The Cell in Development and Inheritance

Fig. 23. — Centrosomes in epithelial and other cells. [A, D, ZiMMERMANN ; E, Heidenhain A. From gastric glands of man ; dead cell at the left. B. Uterine epithelium, man. C. From human duodenum ; goblet-cell, with centrosome in the middle. D. Corneal epithelium of monkev. E. Epithelial cells from mesoblast-somites, embryo duck. F. Red blood-corpuscles from the duck- embryo. The centrosomes are double in nearly all cases. and the recent observations of Henneguy ('98) and Lenhossek (98,1) give reason to believe that the ** basal bodies" to which the ciHa of ciliated epithelium are attached may be the centrosomes.- These facts are of very high significance; for the position of the centro- some, and hence the direction of the axis, is here obviously related to the cell-environment, and it is difficult to avoid the conclusion that the latter must be the determining condition to which the intracellular relations conform. When applied to the germ-cells, this conclusion becomes of high interest ; for the polarity of the Qgg is one of the

primary conditions of development, and we have here, as I beUeve, a clue to its determination.^ In analyzing the structure and functions of the individual cell we are accustomed, as a matter of convenience, to regard it as an inde- pendent elementary organism or organic unit. Actually, however, it is such an organism only in the case of the unicelkilar j)lants and animals and the germ-cells of the multicellular forms. When we consider the tissue-cells of the latter, we must take a somewhat dif- ferent view. As far as structure and origin are concerned the tissue- cell is unquestionably of the same morphological value as the one-celled plant or animal ; and i)i tliis sense the multicellular body is equivalent to a colony or aggregate of one-celled forms. Physi- ologically, however, the tissue-cell can only in a limited sense be regarded as an independent unit ; for its autonomy is merged in a greater or less degree into the general life of the organism. From this point of view the tissue-cell must in fact be treated as merely a localized area of activity, provided it is true with the complete apparatus of cell-life, and even capable of independent action within certain limits, yet nevertheless a part and not a whole.

There is at present no biological question of greater moment than the means by which the individual cell-activities are coordinated, and the organic unity of the body maintained ; for upon this question hangs not only the problem of the transmission of acquired charac- ters, and the nature of development, but our conception of life itself. Schwann, the father of the cell-theory, very clearly perceived this ; and after an admirably lucid discussion of the facts known to him (*39), drew the conclusion that the life of the organism is essentially a composite ; that each cell has its independent life ; and that " the whole organism subsists only by means of the reciprocal action of the single elementary parts." ^ This conclusion, afterward elaborated by Virchow and Haeckel to the theory of the ** cell-state," took a very strong hold on the minds of biological investigators, and is even now widely accepted. It is, however, becoming more and more clearly apparent that this conception expresses only a part of the truth, and that Schwann went too far in denying the influence of the totality of the organism upon the local activities of the cells. It would of course be absurd to maintain that the whole can consist of more than the sum of its parts. Yet, as far as growth and development are con-

^ Cf. pp. 384, 424. We should remember that the germ-cells are themselves epithelial products. 2 Untersuchungen, Trans., p. 181. cerned, it has now been clearly demonstrated that only in a Hmited sense can the cells be regarded as cooperating units. Thcv are rather local centres of a formative power pervading the growing mass as a whole,^ and the physiological autonomy of the individual cell falls into the background. It is true that the cells may acquire a high degree of physiological independence in the later stages of embryological development. The facts to be discussed in the eighth and ninth chapters will, however, show strong reason for the conclu- sion that this is a secondary result of development, through which the cells become, as it were, emancipated in a greater or less degree from the general control. Broadly viewed, therefore, the life of the multicellular organism is to be conceived as a whole ; and the appar- ently composite character which it may exhibit is owing to a second- ary distribution of its energies among local centres of action. ^

In this light the structural relations of tissue-cells become a ques- tion of great interest ; for we have here to seek the means by which the individual cell comes into relation with the totality of the organ- ism, and by which the general equihbrium of the body is maintained. It must be confessed that the results of microscopical research have not thus far given a very certain answer to this question. Though the tissue-cells are often apparently separated from one another by a non-living intercellular substance, which may appear in the form of solid walls, it is by no means certain that their organic continuity is thus actually severed. Many cases are known in which division of the nucleus is not followed by division of the cell-body, so that multi- nuclear cells or syncytia are thus formed, consisting of a continuous mass of protoplasm through which the nuclei are scattered. Heitz- mann long since contended ( '73), though on insufficient evidence, that division is incomplete in nearly all forms of tissue, and that even when cell-walls are formed they are traversed by strands of protoplasm by means of which the cell-bodies remain in organic continuity. The whole body was thus conceived by him as a syncytium, the cells being no more than nodal points in a general reticulum, and the body forming a continuous protoplasmic mass.

This interesting view, long received with scepticism, has been to a considerable extent sustained by later researches, and though it still x^m-d^x^-^siibjiidice, has been definitely accepted in its entirety by some recent workers. The existence of protoplasmic cell-bridges between the sieve-tubes of plants has long been known ; and Tangl's dis- covery, in 1879, of similar connections between the endosperm-cells was followed by the demonstration by Gardiner, Kienitz-Gerloff, A. Meyer, and many others, that in nearly all plant-tissues the cell-walls

are traversed by delicate intercellular bridges. Similar bridges have been conclusively demonstrated by Ranvier, J^izzozero, Rctzius, Flem- ming, Pfitzner, and many later observers in nearly all forms of epithe- lium ( Fig. I ) ; and they are asserted to occur in the smooth muscle-fibres, in cartilage-cells and connective tissue-cells, and in some nerve- cells. Dendy ('88), Paladino ( '90), and Retzius ('89) have endeav- oured to show, further, that the follicle-cells of the ovary are connected by protoplasmic bridges not only with one another, but also with tJic oviiDi ; and similar protoplasmic bridges between germ-cells and somatic cells have been also demonstrated in a number of plants, e.g. by Goroschankin ( '83) and Ikeno ( '98) in the cycads and by A. Meyer ('96) in ]\)hox. On the strength of these observations some recent writers have not hesitated to accept the probability of Heitz- mann's original conception, A. Meyer, for example, expressing the opinion that both the plant and the animal individual are continuous masses of protoplasm, in which the cytoplasmic substance forms a morphological unit, whether in the form of a single cell, a multi- nucleated cell, or a system of cells. ^ Captivating as this hypothesis is, its full acceptance at present would certainly be premature ; and as far as adult animal tissues are concerned, it still remains unde- termined how far the cells are in direct protoplasmic continuity. It is obvious that no such continuity exists in the case of the corpuscles of blood and lymph and the wandering leucocytes and pigment-cells. In case of the nervous system, which from an a priori point of view would seem to be above all others that in which protoplasmic con- tinuity is to be expected, its occurrence and significance are still a subject of debate.

When, however, we turn to the embryonic stages we find strong reason for the belief that a material continuity between cells here exists. This is certainly the case in the early stages of many arthropods, where the whole embryo is at first an unmistakable syncytium ; and Adam Sedgwick has endeavoured to show that in Pcripatns and even in the vertebrates the entire embryonic body, up to a late stage, is a continuous syncytium. I have pointed out ( '93) that even in a total cleavage, such as that of AnipJiioxus or the echi- noderms, the results of experiment on the early stages of cleavage are difficult to explain, save under the assumption that there must be a structural continuity from cell to cell that is broken by mechan- ical displacement of the blastomeres. This conclusion is supported by the recent work of Hammar ( '96, '97), whose observations on sea-urchin eggs I can in the main confirm.

Among the most interesting observations in this direction are those of Mrs. Andrews ('97),^ who asserts that during the cleavage of the echinoderm-egg the blastomeres ** spin " dehcate protoplasmic filaments, by which direct protoplasmic continuity is established between them subsequent to each division. These observations, if correct, are of high importance ; for if protoplasmic connections may be broken and re-formed at will, as it were, the adverse evidence of the blood-corpuscles and wandering cells loses much of its weight. Meyer ('96) adduces evidence that in Volvox the cell-bridges are formed anew after division ; and Flemming has also shown that when leucocytes creep about among epithehal cells they rupture the protoplasmic bridges, which are then formed anew behind them.^

We are still almost wholly ignorant of the precise physiological meaning of the cell-bridges ; but the facts indicate that they are not merely channels of nutrition, as some authors have maintained, but paths of subtler physiological impulse. Beside the facts determined by the isolation of blastomeres, referred to above, may be placed Townsend's recent remarkable experiments on plants, described at page 346. If correct, these experiments give clear evidence of the transference of physiological influences from cell to cell by means of protoplasmic bridges, showing that the nucleus of one cell may thus control the membrane-forming activity in an enucleated fragment of another cell. The field of research opened up by these and related researches seems one of the most promising in view; but until it has been more fully explored, judgment should be reserved regarding the whole question of the occurrence, origin, and physio- logical meaning of the protoplasmic cell-bridges.

Altmann, R.--Die Elementarorganismen und ihre Beziehungen zu den Zellen, 2d ed. Leipzig, 1894. TAnnee Biologique. — /^^7;v>, 1895-96. (Full Reviews and Literature-lists.) Bohm and Davidoff . — Lehrbuch der Histologic des Menschen. Wiesbaden, 1895. Boveri, Th.— (See Lists IV., V.) Biitschli, 0. — Untersuchungen liber mikroskopische Schaume und das Protoplasma. Erlanger, R. v. Neuere Ansichten liber die Struktur des Protoplasmas : ZooL Heidenhain, M. — Uber Kern und Protoplasma : FestscJir. z. ^o-Jii/ir. Doctorjub. 7'Oh

ed. London, 1891. Schiefferdecker & Kossel. — Die Gewebe des Menschlichen Korpers. Braunschweig, I Sc) I . Schwarz, Fr. — Die morphologische und chemische Zusammcnsetzung des Proto- plashias. Jyres/au, 1887. Strasburger, E. — Zcllbildung und Zellteilung. 3d ed. 1880. Id. — Das Botanische Practicum. 3d ed. Jena. 1897. Strasburger, Noll, Schenck, and Schimper. — Lehrbuch der Botanik, 3d ed. Jenay Strieker, S. — Handbuch der Lehre von den Geweben. Leipzig, 1871. Thoma, R. — Text-book of General Pathology and Pathological Anatomy: trans, by

Alex. Bruce. London, 1896. Van Beneden, E. — (See Lists II.. IV.) De Vries, H. — Intracellulare Pangenesis, /ena, 1889. Waldeyer, W. — Die neueren Ansichten liber den Bau und das Wesen der Zelle : Deu'.sch. Med. IVoc/ienschr., Oct., Nov., 1895. Wiesner, J. — Die Elementarstruktur u. das Wachstum der lebenden Substanz : Wood's U oil Biol. Lectures, 1899. Zimmermann, A. — Beitrage zur Morphologic und Physiologic der Pflanzenzelle. Tubingen, 1893. Id. — Die Morphologic und Physiologic des Pflanzlichen Zellkernes. Jena, 1896.

" Wo eine Zelle entsteht, da muss eine Zelle vorausgegangen sein, ebenso wie das Thier nur aus deni Thiere, die Pflanze nur aus der Pflanze entstehen kann. Auf diese Weise ist wenngleich es einzelne Punkte im Korper gibt, wo der strenge Nachweis noch nicht gelie- fert ist, doch das Princip gesichert, dass in der ganzen Reihe alles Lebendigen, dies mogen nun ganze Pflanzen oder thierische Organismen oder integrirende Theile derselben sein, ein ewiges Gesetz der coiithiuir lichen EntzvicklunghQ.%i&\i\.r ViKCHow.^

The law of genetic cellular continuity, first clearly stated by Vir- chow in the above words, has now become one of the primary data of biology, and the advance of research is ever adding weight to the conclusion that the cell has no other mode of origin than by division of a preexisting cell. In the multicellular organism all the tissue- cells arise by continued division from the original germ-cell, and this in its turn arises by the division of a cell preexisting in the parent-body. By cell-division, accordingly, the hereditary substance is split off from the parent-body ; and by cell-division, again, this substance is handed on by the fertilized egg-cell or oosperm to every part of the body arising from it.^ Cell-division is, therefore, one of the central facts of development and inheritance.

The first two decades after Schleiden and Schwann ('40-'6o) were occupied with researches, on the part both of botanists and of zool- ogists, which finally demonstrated the universality of this process and showed the authors of the cell-theory to have been in error in asserting the independent origin of cells out of a formative blastema.'"^ The mechanism of cell-division was not precisely investigated until long afterward, but the researches of Remak ('41), Kolliker (44), and others showed that an essential part of the process is a division of both the nucleus and the cell-body. In 1855 {I.e., pp. 174, 175), and again in 1858, Remak gave as the' general result of his researches the following synopsis or scheme of cell-division. Cell-division, he asserted, proceeds from the centre toward the periphery. It begins with the division of the nucleolus, is continued by simple constriction and division of the nucleus, and is completed by division of the cell-

3 For a full historical account of this period, see Remak, Untersttchungen iiher die Ent- wickhmg derWirbelthiere, 1855, PP- ^ 64-1 80. Also Tyson on the Cell-doctrine and Sachs's Geschichte der Botajiik. body and membrane (Fig. 24). For many years this account was accepted, and no essential advance beyond Rcmak's scheme was made for nearly twenty years. A number of isolated observations were, however, from time to time made, even at a very early period, which seemed to show that cell-division was by no means so simple an operation as Remak believed. In some cases the nucleus seemed to disappear entirely before cell-division (the germinal vesicle of the ovum, according to Reichert, Von Ikier, Robin, etc.); in others to become lobed or star-shaped, as described by Virchow and by Remak himself (Fig. 24,/). It was not until 1873 that the way was opened for a better understanding of the matter. \\\ this year the discoveries of Anton Schneider, quickly followed by others in the same direction by Biitschli, Fol, Strasburger, Van Beneden, Flemming, and Hertwig, showed cell-division to be a far more elaborate process than had been

supposed, and to involve a com- plicated transformation of the nucleus to which Schleicher ('78) afterward gave the name of karyokincsis. It soon ap- peared, however, that this mode of division was not of universal occurrence ; and that cell-divi- sion is of two widely different types, which Van Beneden {'j^^ distinguished as fnii^Di 01 ta t io n , T?;„ . T^- ♦ ^- • • f n^-,^ .oUc ;^ Corresponding nearly to the the cmbrvo chick, illustrating Remak's scheme. simple proCCSS described by

of karyokincsis. Three years later Flemming ('79) proposed to substitute for these the terms direct and iJidircct division, which are still used. Still later ('82) the same author suggested the terms mitosis (indirect or karyokinetic division) and auiitosis (direct or akinetic division), which have rapidly made their w^ay into general use, though the earlier terms are often em- ployed. ' Modern research has demonstrated the fact that amitosis or direct division, regarded by Remak and his immediate followers as of uni- versal occurrence, is in reality a rare and exceptional process ; and there is reason to believe, furthermore, that it is especially char- acteristic of highly specialized cells incapable of long-continued multiplication or such as are in the early stages of degeneration, for instance, in glandular epithelia and in the cells of transitory embry- onic envelopes, where it is of frequent occurrence. Whether this

view be well founded or not, it is certain that in all the hifjjher and in many of the lower forms of Ufe, indirect division or mitosis is the typical mode of cell-division. It is by mitotic division that the germ- cells arise and are prepared for their union during the process of maturation, and by the same process the oosperm segments and gives rise to the tissue-cells. It occurs not only in the highest forms of plants and animals, but also in such simple forms as the rhizopods, flagellates, and diatoms. We may, therefore, justly regard it as the most general expression of the "eternal law of continuous develop- ment" on which Virchow insisted.

In the present state of knowledge it is somewhat difficult to give a connected general account of mitosis, owing to the uncertainty that hangs over the nature and functions of the centrosome. For the pur- pose of the following preliminary outline, we shall take as a type mitosis in which a distinct and persistent centrosome is present, as has been most clearly determined in the maturation and cleavage of various animal eggs, and in the division of the testis-cells. In such cases the process involves three parallel series of changes, which affect the nucleus, the centrosome, and the cytoplasm of the cell-body respectively. For descriptive purposes it may conveniently be divided into a series of successive stages or phases, which, however, graduate into one another and are separated by no well-defined limits. These are: (i) The Pi'opJiases, or preparatory changes; (2) the Mctaphasc, which involves the most essential step in the division of the nucleus ; (3) the Anaphases, in which the nuclear material is distributed ; (4) the Telophases, in which the entire cell divides and the daughter-cells are formed.

I. Prophases. — {a) The Nticletis. As the cell prepares for division, the most conspicuous fact is a transformation of the nuclear substance, involving both physical and chemical changes. The chromatin-sub- stance rapidly increases in staining-power, loses its net-like arrange- ment, and finally gives rise to a definite number of separate intensely staining bodies, usually rod-shaped, known as cliroiuosoines. As a rule this process, exemplified by the dividing cells of the salamander-epi- dermis (Fig. i) or those of plant-meristem (Fig. 2), takes place as fol- lows. The chromatin resolves itself little by little into a more or less convoluted thread, known as the 5/r/;/(Knauel)or spireme, and its sub- stance stains far more intensely than that of the reticulum (Fig. 25). The spireme-thread is at first fine and closely convoluted, forming the close spireme." Later the thread thickens and shortens and the

convolution becomes more open ('* open spireme"). In some cases there is but a single continuous thread ; in others, the thread is from A. Resting cell with reticular nucleus and true nucleolus; at c the attraction-sphere containing two centrosomes. B. Early prophase ; the chromatin forming a continuous spireme, nucleolus still present : abo%-e, the amphiaster {a). C. D. Two different types of later prophases. C. Disappear- ance of the primary spindle, divergence of the centrosomes to opposite poles of the nucleus (exam- ples, some plant-cells, cleavage-stages of many eggs). D. Persistence of the primary spindle (to form in some cases the " central spindle"), fading of the nuclear membrane, ingrowth of the astral rays, segmentation of the spireme-thread to form the chromosomes (examples, epidermal cells of salamander, formation of the polar bodies). E. Later prophase of type C\ fading of the nuclear membrane at the poles, formation of a new spindle inside the nucleus; precocious splitting of the chromosomes (the latter not characteristic of this tvpe alone). E. The mitotic figure established; e.f. the equatorial plate of chromosomes. {€/. Figs. 21, 27, 32, etc.)

its first appearance divided into a number of separate pieces or seg- ments, forming a segmented spireme. In either case it ultimately breaks transversely to form the chromosomes, which in most cases have the form of rods, straight or curved, though they are sometimes spher- ical or ovoidal, and in certain cases may be joined together in the form of rings. The staining-povver of the chromatin is now at a maxi- mum. As a rule "the nuclear membrane meanwhile fades away and finally disappears, though there are some cases in which it persists more or less completely through all the phases of division. The chromosomes now lie naked in the cell, and the ground-substance of the nucleus becomes continuous with the surrounding cytoplasm

The remarkable fact has now been established with high probability that eveiy species of plant or animal has a fixed and characteristic niDH- ber of cJiromosomes, zvhicJi regularly recurs in the division of all of its cells ; and in all forms arising by sexual reproduction the number is even. Thus, in some of the sharks the number is 36 ; in certain gas- teropods it is 32 ; in the mouse, the salamander, the trout, the lily, 24 ; in the worm Sagitta, 18 ; in the ox, guinea-pig, and in man - the num- ber is said to be 16, and the same number is characteristic of the onion. In the grasshopper it is 12; in the hepatic Pallavicinia and some of the nematodes, 8 ; and in Ascaris, another thread-worm, 4 or 2. In the crustacean Artemia it is 168.^ Under certain conditions, it is true, the number of chromosomes may be less than the normal in a given species; but these variations are only apparent exceptions (p. %'j). The even number of chromosomes is a most interesting fact, which, as will appear hereafter (p. 205 ), is due to the derivation of one-half the number from each of the parents.

The nucleoli differ in their behaviour in different cases. Net-knots, or chromatin-nucleoli, contribute to the formation of the chromosomes; and in cases such as Spirogyra {ViQ.\m\Q.x, "^6, and Moll, '93 ) or .-^r//- nosphcerium (R. Hertwig, '99), where the whole of the chromatin is at one period concentrated into a single mass, the whole chromatic figure thus appears to arise from a "nucleolus." True nucleoli or plasmo- somes sooner or later disappear ; and the greater number of observers agree that they do not take part in the chromosome-formation. In a considerable number of forms {e.g. during the formation of the j^olar

1 The spireme-formation is by no means an invariable occurrence in mit»)sis. In a consid- erable number of cases the chromatin-network resolves itself ilirectly into tlie chromosomes, the chromatic substance becoming concentrated in separate masses which never form a con- tinuous thread. Such cases are connected by various gradations with the " segmented spi- 2 Flemmino believes the number in man to be considerably greater than 16. ^ For a more complete list see p. 206.

bodies in various eggs) the nucleolus is cast out into the cytoplasm as the spindle forms, to persist as a " metanucleus " for some time before its final disappearance (Fig. 104). More commonly the nucleolus fades away /;/ s/t//, sometimes breaking into fragments meanwhile, while the chromosomes and spindle are forming. The fate of the material is in this case only conjectural. An interesting view is that of Strasburger ('95, '97), who suggests that the true nucleoli are to be regarded as storehouses of " kinoplasmic " material, which is either directly used in the formation of the spindle, or, upon being cast out of the nucleus, adds to the cytnj-)lasmic store of " kinoplasm " avail- able for future mitosis.

{d) The AinpJiiastcr. Meanwhile, more or less nearly parallel with these changes in the chromatin, a complicated structure known as the avipJiiastcr {Yo\, '77) makes its appearance in the position formerly occupied by the nucleus (Fig. 2^,B-F). This structure consists of a fibrous spindle-shaped bodv, the spindle, at either pole of which is a star or aster formed of rays or astral fibres radiating into the sur- rounding cytoplasm, the whole strongly suggesting the arrangement of iron filings in the field of a horseshoe magnet. The centre of each aster is occupied by a minute body, known as the centrosoine (Boveri, 'ZZ\ which may be surrounded by a spherical mass known as the ^rw/rt^j/Z/^/r (Strasburger, '93). As the amphiaster forms, the chro- mosomes group themselves in a plane passing through the equator of the spindle, and thus form what is known as the equatorial plate.

The amphiaster arises under the influence of the centrosome of the resting cell, which divides into two similar halves, an aster being developed around each while a spindle stretches between them (Figs. 25, 27). In most cases this process begins outside the nucleus, but the subsequent phenomena vary considerably in different forms. In some forms (tissue-cells of the salamander) the amj^hiaster at first lies tangentially outside the nucleus, and as the nuclear membrane fades away, some of the astral rays grow into the nucleus from the side, become attached to the chromosomes, and finally pull them into posi- tion around the equator of the spindle, which is here called the ecu- tral spiudle (Figs. 25, D, F ; 27). In other cases the original spindle disappears, and the two asters pass to o]:>])osite poles of the nucleus (some plant mitoses and in many animal-cells). A spindle is now formed from ravs that grow into the nucleus from each aster, the nuclear membrane fading away at the poles, though in some cases it may be pushed in by the spindle-fibres for some distance before its disappearance (Figs. 25, 32). In this case there is apparently ho central spindle. In a few exceptional cases, finally, the amphiaster may arise inside the nucleus (p. 304).