the number of chromosomes. In the meantime the nucleus of the macro- gamete moves to the surface and there expels a portion of its chro- matin, after which union of the nuclei takes place. Interesting facts have been observed in unicellular plants which indicate that the reduction may here occur either before (diatoms) or after (desmids) fusion of the conjugating nuclei. In the ioxxwitx {Rliopalodina) Klebahn ('96) finds that each nucleus divides twice, as in many Infusoria, giving rise to two large and two small nuclei. Each of the conjugates then divides, each daughter-cell receiving one large and one small nucleus. The four resulting individuals then conjugate, two and two, the large nuclei fusing while the small (polar bodies) degenerate. The com- ])arison of this case with that of the Infusoria is highly interesting. In the desmids on the other hand (^'/^^•/mV/;;/ and Cosmariuin, Fig. 140), according to Klebahn ('92), the nuclei first unite to form a cleavage- nucleus, after which the zygote divides into two. Each of the new nuclei now divides, one of the products persisting as the perma- nent nucleus, while the other degenerates and disappears. Chmie- lewski asserts that a similar process occurs in Spirogyra.

Although the numerical relations of the chromosomes have not been determined in these cases, it appears probable that the elimination of a nucleus in each cell is a process of reduction occurring after fertilization. The maturation of eggs that develop without fertilization is a sub- ject of special interest, partly because of its bearing on the general theory of fertilization, partly because it is here, as I believe, that one of the strongest supports is found for the hypothesis of the individ- uality of chromosomes. In an early article by Minot {'yj) on the

a number (Tive lo twelve) of " jirimary cysts." each containing one of the remaining nuclei. Kach primary cyst divides by mitosis to form two gametes ("secondary cysts "), whicii, after forming the polar bodies, reunite, their nuclei fusing to form a single one. The resulting cell soon creeps out of the cyst-wall and assumes the active life, its nucleus meanwhile mul- tiplying to produce the multinuclear condition characteristic of the adult animal. What is here the physiological motive for the formation of the polar bodies, and how shall it be explained under the Weismann hypothesis?

theoretical meaning of maturation, the suggestion is made that parthenogenesis may be due to failure on the part of the Kt^^y:^ to form the polar bodies, the egg-nucleus thus remaining hermaphrodite, and hence capable of development without fertiHzation. This sug- gestion forms the germ of all later theories of parthenogenesis. lial- four ('80) suggested that the function of forming polar cells has been acquired by the ovum for the express purpose of preventing parthe- nogenesis, and a nearly similar view was afterward maintained by Van Beneden.i These authors assumed accordingly that in par- thenogenetic eggs no polar bodies are formed. Weismann ('86) soon discovered, however, that the parthenogenetic eggs of Polyphemus (one of the Daphnidae) produce a single polar body. This observa- tion was quickly followed by the still more significant discovery by Blochmann i^^^) that in Aphis the partJienogcnetic eggs produce a single polar body, zvhile the fertilized eggs produce tzvo. Weismann was al^le to determine the same fact in ostracodes and Rotifera, and was thus led to the view^ which later researches have entirely confirmed, that it is the second polar body that is of special significance in partheno- genesis. Blochmann observed that in insects the polar bodies were not actually thrown out of the ^gg, but remained embedded in its substance near the periphery. At the same time Boveri {'Sy, i) dis- covered that in Ascaris the second polar body might in exceptional cases remain in the egg and there give rise to a resting-nucleus indis- tinguishable from the egg-nucleus or sperm-nucleus. He was thus led to the interesting suggestion that parthenogenesis might be due to the retention of the second polar body in the Qgg and its union with the egg-nucleus.

" The second polar body would thus, in a certain sense, assume the role of the spermatozoon, and it might not without reason be said : ^^ PartJienogenesis is the result of fertilizaiiou by the second polar body'' ^ This conclusion received a brilliant confirmation through the obser- vations of Brauer ('93) on the parthenogenetic egg of Arteviia, though it appeared that Boveri arrived at only a part of the truth. Blochmann ('88-89) had found that in the parthenogenetic eggs of the honey-bee tzvo polar bodies are formed, and Platner discov- ered the same fact in the butterfly Liparis ('89) — a fact which seemed to contradict Boveri's hypothesis. Brauer's beautiful re- searches resolved the contradiction by showing that there are txvo types oi partJienogenesis \M\i\Q\\ may occur in the same animal. In the one case Boveri's conception is exactly realized, while the other is easily brought into relation with it.

{a) In both modes typical tetrads are formed in the germ-nucleus to the number of eighty-four. In the first and more frequent case {Y'v^. 141 ) but one polar body is formed, which removes eighty-four dyads, leaving eighty-four in the ^g^2,. There may be an abortive attempt to form a second polar spindle, but no division results, and the eighty-four dyads give rise to a reticular cleavage-nucleus. From Fig. 141. — First type of maturation in the parthenogenetic egg of Artemia. [BraUER.]

A, The first polar spindle; the equatorial plate contains 84 tetrads. D. C. Formation of the first polar body ; 84 dyads remain in the egg, and these give rise to the egg-nucleus, shown in D. F. Appearance of the egg-centrosome and aster. E. G. Division of the aster and formation of the cleavage-figure ; the equatorial plate consists of 84 apparently single but in reality bivalent chromosomes. this arise eighty-four thread-like chromosomes, and t/ic same juimbcr appears in later cleavage-stages.

{b) It is the second and rarer mode that realizes Boveri's concep- tion (Fig. 142). Both polar bodies are formed, the first removing eighty-four dyads and leaving the same number in the (tgg. In the formation of the second, the eighty-four dyads are halved to form two daughter-groups, each containing eighty-four single chromosomes. Both these groups reviain in the egg, and eaeh gives rise to a singli i^eticular ujccleiis, as described by Boveri in Ascaris. These tzvo niiclei place themselves side by side in the cleavage-figure, and give rise each to eighty-four cJironiosomes, precisely like two germ-nuclei in ordinary fertilization. The one hundred and sixty-eight chromosomes split

Fig. 142. — Second type of maturation in the parthenogenetic egg of Arh-mia. [BraL'ER.] A. Formation of second polar body. D. Return of the second polar nucleus ( />. b?) into the egg; development of the egg-amphiaster. C. Union of the egg-nucleus (?) with the second polar nucleus {p. b?-). D. Cleavage-nucleus and amphiaster. /:". First .l.Mvage-figure wiili equatorial plate containing 168 chromosomes in two groups of 84 each. lengthwise, and are distributed in the usual manner, and reappear in the same number in later stages. In other words, the second polar body here plays the part of a sperm-nucleus precisely as maintained

In all individuals arising from eggs of the first type, therefore, the somatic number of chromosomes is eighty-four; in all those arising from eggs of the second type, it is one hundred and sixty-eight. This difference is clearly due to the fact that in the latter case the chromo- somes are single or univalent, while in the former they are bivalent (actually arising from dyads or double chromosomes). The remark- able feature, on which too much emphasis cannot be laid, is that the numerical difference should persist despite the fact that the mass, and, as far as we can see, the quahty, of the chromatin is the same in both cases. In this fact we must recognize a strong support, not only of Hacker's and Vom Rath's conception of bivalent chromosomes, but also of the more general hypothesis of the individuality of chromo- somes (Chapter VI.).

It is necessary to touch here on the nature of the so-called " Sertoli-cells," or sup- porting cells of the testis in mammals, partly because of the theoretical ^ significance attached to them by Minot. partly because of their relations to the question of amito- sis in the testis. In the seminiferous tubules of the mammalian testis, the parent- cells of the spermatozoa develop from the periphery inwards toward the lumen, where the spermatozoa are finally formed and set free. At the periphery is a layer of cells next the basement-membrane, having flat, oval nuclei. Within this, the cells are arranged in columns alternating more or less regularly with long, clear cells, con- tainin'g large nuclei. The latter are the Sertoli-cells, or supporting cells :^ they extend nearly^throligh from the basement-membrane to the lumen, and to their inner ends the young sp'ermatozoa are attached by their heads, and there complete their growth. The' spermatozoa are developed from cells which lie in columns between the Sertoh- cells. and which undoubtedly represent spermatogonia, spermatocytes, and sperma- tids, though their precise relationship is, to some extent, in doubt. The innermost of these cells, next the lumen, are spermatids, which, after their formation, are found attached to the Sertoli-cells, and are there converted into spermatozoa without further division. The deeper cells from which they arise are spermatocytes, and the sper- matogonia lie deeper still, being probably represented by the large, rounded cells.

Two entirely diiferent interpretations of the Sertoli-cells were advanced as long ago as 1 87 1, and both views still have their adherents. Von Ebner ("71) at first regarded the'Sertoli-cell as the parent-cell of the group of spermatozoa attached to it, and the same view was afterward especially advocated by Biondi ("85) and by Minot (■92). the latter of whom regarded the nucleus of the Sertoli-cell as the physiological analogue of the polar bodies, i.e. as containing the female nuclear substance (92. p. 77). According to the opposing view, first suggested by Merkel ('70- the Sertoli- cell is not the parent-cell, but a nurse-cell, the spermatozoa developing from the columns of rounded cells, and becoming secondarily attached to the Sertoli-cell, which serves merely as a support and a means of conveying nourishment to the growing spermatozoa. This view was advocated by Brown ('85), and especially by Benda ("87). In the following year ("88), von Ebner himself abandoned his early hypothesis and strongly advocated Benda s views, adding the very significant result that/^;/r spermatids arise from each spermatocyte, precisely as was afterward shown to be the case in Ascaris, etc. The very careful and thorough work of Benda and von Ebner. confirmed by that of Lenhossek (98. 2). leaves no doubt that mamma- lian spermatogenesis conforms, in its main outlines, with that of Ascafis. the sala- mander, and other forms, and that Biondi's account is untenable. Minot's theoretical interpretation of the Sertoli-cell. as the physiological equivalent of the polar bodies, therefore collapses.

Whether the progenitors of the germ-cells ever divide amitotically is a question of high theoretical interest. Numerous observers have described amitotic division in testis-cells. and a few also in those of the ovary. The recent observations of Meves ('91), Vom Rath ('93), and others leave no doubt whatever that such divisions occur in the testis of many animals. Vom Rath maintains, after an extended inves- tigation, that all cells so dividing do not belong in the cycle of develo]jmcnt of the germ-cells ('93, p. 164) : that amitosis occurs only in the supporting (;r nutritive cells (Sertoli-cells, etc.), or in such as are destined to degenerate, like the •• residual bodies'' of Van Beneden. Meves has, however, produced strong evidence ("94) that in the salamander the spermatogonia may, in the autumn, divide by amitosi.s, and in the ensuing spring may again resume the process of mitotic division, and give rise to functional spermatozoa. On the strength of these observations Flemming (93) him- self now admits the possibility that amitosis may form part of a normal cycle of devel- opment.^

The one fact of maturation that stands out with perfect clearness and certainty amid all the controversies surrounding it is a rcdiiction of the number of chromosoiiies in tJie tdtiniate gerni-cells to one-lialf tJic nnniber cJiaracteristic of the somatic cells. It is equally clear that this reduction is a preparation of the germ-cells for their subsequent union, and a means by which the number of chromosomes is held constant in the species. With a few exceptions the first indication of the numerical reduction appears through the segmentation of the spireme- thread, or the resolution of the nuclear reticulum, into a number of masses one-Jialf that of the somatic cJiromosomes. In nearly all higher animals this process first takes place two cell-generations before the formation of the definitive germ-cells, and the process of reduction is completed by two rapidly succeeding "maturation-divisions," giving rise to four cells, all of which become functional in the male, while in the female only one becomes the ^gg, while the other three — the polar bodies or their analogues — are cast aside. During these two divisions each of the original chromatin-masses gives rise to four chromosomes, of which each of the four daughter-cells receives one ; hence, each of the latter receives one-half the somatic number of chromosomes. In the higher plants, however, the two maturation- divisions are followed by a number of others, in which the reduced number of chromosomes persists, a process most strikingly shown in the pteridophytes, where a separate sexual generation (prothallium) thus arises, all the cells of which show the reduced number.

Two general types of maturation may be distinguished according to the manner in which the primary chromatin-masses 'divide. In one, 1 For more recent literature on this subject see Meves, Zelltheilung, in Mcrkel and Bon- net's Ergebnisse, VIIL, 1 898. typically represented by Ascaris and the arthropods, each of these masses divides into four to form a tetrad, thus preparing at once for two rapidly succeeding divisions, which are not separated by a recon- struction of the daughter-nuclei during an intervening resting period. In the other, examples of which are given by the flowering plants and the spermatogenesis of the Amphibia, no true tetrads are formed, the primary chromatin-masses dividing separately for each of the matura- tion-divisions, which are separated by a period in which the nuclei regress toward the resting state, though often not completely return- ing to the reticular condition. These two types differ, however, only in degree, and with few exceptions they agree in the fact that during the prophases of the first division the chromatin-bodies assume the form of rings, the mitosis thus being of the heterotypical form, and each ring having the prospective value of four chromosomes.

Thus far the phenomena present no difficulty, and they give us a clear view of the process by which the numerical reduction of the chromosomes is effected. The confusion of the subject arises, on the one hand, from its complication with theories regarding the individu- ality of the chromosomes and the functions of chromatin in inheri- tance, on the other through conflicting results of observation on the mode of tetrad-formation and the character of the maturation-divisions. Regarding the latter question nearly all observers are now agreed that one of these divisions, usually the first, is a longitudinal or equation- division, essentially like that occurring in ordinary mitosis. The main question turns upon the other division, which has been shown in some cases to be transverse and not longitudinal, and thus separates what were originally different regions of the spireme-thread or nuclear substance. The evidence in favour of such a division seems at present well-nigh demonstrative in the case of insects and copepods, and hardly less convincing in the turbellarians, annelids, and mollusks. On the other hand, both divisions are regarded as longitudinal by most of those who have investigated the phenomena in Ascaris and in the vertebrates, and by some of the most competent investigators of the flowering plants.

The evidence as it stands is so evenly balanced that the subject is hardly yet ripe for discussion. The principle for which Weismann contended in his theory of reducing div^ision has received strong support in fact; yet should it be finally estabUshed that numerical reduction may be effected either with or without transverse division, as now seems probable, not only will that theory have to be aban- doned or wholly remodelled, but we shall have to seek a new basis for the interpretation of mitosis in general. Weismann's theory is no doubt of a highly artificial character ; but this should not close our eyes to the great interest of the problem that it attempted to solve.

The existing contradiction of results has led to the opinion, expressed by a number of recent writers, that the difference between longitudinal or transverse division is of minor importance, and that the entire question of reduction is a barren one. This opinion fails to reckon with the facts on which rests the hypothesis of the individuality of chromosomes (Chap. VI.); but these facts cannot be left out of account. We must find a common basis of interpretation for them and for the phenomena of reduction ; yet how shall we reconcile them with reduction by longitudinal division only } I cannot, there- fore, share the opinion that we are dealing with a barren problem. The peculiarities of the maturation-mitoses are obviously correlated in some way with the numerical reduction, and the fact that they differ in so many ways from the characters of ordinary mitosis gives ground to hope that their exhaustive study will throw further light not only on the reduction-problem itself but also on mitosis in general and on still wider problems relating to the individuality of the chromo- somes and the morphological organization of the nucleus. It is indeed very probable that Weismann's theory is but a rude attempt to attack the problem, and one that may prove to have been futile. The prob- lem itself cannot be ignored, nor can it be dissociated from the series of kindred problems of which it forms a part.

Van Beneden, E. — Recherches sur la maturation de Toeuf, la fdcondation et la division (List IV.). Brauer, A. — Ziir Kenntniss der Spermatogenese von Ascaris viegaloccphala : An/i. mik. Anat., XLII. 1893. Id. — Zur Kenntniss der Reifung der parthenogenetlsch sich entwickelnden Lies von Artemui Salina : Arch. mik. A?iat., XLIII. 1894. Guignard, L. — Le developpement du pollen et la reduction chromatique dans Ic Naias: Arch. Anat. Mic. II. 1899. (Full literature on reduction in plants.) Griffin, B. B. — See Literature, IV. Hacker, v. — Die Vorstadien der Eireifung (General Review): Arch. mik. A/uii.,

Id. — Uber weitere Ubereinstimmungen zwischen den Fertpflan/.ungsvorgangen der Thiere und Pflanzen : Bio/. Centrally.. X\' II. 1897. Id. — Uber vorbereitende Theilungsvorgange bei Thieren und Prianzcn : \ erh. dentsch. Zool. Ges., VIII. 1898. Id. — Die Reifungserscheinungen : Merkel nnd Bonnet's Ergebnisse. \' 1 1 1 . i S9S . Hertwig, 0.— Vergleich der Ei- und Samenbildung bei Nematoden. Eine Crund- lage fiir cellulaire Streitfragen : Arch. mik. Anat.. XXWI. 1890. Mark, E. L. — (See List IV.) Peter, K. — Die Bedeutung der Nahrzcllen im Hoden : Arch. mik. A>i<7f.. LI 11. ibgb

Plainer, G.— Uberdie Bedeutung der Richtungskorperchen : Biol. Centralb.,\\\\. Vom Rath, 0. — Zur Kenntniss der Spermatogenese von Gryllotaipa vulgaris : Arch. Id. — Neue Beitrage zur Frage der Chromatinreduktion in der Samen- und Eireife : Riickert, J.— Die Chromatinreduktion der Chromosomenzahl nn Entwicklungsgang der Oro-anismen: Ergebn. d. Anat. u. Entwick.. III. 1893 (1894). Strasburgert E. — Uber periodische Reduktion der Chromosomenzalil im Entwick-

" Wir miissen deshalb den lebenden Zellen, abgesehen von der Molecularstructur der organischen Verbindungen, welche sie enthalt, noch eine andere und in anderer Wcise coni- plicirte Structur zuschieiben, und diese es ist, welche wir mit dem Namen Organization bezeichnen." Brl'cke.^ " Was diese Zelle eigentlich ist, dariiber existieren sehr verschiedene Ansichten." The remarkable history of the chromatic substance in the matura- tion of the germ-cells forces upon our attention the problem of the ultimate morphological organization of the nucleus, and this in its turn involves our whole conception of protoplasm and the cell. The grosser and more obvious organization is revealed to us by the micro- scope as a differentiation of its substance into nucleus, cytoplasm, and the like. But, as Strasburger has well said, it would indeed be a strange accident if the highest powers of our present microscopes had laid bare the ultimate organization of the cell. Briicke insisted more than thirty years ago that protoplasm must possess a far more com- plicated morphological organization than is revealed to us in the visible structure of the cell, repeating, though without accepting, an earlier suggestion of Henle's('4i) that the cell might be composed of more elementary vital units ranking between the molecule and the cell. Many biological thinkers since Briicke's time have in one form or other accepted this conception, which indeed lies at the root of nearly all recent attempts to analyze exhaustively the phenomena of cell-life. Without attempting to follow out the history of opinion in detail or to give any extended review of the various theories,^ it may be pointed out that this conception was based both on theoretical a priori grounds and on the observed facts of cell-structure. On the former basis it was developed by Herbert Spencer'' in his theory of '' physiological units " by which he endeavoured to explain the phe- nomena of regeneration, development, and heredity ; while Xageli ('84) developed on the same general lines his theory of micclUc which

3 For an exhaustive review see Yves Delage, La structure du protoplasma et Us theories sui Vheredite. Paris, 1895. * Principles of Biology, 1864. has been so widely accepted by botanists. In the meantime Darwin ^ introduced a new element into the speculative edifice in his celebrated hypothesis of pangenesis, where for the first time appear the two assumptions of specific differences in the ultra-microscopic corpuscles C'gemmules ") and the power of self-propagation by division. Dar- win did not, however, definitely maintain that protoplasm was actually built of such bodies. The latter hypothesis was added by De Vries ('89), who remodelled the theory of pangenesis on this assumption, thus laying the basis for the theories of development which reached their climax in the writings of Hertwig and Weismann.

The views of Spencer and Darwin were based on purely theoretical grounds derived from the general phenomena of growth and inheri- tance.^ Those of Nageh, De Vries, Wiesner, Altmann, and others were more directly based on the results of microscopical investigation. The view was first suggested by Henle ('41), and at a later period developed by Bechamp and Estor, by Maggi and especially by Alt- mann, that the protoplasmic granules might be actually organic units or bioblasts, capable of assimilation, growth, and division, and hence to be regarded as elementary units of structure standing between the cell and the ultimate molecules of living matter. By Altmann, espe- cially, this view was pushed to an extreme limit, which lay far beyond anything justified by the known facts; and the theory of genetic con- tinuity expressed by Redi in the aphorism ^^ oiniie viviini ex vivo,'' reduced by Virchow to '^ omnis cellula e celhda,'" finally appears in the writings of Altmann as ^^ omne gramdiini e graiuilo'' /^

Altmann's premature generalization rested upon a very insecure foundation and was received with just scepticism. Except in the case of plastids, the division of the cytoplasmic granules was and still remains a pure assumption, and furthermore many of Altmann's ** granules" (zymogen-granules of gland-cells, etc.) are undoubtedly metaplasmic bodies.'^ Yet the beautiful discoveries of Schimper ('85) and others on the origin of plastids in plant-cells give evidence that these cells do in fact contain large numbers of bodies, other than the nuclei, that possess the power of growth and division. The division of the chlorophyll-bodies, observed long ago by Mohl, was shown by Schmitz and Schimper to be their usual if not their only mode of ori- gin ; and Schimper was able to trace them back to minute colourless plastids, scarcely larger than '* microsomes," that are present in large numbers in the protoplasm of the embryonic cells and of the Qgg, and give rise not only to chlorophyll-bodies but also to the amyloplasts or starch-formers and the chromoplasts or pigment-bodies. While it still remains doubtful whether the plastids arise solely by division or also

by new formation (as now seems to be the case with the centrosome), the foregoing observations on the plastids give a substantial basis for the hypothesis that protoplasm may be built of minute dividing bodies which form its ultimate structural basis. It was these facts, taken in connection with the phenomena of particulate inheritance and varia- tion (Galton), that led De Vries and his followers to the fundamental assumption of '' pangens," ''plasomes," " biophores," and the like as final protoplasmic units ; ^ but these were conceived not as the visible granules, plastids, etc., but as much smaller bodies, lying far bevond the limits of present microscopical vision, through the growth or aggregation of which the visible structures arise. This assumption has been harshly criticised; yet when we recall that in one form or another it has been accepted by such men as Spencer, Darwin, Beale, Hackel, Michael Foster, Nageli, De Vries, Wiesner, Roux, Weis- mann, Oscar Hertwig, Verworn, and Whitman, and on evidence drawn from sources so diverse, we must admit that despite its highly specula- tive character it is not to be lightly rejected. In the present chapter we may inquire how far the known facts of cell-structure speak for or against this hypothesis, incidentally considering a number of detailed questions of cell-morphology which have not hitherto found a place.

The cell is, in Briicke's words, an elementary organism, which may by itself perform all the characteristic operations of Hfe, as is the case with the unicellular organisms, and in a sense also with the germ-cells. Even when the cell is but a constituent unit of a hifrher sfrade of organization, as in multicellular forms, it is no less truly an organism, and in a measure leads an independent life, even though its functions be restricted and subordinated to the common life. It is true that the earlier conception of the multicellular body as a colony of one-celled forms cannot be accepted without certain reservations.- Neverthe- less, all the facts at our command indicate that the tissue-cell possesses the same morphological organization as the egg-cell, or the protozoan, and the same fundamental physiological properties as well. Like these the tissue-cell has its differentiated structural parts or organs, and we have now to inquire how these cell-organs are to be conceived.

1 The following list includes only some of the various names that have been «;ivcn to these hypothetical units by modern writers : Physiological iDiiis (Spencer) ; geinmuU's (Darwin); pangens (De Vries); plasomes (Wiesner); miccllie (Nageli); ,plastiiiules (Hackel and Elssberg) ; inotagmata (Engelmann); hiophores (Weismann); bioblasfs (Beale); so7nacitles {Yo?X.(tx)\ idioblasts (Hertwig); icUosomes (Whitman); biogens (Ver- worn); microzymas (Bechamp and Estor) ; gemnuc (Haacke). These names are not strictly synonymous, nor do all of the writers cited assume the power of division in the units. - Cf. p. 58.