A textbook of general embryology (1913) 2

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Kellicott WE. A Textbook of General Embryology (1913) Henry Holt and Co., New York.

Kellicott (1913): 1 Ontogeny | 2 The cell and cell division | 3 The germ cells and their formation | 4 Maturation | 5 Fertilization | 6 Cleavage | 7 The germ cells and the processes of differentiation, heredity, and sex determination | 8 The blastxtla, gastrula, and germ layers. Morphogenetic processes

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Chapter II. The Cell and Cell Division

Two universal characteristics of living things are the possession of protoplasm and a cellular composition. Recognition of these fundamental facts was dependent upon the use of the compound microscope, and so we find them comparatively late acquirements in the history of biology. The cell-unit structure of an organic tissue was first described in plants (cork tissue), by Robert Hooke in 1665, and quite naturally, therefore, emphasis was laid upon what we now know as the cell walls." As a consequence the term "cell" was applied to these small box-Hke units which seemed to resemble the cells of a honey comb. Wh,en, about the middle of the nineteenth century, it became apparent that the cell content, and not the cell wall, was the important thing, the word "cell" had become so definitely fixed that it could not but be retained, although its utter inaptness was fully recognized.

This is not the place to discuss the general importance and significance of the Cell Theory of Schleiden and Schwann (1839) and their successors. It will become clear as we proceed that the cell, in structure and in action, is the basis of modem Embryology. Most of the early processes of organic development are strictly cell processes and must be studied from the standpoints of both Cytology and Embryology, from neither alone, and throughout development constant reference must be had to cellular phenomena.

As known to-day cells of different organisms and different tissues exhibit an unending variety in size, form, structure, and function (Figs. 14, 15), but throughout there are two essentials of structure expressed by the definition of a cell given by Leydig (1852) and by Schultze (1861) as "a mass of protoplasm containing a nucleus/' To-day we modify this but slightly and define a cell as a limited mass of protoplasm containing nuclear material.

Fig. 14, — Vuious forms of cells. IV-IX, irons Dahlgren and Kepoer, X, after Prenant and Bouin. I, II. Human leucocyte, X 350. III. Human red blood-corpuscle, X 350. IV. Cell from root cap of calla lily, X 350; p, plastids. V. Epidermis of earthworm, showing four mucous cells iu various stages of lecretion, and cu. cuticle, X 660, VI, Fat cells in skin of chicken; n, nucleus. X 435, VII. Ovarian ovum (oficytc) of cat; cm, cell membrane; mi, microsomes; nm, nuclear membrane; o, nucleolus; v. yolk alveoli. VIII. Connective tissue cells from the lobster; cv, cytjiplasmic mass; pr, cytoplasmic processes; per, peripheral layer of cytoplasm upon which the rigid material of the tissue is laid down. IX. Pigment cell from the peritoneum of the fish, Ammodi/Ut. Fully extended; the processes can be completely retracted. Two nuclei. X 90. X. Stratified epithelium from human pharynx, showinB intercellulfu coniiMtioiM or bridges, X 37fi.

Fig. 15. — VariouB forms of cells, continued. XI-XIII, after Prenuit tutd Bouin, XIV-XVI, from Dahlgren and Kepner. XI. Ganglion cell from human spinal cord; n. nucleus. X 250. XII. Sensoiy (receptor) cells from humtui olfactory epithelium, X 175. XIII, Multipolar ganglion cell from optic RanglioQ of horse. (The procesaeB of the right side are cut off.) o. Aioq. X 63. XIV. Part of muscle cell from the Gsh, Cofasfomus; e. capillary containing blood cells and platelets, X SOO. XV. Ciliated cells from the digestive tract of the Mollusc, Cydm. XVI. Gland cell from the leech, Pitcicola; ch, cytoplasmic channels containing the secretion granules; dt, dischar^ng tubes; n, aucleus; 0, nucleolus; s, secreted materiala in various Btages of elaboration.

The mass of the cell is usually very small. The smallest cells known are the Bacteria, some of which may be only 0.001 mm. (1 micron, or 1/25000 inch) in length (Streptococci), or even less (Staphylococci). But among the Metazoa, cells are never so minute. The human white blood corpuscle or leucocyte (Fig. 14, 7), is perhaps of average size, measuring something less than 0.01 mm. (8-10 micro) in diameter. Tissue cells having a diameter of 0.05 mm. (50 micro) are considered large, although a few specialized cells may far exceed this {e.g., muscle or nerve cells). The egg cells of animals are usually larger than tissue cells, but this is a special condition, and is frequently due to the apcumulation of stored food substance, rather than to the possession of a larger amount of protoplasm. Within the species the sizes of specific varieties of cells are very constant. The size of an organ or of an individual is related to the number of its component cells rather than to their size (Amelung, Conklin).

We may proceed now to describe the essentials of structure exhibited by a typical cell— an imaginary thing which has no more real existence than the *' average man." Such a cell would consist of a spheroidal or irregular mass of protoplasm, limited by a definite cell membrane or cell wall. The wall may be a surface condensation of the protoplasm or, more frequently, a true secretion of the cell body, either membranous as in most animal tissues, or thick and rigid, like the cellulose walls of most plants. In many cells the viscid transparent protoplasm just within and in contact with the cell wall forms a thin layer, the ectosarc or ectoplasm (Fig. 16), clearer than the granular and more refractive central endosorc or endopUism, which contains, besides the granules, many cell organs and inclusions.

The protoplasm itself is made up of a combination of two forms, perhaps two kinds, of material plainly differing in density and arrangement (Fig. 17). The denser material called the mitom^f spongioplasm, reticulum, or filar substance, forms a sort of complex framework or fine network of irregularly woven paths along which are scattered minute granules called microsomes. The spaces or meshes of this spongioplasmic network are filled with the less dense ground avbstance or cell sap, called also the hyaloplasm, paraplasm, or irUerfilar substance. The

Fig. 18. — Di&gTEim of a typical cell, a, aster; e, centroaome (centriole); tK, chromatin; cr. chromidia; o, ceatrosphere ; d. deutoplasmic granules; en, endoplaam; ex, eioplasm (cortical pi asm) ; hy, hyaloplasm; k, karyoBOme ; I, linin network; m, cell membrane; n, nucleus; nm. nuclear membrane; o, nucleolus; p, plaatids; ap, apoogioplasm; s. Quid vacuolea (metaplaam). <

actual relation of these two kinds of substances varies in different' kinds of cells or even at different times in the same cell. A frequent arrangement is that of a reticulum just described, in which the spongioplasm is definitely fibrous, forming a felt-work holding the more fluid hyaloplasm. In other cells, or at other times, protoplasm has a distinctly alveolar structure resembling a fine emulsion. Here the hyaloplasm is in the form of minute drops or cdveoli, while their walls or the irregular interalveolar spaces are of the denser material. Occasionally other structural relations are seen, such as the granular, where the fibrous reticulum is represented by rows of excessively minute granules, and the fibrillar, where the fibers

Fig. 17 — Alveolar protoplasmic structure in the egg of the Bea-urclim, ToxojnteusCes, one and one-half mioutCB after the entrance of the BpermBtozooD. From Wilson, "Cell," X about 2000. The protoplasm conBistB of alveoli surrounded by microsomes. In the middle 18 the centriole, surrounded by the ceutrosphere, while radiating from it are the rays of the aster. The large and small black massea are the sperm head and middle-piece.

of the reticulum are larger, longer, and less branched than in the ordinary reticulum. It is still uncertain how exactly the real structure of living protoplasm is represented by its appearance after it has been killed, in preparing it for examination. It should be remembered that in living protoplasm these fibrils, reticula, etc., are in all probability fluid structures of greater density than the ground substance. The cell is far from being a simple unit, for it contains a variety of structures and materials differing chemically and functionally; these may not all be directly visible as organized structures. The only constantly differentiated substance is the nuclear material which is usually contained in a definitely formed body, the nucleus, though it may be scattered through the protoplasm. There are many reasons for believing that primitively the nuclear substance was not thus organized into a definite nucleus, but that it was distributed through the cytoplasm in the form of small granules, as it is still in many of the simplest organisms, and that gradually these became aggregated into fewer larger masses. The Protozoa show many stages in the gradual enlargement and numerical reduction of the nuclear elements, but in the Metazoa the nuclear material is nearly always collected into a single body. The nucleus is to be regarded as a specialized portion of the protoplasm of the cell, highly differentiated in structure, chemical composition, function, and behavior. All cell activities seem to involve mutual interaction between the nucleus and the remainder of the cell and neither is able long to function normally without the other. But the action of the nucleus is primary and directive, to a large extent controlling and regulating cell activities and cell life as a whole. In most cases the nucleus is a spherical or ovoid body of fixed form; in some very active cells it may be elongated or of irregular form, or even branched, ramifsdng all through the cell; in a few rare instances the nucleus may be amoeboid (Figs. 14, 15).

Typically this complex center of cell activity shows much the same fundamental structure as the remiainder of the protoplasm, which in distinction from the nucleus is called the cytoplasm. The nucleus is limited by a definite nuclear wall or membrane formed either from the cytoplasm or from the nucleus itself. The substance of the nucleus as a whole is termed the karyoplasm. The equivalent of the spongioplasmic reticulum of the cytoplasm is here termed the linin network, and the hyaloplasmic ground substance is known as the nudear sap, or karyolymph, or paralinin. The chief distinction of the nucleus is the presence of a very special nucleo-protein substance called chromatin, a name given it on account of the ease with which it is colored with such dyes as hsematoxyhn (logwood) or carmine. This chromatin is ordinarily in the form of small flakes or granules of variable size, tenned chromioles (Eisen). These are always distributed along the linin fibers, which stain, less readily and are therefore described as achromdtic. Sometimes a few large masses of chromatin called karyosomes may be seen within the nucleus. Apparently the chromatin may be rapidly dissolved or condensed, or even formed anew, so that its visible amount and condition vary considerably from time to time.

The structural differentiation of the nucleus is frequently complicated fm-ther by the presence of one or more bodies called plasmosomes or nucleoli, which often superficially resemble the karyosomes, at least morphologically, although chemically and physiologically they are quite unlike (Fig. 16). The Xiucleoli are spheroidal bodies, staining densely, though not of the same material as the true chromatin. The karyosomes are sometimes called chromatin nucleoli. The nucleoli vary considerably in number, size, and form, and their significance in the nucleus is not altogether understood; probably several unlike bodies having different functions have been included under this single term.

Another organ of the cell is the centrosome. This is not always to be seen for it may be lost from the cell at certain times and reappear later. While commonly a cytoplasmic structure lying just outside the nucleus, in some forms it is an intra-nuclear organ and it is quite possible that primitively it was an ^essential part of the nucleus, as it still is in many Protozoa (Figs. 29, 30). The centrosome is a minute densely staining granule or pair of granules, sometimes hardly larger than the granules or microsomes of the cytoplasmic reticulum. Occasionally it consists of several separate granules closely associated. The cytoplasm in the neighborhood of the centrosome and directly under its influence is ordinarily differentiated as the archoplasm. This substance consists typically of two portions. A medullary region forming a small spheroidal mass called the centrosphere or attraction sphere immediately surrounds the centrosome, and peripherally, radiating from this out into the cytoplasm, there is at times a collection of diverging rays or fibers called collectively the aster. In the ordinary vegetative cell the centrosome and archoplasmic structures are usually reduced in size and perhaps even absent in some cells, but in the dividing cell they may be very large and prominent organs extending nearly throughout the cell, for their chief function is in connection with the process of cell division. Similar dense granules and fibers are found associated with organs of the cell which are motile, for example, the granules at the bases of ciha and flagella, the axial filaments of some flagella, undulating membranes, and some pseudopodia, the fibrillse of muscle cells, etc. All such modifications of protoplasm connected particularly with the production or regulation of motion are included under the general term kinoplasm (Strasburger). It has been suggested that the kinoplasm of the cell is of the nature of a definite and permanent cell organ. In many cells the centrosome is such a permanent organ, but many other kinoplasmic structures seem to be more or less temporary and may disappear or be formed anew at the time of cell division or at other times. The rays of the aster, for example, in many cases apparently are formed from the enlargement and rearrangement of the cytoplasmic reticulum resulting from the activity (probably chemical) of the centrosome.

In addition to these nearly constant cell structures of comparatively uniform characteristics, there are various other organs or bodies which are peculiar to certain special kinds of cells. Among these are the bodies called in general plastids (Fig. 14, IV)fOi which the more familiar are the pigment bodies, such as chhropUistids or chlorophyl bodies^ the chromoplastids or colored bodies not containing chlorophyl, amyloplastids or starch-forming bodies, protein-forming plastids, and others. Vacuoles of various sizes and kinds are very common, such as the digestive, excretory, food, water, and f6od-storage vacuoles; occasionally one or more are specialized as contractile or pulsating vacuoles. Nutritive substances are often stored temporarily in cells. These materials are collectively known as deutoplasm, metaplasm, or paraplasm, and may be starch or protein granules, yolk plates, oil drops, etc. There are also granules of various other kinds — of materials being secreted or excreted, of food in various stages of ingestion or digestion, pigment granules, crystals, and formed substances of many kinds, usually specific for the organism or tissue. In many cells true chromatic structures are found in the cytoplasm outside the definite nucleus. These are usually small granules and bits of chromatin of varied form and significance in different cells. Collectively they are termed chromidia; for the most part they are not formed in situ, but . are derived directly from the nucleus (Fig. 18). They are most frequent in very active cells such as gland cells, the rapidly growing germ cells, and many others. Many forms of chromidia, functionally as well as structurally distinct, have been given special names such as chondriosomeSj ^^ Chondromiten,^\ idiochromidia, mitochondria, pseudochromosomes, ^^Nehenkem,^^ " Trophospongien,^^ etc.

It has been said already that no single cell shows typically all the structures described above and illustrated in Fig. 16. Without stopping for further description many of the details of structure truly characteristic of a few varied types of cells are shown for comparison in Figs. 14, 15. (For descriptions and further details the student should consult the standard texts of Cytology and Histology, e.g,, Dahlgren and Kepner's "Principles of Animal Histology," New York, 1908.)

One further aspect of cell structure remains to be mentioned. This is the important fact that there is in most tissues a fairly definite cell form combined with a nearly constant arrangement of the cell organs and contents (Van Beneden, Rabl, Heidenhain). In any epithelial cell the different physiological conditions at the free and the attached surfaces lead to this definite relation of cell structures which is called polarity. In such cells the nucleus lies toward the basal end of the cell, the centrosome either toward the free end or on that side of the nucleus. Thus an imaginary axis may be passed through the centrosome and nucleus perpendicular to the free surface, about which the cell organs are arranged symmetrically, either bilaterally or radially (rotatorially ) . This polarity extends also to other less constant structures and even occasionally to non-epithelial cells. It may be seen for example in the arrangement of cilia, cuticle, conductile and contractile fibers, granules or drops of substances being excreted or secreted, and yolk or other stored materials (Figs. 14, 15, 16, etc.). In the germ cells, as we shall see later, this fact of polarity becomes of the greatest importance, for it is frequently related closely to the symmetry of the mature organism.

While we may thus describe the Metazoan tissue cell as a separate morphological unit, complete in itself, it is also true that in a great many tissues the cells are in direct material continuity with one another through minute protoplasmic connections or bridges which pass through fine perforations in the cell walls. These have been observed in a great variety of tissues in many different forms (Fig. 14, X) ; whether or not they are present in all tissues it is yet impossible to say definitely, and we must recognize clearly that in the physiology of the organism the cells do not behave as completely autonomic units. While each represents a localized field of activity, the life of the cell is subordinated to the life of the organism as a whole — a fact that comes out with especial clearness in the development of the organism. In some way not yet understood the cell, or groups of cells, influence the activities of other cells, and are in turn influenced by them. • The activities of the Metazoan organism of course equal the sum of the activities of its component cells; but these combined activities are organized and unified into a whole in such a way that this represents more than the unit activities when considered separately, just as the action of a community represents something beyond the sum of the actions of its members taken individually.

From the embryological point of view one of the most important phases of cell activity is cell reproduction or cell division. For cells arise only from preexisting cells. The history of opinion regarding the genesis of cells parallels roughly that regarding the genesis of organisms. It was Virchow who finally demonstrated convincingly (1855, 1858) the universality of the fact of cell continuity in the tissues of a single organism, and further the fact that in a succession of generations of organisms the process of the formation of cells from preexisting cells is not interrupted. We know now that in this process of cell division all the essential organs of the cell take an active part — the cytoplasm, nucleus, the centrosome, and even many of the plastids. So that the final result of the process is typically the formation of two daughter cells similar to each other and also to the parent cell in all essential respects save in size.

The division of cells occurs in two quite dissimilar ways. The simpler method, and the less frequent, is termed direct division or amitosis (Flemming). Here the first step is sometimes the elongation and constriction of the nucleolus, when this is present, into two separate daughter nucleoli, or in other cases the appearance of a new second nucleolus (Fig. 19). Next the whole nucleus divides into two, sometimes by simple constriction into two separate elements, sometimes by the ingrowth of a partition wall, or in still other cases, by the formation of two new nuclear membranes within the original membrane, 'the disappearance of the latter freeing the two daughter nuclei. This division of a nucleus is typically followed by the division of the cytoplasmic portion of the cell which is ordinarily accomplished by the development of a cell wall between the two daughter nuclei. Very frequently, however, division of the cell body does not follow and the cell remains binucleate; or this process of nuclear fission may be repeated, a multinucleate cell resulting, such as a striated muscle cell. In such cases of incomplete cell division the essence of the process seems to be the rapid increase of nuclear surface and then volume; it is usually associated with special forms of cell activity. Conditions in some of the Protozoa suggest that primitively division of the nucleus or the multiphcation of the nuclear bodies might not have been associated with a corresponding division of the cytoplasmic body, but that these originally independent divisions have gradually come to be uniformly associated.

Fig. 19. — Amitosis in tendon cells of a new-born mouse. After Nowikofif, X 800. nc, nucleolus.

It has commonly been supposed that the direct form of cell division occurs but rarely and then usually in cells which are moribimd. It is becoming clear, however, that amitosis is in reality not particularly infrequent. It seems to occur normally in many tissues (mesenchyme), and often where there is an unusual or a sudden increase in nuclear activity and energy expenditure on the part of the whole cell, as in ovarian follicle cells and tapetal cells, muscle cells, rapidly growing or redifferentiating cells in regenerating tissues ; it is also true that amitosis is frequent in such tissues as stratified epithelia whose cells are nearing the end of their Ufe or activity.

The second and more usual method of cell fission is that termed indirect cell division, or mitosis (Flemming), or karyokinesis (Schleicher). This is a complicated process involving the establishment and operation of an intricate mechanism within the cell, shared in by nearly all its living parts. The essential result of the division of the cell by the action of this complex mechanism concerns in particular the chromatic substance of the nucleus, for in nearly all known instances the chromatin sharing in the process is very precisely divided into two equal portions, each of which goes to one of the daughter cells. We may give here only a brief outline of the essentials of this process of mitosis, again describing an imaginary schema with which to compare later some of the variations in detail shown by actual cells.

As the first step in mitosis we should consider the division of the centrosome into two, which remain lyiiig together within the undivided kinoplasmic centrosphere. This division of the centrosome is usually quite removed in point of time from the other phenomena of mitosis, for it occurs normally during the reconstruction of a daughter cell immediately after its formation, and so is separated by a considerable intervening vegetative period from the other events of mitosis or the doubling of other parts (Fig. 20, A). This vegetative phase of cell life is frequently referred to as the ^^ resting'^ period or interkinesis; a state of inaction is not impUed by the term "resting/' for during this period the cell is performing its normal and characteristic functions as a tissue cell; the word merely indicates that the cell is not undergoing any active phase of division. The termination of the vegetative phase and the immediate inauguration of mitosis is ordinarily first distinguishable in the structure of the nucleus. The chromatin granules become more distinct, enlarge rapidly, and undergo some change in chemical constitution indicated by an increase in staining capacity (Fig. 20, A; 22, A). As the chromatin increases some of the granules or flakes come to be arranged in a linear, or sometimes bilinear series, still upon some of the linin threads which share in this arrangement. Thus the chromatin and linin form a tangled thread or ribbon called the skein or spireme (Figs. 20, B; 21, B; 22, B).

We should note here that at this time the chromatin of the nucleus which is not included in the spireme, often indeed the greater part of the whole amount of this material, is thrown out into the cytoplasm and dissolves (Fig. 32) ; the more fluid parts of the nucleus are also thrown imo the cytoplasm by the dissolution of the nuclear membrane. It may thus be only a comparatively small part of the whole nuclear structure that is formed into the spireme proper.

The spireme may be quite continuous throughout the nucleus, or it may appear from the first as a fragmented thread composed of several short pieces; when in this latter condition it is spoken of as a segmented spireme. In a few cases the spireme stage is largely suppressed and the chromatin granules collect immediately into compact groups without indication of a skein stage. The linin network in part becomes a sort of fine core throughout the spireme and in the extra-chromatic region remains as a network of naked fibers. The latter portion soon becomes polarized so that its fibers converge, more or less

Fig. 20. — Mitosis in cells of Salamandra maculoea. After Prenant and Bouiu. D, H. Primary Bpermatocytea, othcra, epermatogonia. A, B, C, X 1000, otharB, X 800. A. InterkineBis or resting stage. B. Early prophase; Bpireme continutudin&l aplittine of chroi continued divergence o equatorial plate. Polar view. Only a few of the chromoaon aomes diverging, still united

Begmented i

to diverge; spindle forming

E. DiaappeBrBncB of nuclBst

and asters. F. Mesophaae; formation of

omosoroeB V-ahaped. G. Same in aide view.

■e shown. H. Anaphase: daughter chromo I. Anaphase; continued divergen

chromosomes, now entirely separated. J, I.ate anaphase; complete divergence ot chromoBomeB. Spindle breaking down, asters disappearing. K. Telophase; beginning of reconatruction of daughter nuclei. Chromosomes diaintegrating. L Late telophase; division completed. Nuclei reconstructed; eentriole divided; cell walla completed. Nuclear membrane forming, c, Centriolea;ca, centroapbere; R, nucleus; >, spindle temsinaiVi Bpindle fibers cut acroM.

Fig. 21.— Diagrams of the process of mitosis. From Wilson, *'Cell,** slightly modified. 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; above, the amphiastCT (a). CD, Two different types of later prophases; C. Disappearance of the primary spindle, divergence of the centrosomes to opposite poles of the nucleus (examples, many plant-cells, cleavage-stages of many eggs). D. Perastence of the primary spindle (to form in some cases the "central spindle"), fading of the

distinctly, toward the region of the centrosphere; in many cells such a polarization of the linin toward the centrosome exists throughout the vegetative phase. The two centrosomes now begin to diverge and the surrounding centrosphere pulls in two, one portion accompanying each centrosome (Figs. 20, E; 21, Z>). As the centrospheres diverge they enlarge, and within each appear fibers radiating from the centrosome as a center and producing the asters. While the chromatic and achromatic parts of the nucleus have been passing through these early stages of mitosis, the nucleolus when present becomes vacuolated, commences to dissolve and finally disappears. Soon the nuclear membrane also commences to break down and dissolve, first in the region of the asters, leaving the nuclear substance free in the cytoplasm. Next the chromatin thread shortens and thickens, breaking in the case of the continuous spireme, into a number of separate segments or rods; or if the spireme itself is of the segmented type, its elements now shorten and thicken. When the spu-eme segments, the linin thread upon which the chromatin granules are strung may remain continuous between as well as through the chromatic rods. These chromatic segments now become quite homogeneous, clearly differentiated structures called the chromosomes (Figs. 20 C, E; 21 J D; 22, C). Strictly speaking, each chromosome consists of a dense mass of fused chromatin granules with a portion of linm embedded.

In practically all organisms in which the nucleus is a definitely formed structure, the number of chromosomes appearing during mitosis is fixed, and is constant throughout all divisions of

nuclear membrane, ingrowth of the astral rays, segmentation of the spiremethread 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 type alone). F, The mitotic figure established. G. Metaphase; splitting of the chromosomes (e.p.) ; n, the cast-off nucleolus. H, Four stages in the divergence of the two halves of a chromosome. /. Anaphase; the daughter-chromosomes diverging, between them the interzonal fibers (i./.). or central spindle; centrosomes already doubled in anticipation of the ensuing division. J. Late anaphase or telophase, showing division of the cell-body, mid-body at the equator of the spindle and beginning reconstruction of the daughter-nuclei. K. Division completed.

Fig. 22. — MitoMB in the aegmenting egg of the clam, Vnio. From Dahlgren and Kepner. A. Prophase of the fourth cleavage. Chromatin reticulum; centroBomes on opposite Bidea of nucleus. B, Prophase. Spireme bejpnning to segmeat ioto chromosomes; nuclear membrane disappearing; spindle forming. C. Late prophase. Cbromosomes formed; spindle becoming completed; nucleolus nearly disappeared. D. Mesophase. Chromosomes in equatorial plate. E. Early anaphase. Divergence of the daughter chromosome groups. F. Telophase, Nuclear division completed and daughter nuclei reformed; 03^0plasmic division commencing.

somatic cells. In all but a few groups the chromosomes appear in an even number, ordinarily between twelve and thirty-six, although these Umits are frequently passed. The chromosomes are at present considered the most important elements in the cell, and interest in the whole process of mitosis centers in their behavior. The details of the process of mitosis seem directed toward the exactly equal division and distribution of these elements, the importance of which justifies the more detailed consideration which we give them after the general description of mitosis is completed.

While the chromosomes are forming, the centrosomes and asters continue to diverge, passing around toward opposite sides of the nucleus. The linin fibers of the nucleus tend throughout to remain polarized toward the centrosomes and the separation of these bodies from one another draws out the linin fibers into an elongated bundle converging at each end toward the centrosome. Finally the centrosomes come to lie on exactly opposite sides of the nuclear structures and as the nuclear membrane disappears completely we find the rays of the asters penetrating into the nuclear region and forming, together with the linin, a spindle-shaped structure Ijdng between the centrosomes, its component fibers passing among the chromosomes (Figs. 20, G; 21, F; 22, P). In many cells the centrosomes do not thus migrate to opposite sides of the nucleus, but separate directly; the nucleus in this case is simply drawn up to lie between them (Fig. 21, D). The result is the same, but the difiference in relative behavior of the centrosomes and nucleus is real and must be taken into account in some cases. The spindle and asters now form a figure resembling a diagram of the lines of force within a simple bipolar magnetic field. This figure is called the amphiasterf sometimes the achromatic figure ^ emphasizing its distinctness from the chromatic portion of the nucleus, now all or largely included in the chromosomes. The parts of the linin network directly continuous with those upon which the chromosomes were formed originally, seem to have a somewhat different history from the remainder of the linin. They remain attached to the chromosomes and extend thence toward the centrosomes, forming in this stage a superficial sheath around a central portion of the spindle, and are hence termed the mantle fibers. The central core of the spindle seems in many cases to be formed largely from the remainder of the nuclear linin, though in other cases this is formed in the same way that the asters are, and from cytoplasmic materials. Thus the amphiaster is usually of mixed origin, nuclear and cytoplasmic, though in some cases the spindle at least seems to be wholly nuclear (li^i^); tbe asters are always cjrtoplasmic in origin. All these fibers form definite threads, enlarged as compared with the original linin reticulum.

The definite formation of the chromatic portion of the nucleus into chromosomes and the achromatic substance into the amphiaster marks the termination of the first phase of mitosis which is known as the prophase. During the prophase there has occurred the actual division of only the centrosome and centrosphere; the other important changes have been preparatory to further divisions — the dissolution of the nuclear membrane, the enlargement and rearrangement of the chromatin granules, the formation of definite chromosomes, and the establishment of the achromatic figure. We should remember that the nucleolus meanwhile has fragmented and, together with a large or small amount of chromatin which is not formed into chromosomes, has passed out into the cytoplasm and disappeared.

The arrangement of the materials forming the achromatic figure is evidently the result of certain tensions within the cell, the effect of which is first to draw the chromosomes, until now distributed irregularly, into a circle about the equator of the spindle. When in this position the chromosomes are said to form the equatorial plate (Figs. 20, F, G;21, F). This phase of mitosis is also in general preparatory to actual division but it is carried on after the division mechanism is completely established. This period of division is known as the mesopha^e (Lillie).

Following the mesophase is the metaphase. The chief event of this phase is the longitudinal splitting or division of each chromosome into two parallel halves. This forms two equal and similar groups of daughter chromosomes, each group similar to the original group except in size. As a matter of fact this longitudinal splitting of the chromosomes is by no means always deferred until this time, for frequently it occurs during the prophase of division, even in the spireme stage; or rarely the chromatin granules may divide even before a definite spireme is constituted {Fig. 23). In such cases the chromosomes are in the form of double rods throughout the entire prophase and mesophase; the metaphase is then present only virtually.

The mantle fibers from the opposite poles of the spindle are now y \ attached to the daughter chromosomes, usually in their middles. Next the mantle fibers begin to shorten as the result of some process centering in or about the centrosomes at the poles of the spindle. As the poles are relatively fixed, perhaps by the anchoring asters or through the rigidity of the central n^^^,/'"'^ part of the spindle, the result is the

Fig. 23.-Long!tudinai fi««ion

separation of the two groups of of the spireme in the division of , , . , 1 ■ 1 Bpore-mother-cell in LiliMtn canr

daughter chromosomes which move m^^, Arter Farmer and Moora.

along the central spindle fibers toward the regions of the centrosomes. If the mantle fibers are attached to the middle of the chromosome it is first drawn out into a 3- or >-shape; frequently this formes assumed during the mesophase, the apex of the > pointing centrally in the equatorial plate (Figs. 20, F, H; 21). The period of mitosis occupied by the divergence of the two chromosome groups is called the anaphase; this is usually very brief as the chromosomes diverge rapidly. Their divergence exposes the central fibers, of the spindle which are then called the interzonal or conneciing Jibers, and which frequently come to have an important share in the formation of later structures (Figs. 21, H; 22, E). In their divergence the chromosomes themselves seem to be entirely passive, except in a very few isolated instances where they are definitely amoeboid (Opalind) (Fig. 31). The two chromosome groups are finally drawn completely to the opposite poles of the spindle and the process of mitosis then enters upon its final period, the telophase.

During this phase the cytoplasmic portion of the cell becomes divided into two parts, usually equal, though occasionally extremely unequal. Sometimes, as in many animal cells, this division of the cytoplasm results from its peripheral constriction in a plane corresponding with that of the equatorial plate, the constriction deepening until the cell body is completely severed and the two daughter cells formed (Figs. 20, L; 21, /, J; 22, F). More frequently, in some animal and in practically all plant cells, the division of the cytoplasm results from the formation of a partitioning cell wall, in the formation of which the interzonal fibers of the spindle usually take an important part. These seem to increase in number and to thicken in the middle, ultimately fusing and forming a transverse plate which is the rudiment of the future cell wall; the remainder of the wall forms as a distinct secretion of the cytoplasm in that region.

As the diverging chromosomes approach the poles of the spindle they lose their distinct outlines, become vesicular, and gradually lose their visible identity and separateness to a large extent; with a few exceptions they finally seem to disintegrate completely and form scattered granules, and a new typical nucleus is constituted in each daughter cell. Meanwhile the centrospheres and asters have diminished in extent and clearness and have returned again to the condition which is characteristic of the interkinesis. About the new nucleus a membrane is formed, either from the nucleus or the cytoplasm, and the mitosis is accomplished (Figs. 20, L; 21, H-J). In most cases just about this time the centrosome divides in preparation for the next mitosis. Dining the interkinesis the nucleus and cytoplasm increase in size and soon the process of division is repeated.

The length of time occupied by the whole process of mitosis varies greatly. In the division of some egg cells it may be completed in fifteen minutes, or it may occupy one or even two hours, and in some special cases a much longer period.

This account of mitosis, although brief and including only some of the essentials, brings out clearly the unity of the nucleus as an organ; it behaves as a more or less separate unit of cell organization throughout all this intricate process. And we see clearly this extremely important fact, that the nucleus of a cell is formed from a preexisting nucleus of the same constitution. Nuclei arise only from preexisting nuclei; there is a nuclear continuity quite parallel with cell continuity. And going one step farther, it is probable that chromosomes are derived only from preexisting chromosomes. This idea of genetic continuity is not completely applicable to all cell organs, however, for occasionally the centrosomes are not derived from preexisting centrosomes, but may arise de novo, and in the development of the new organism the centrosome is typically derived from the sperm cell alone. Among the plants many of the plastids seem to be genetically related and to be formed by the division of preexisting plastids. The other less living cell structures are usually distributed passively to the daughter cells, and such structures may be formed anew in the new cells.

The relation between the direction of the plane of cell division and the general morphology of the cell body demands a word. From the preceding account it is obvious that the plane of division is at right angles to the longitudinal axis of the spindle, but the position which the spindle assumes is itself predetermined. The position of the spindle axis is fixed by the location of the centrosomes. When the single centrosome divides and the daughter centrosomes pass to opposite sides of the nucleus, they usually migrate equal distances from the original position of the centrosome; it follows, therefore, that this region falls within the plane of the equator of the spindle and consequently in the plane of the new division. When the centrosome does not alter materially its relative position in the cell, the next division, again being through the plane occupied by the centrosome and the center of the nucleus^ will be in general at right angles to the preceding division (Fig. 24). . Thus the planes of succeeding divisions tend to alternate, each perpendicular to the preceding. Any other relation involves the migration of the centrosome from the position originally occupied by it in the daughter cell; or the spindle may change its position during or after its formation, and this regular relation thus be disturbed. Typically the spindle takes such a position that its long axis lies in the direction of the greatest protoplasmic extent of the cell (0. Hertwig), a position which would result from the tensions between a comparatively elongated body and a fluid medium in which it is suspended

Fig. 24. — Diagram illustrating the relation between the position of the centrosome and the plane of cell division. Symmetrical motion of the daughter centrosomes results in the regular alternation, at right angles, of successive division planes.

and free to move in any direction. There are many exceptions to these two general rules in special conditions, such as simple columnar epithelia, stratified epithelia, the maturing germ cells, etc,; these indicate perhaps that a more fundamental cause of the direction of cell division remains to be discovered.

Some important modifications of this schema of cell division given above will be noted in other connections, but a few special conditions are conveniently mentioned here. The most important and fundamental modifications are doubtless those which occur during the forming and maturing of the germ cells ; these are to be described in detail in Chapter IV, and here we should only note that the behavior of the chromosomes

In these divisions is very complex; that there is here only one-half the number of these bodies formed in the cells of the somatic tissues; that mitoses may occur without an intervening interkinesis; that in the case of the egg cell the division may be of extreme inequality, so that one of the daughter cells is like an extremely small bud, although with respect to nuclear structure the two cells are alike ; and that in certain special mitoses one or inore of the chromosomes may fail to divide and therefore may be unrepresented in one of the daughter cells.

Among the higher plants an important characteristic is the absence of definite centrosomes and asters, although these structures are normally present among the lower plants. The absence of centrosomes results in the formation of a characteristically blunt or truncated spindle in most of the higher plants. In some animals the spindle is rather truncated also, but this is usually found to be in reality multipolar, composeif of many small bundles of spindle fibers terminating in a row of centrosomes or centrosome-like bodies {Fig. 25, A). In the tissue cells of most animals the asters are relatively small, though the spindle remains lai^e and distinct; in a few cases it seems that even in animal cells division may be effected in the absence of centrosomes.

Fig. 25. — A. Multipolar spindle in spore-mother-eell of EQmgelum. From Wilson. "Cell." after Osterhout. B. Intranuclear spindle in the oocyte of the Copep<>d, CantKoram'^us aJapAyKnue. From Hegner. X 850.

Many significant modifications of mitosis occur among the Protozoa, where we find certain conditions which seem to offer suggestions aa to the evolution of the mitotic figure and process, as well as of some of the chief cell oi^ans themselves. Among these forms the process of division is often complicated through its being at the same time the essential step in reproduction, rather than merely a step in or condition of differentiation, as in the Metazoan. Thus the process of budding or gemma* tion is essentially an unequal celt divi»on; and in brood ("spore") formation we see a form of cell division in which the nucleus divides many timee without any correepondii^ cytoplasmic divisions, until finally, all at once, the cytoplasm is cut into a large number of small cells. In these forms of division no mitotic figure is formed ordinarily, and one of the modifications due to association with, reproduction is seen in the fact that the resulting bud, or brood-cell, may have a form and structure entirely unUke that of the mother cell. In budding, especially in that form called bud-fission, the nucleus does not ordinarily divide until after the bud is practically completed and ready to be cut off, i.e., cytoplasmic division tends to precede nuclear division. Another complication due to the same association is the frequent differentiation of two forms of chromatic substance in the nucleus. These are the reproductive chromatin, or idiochTomalin, and the vegetative, or trophochromatin (Dobell) ; in some Protozoa these may come to be organized into two separate nuclei which are sometimes equivalent to what are called the micro- and macronucleus respectively. We have already mentioned the distributed nucleus of many unicellular organisms in which the chromatin is not organized into a definite nuclear organ, but is in the form of scattered granules or collections of granules throughout the cell (Fig. 26, A). Such a condition indicates strongly that the nuclear and cytoplasmic parts of the cell have arisen through the gradual differentiation of a common protoplasmic basis. In cell division e^ch of these chromatic bodies may first divide into two (PUeptus), though the members of the resulting pair are not distributed to different daughter cells, for the accompan3dng division of the cell is completed without any rearrangement of the chromatin granules (Fig. 26, B). In other forms with distributed nuclei {Tetramitus), the scattered granules collect about an active kinoplasmic organ termed the division center (Fig. 27) ; this divides, and the two products separate, each accompanied by a group of chromatic granules which are then redistributed equally to the daughter cells, although no definite mitotic figure is formed. Even when a definite nucleus does come to be established, much of the chromatin of the cell may not be contained within it but may remain distributed (Heliozoa, Radiolaria) . Finally, of course, all of the chromatin becomes contained within one or more definite nuclear structures which may be simple spherical bodies, or they may show considerable complication and variation in form. A definite nuclear membrane may be absent at first, as in ChUomonas and TrcLchelomonaSy though it is formed in practically all cases where the chromatin granules form definite nuclear groups. Within the nucleus the chromatic substance may not be definitely organized into chromosomes, or these bodies may appear only in certain divisions associated with gametic reproduction {Paramcecium) ; in some Protozoa, however, definite chromosomes are typically established and become clearly marked during each mitosis (Fig. 28).

Fig. 26, — Nuolear division in the Ciliate, Dileplvt. From Calkins, " ProtolOology." A. Vegetative form. Nucleus in tlie form of lihromatin granules scattered througli the greater part ol the cell ("distributed nucleus"). B. . During division. Each chromatin granule elongates and divides into two.

Fig. 27. — Cell division in the Flagellate, Tetramitus, After Calkins. A Vegetative condition showing scattered chromatin granules (distributed nucleus) and division center. B. Collection of chromatin granules preparatory to division. C. Fission of the division center. D. Separation of the division centers accompanied by the daughter groups of granules (nuclei).

Probably the most interesting mcMlificatious of mitosis among the Protozoa are tboee connected with the formation and behavior of the centrosome and mitotic spindle, upon the origin of which they may perhaps throw some light. In some species there is no indication of a, specialized organ concerned particularly with division. In a few forms, as mentioned above, a division center may be formed, although no definite nucleus is present, the distributed chromatin granules collecting into a single group only at the time of cell fission. In some species of Amaba, and in a few other forms, one of the large chromatin bodies, or karyoaomea, xeiihin the nucleus is specialized as an organ of division, called the central body and functionally equivalent to a centrosome (Fig. 29). This body does not lose its chromatic character, may be surrounded by a definite membrane, and appears to have functions other than those of a ceutrosome which it exercises during the intervals between divisions. In cell division this central body remains wholly or in part intranuclear. Apparently this represents a very early stage

Fig. 28. — Nuclear divieion in the Rhiiopod, Eaolypha. After SchewiakofT. X 800. A, Coarae network of chromatin. B. Cootraction of chromatin fibers and beKinniQK of formation of loops (chromosomea). C. Arrangement of chro' mosomes in equatorial plate. Polar view. D. Equatorial plate. Lateral view. Spindle (orroing. E. Splitting of chromosomes and beginning of divergence. F. Continued divergence of cbromoaomes. Q. Division nearly completed.

Fig. 29. — Nuclear division in the Rhizopod, Centropt/ma aculeala. After Bchaudinn (Doflein). A. Nucleus in vegetative stage. B. Appearance of centriole. C. Equatorial plate. Spindle with centrosomes; plasma radiations. D. Beginning of reconstruction of daughter nuclei.

Fig, 30. — Nuclear diviaioii in the Rhiiopod. CMamudophrv' itercorea. After SchBudinn (Doflein). A, Nucleus with central body and chromatia threads.

B. Elongation of central body and beginning of formation of equatorial plate.

C. Equatorial plate. Central body spindle-abaped with polar ceotrosome-like thickenings. D. Equatorial plate divided into two. Plasma radiations from the "centrosomes." E, Fission of central body and chromatiQ masses. F. DividoD completed. Daughter nuclei being reconstructed.

in the differentiation from a chromatic body of the centrosome which later becomes wholly achromatic and typically extranuclear. Several other forms have a dynamic division center equivalent to the centrosome but intranuclear; a nucleus of this type is known as a centromicleus (Fig. 30; see also Fig. 25, B). In the division of such nuclei the nuclear membrane may remain entirely intact {Evglena) or, as in Nocliluca, the nuclear membrane may partly break down during mitosis. In several forms possessing a definite extranuclear centrosome this body remains, undivided in cell fission, passing to one daughter cell alone, while it new centrosome develops in the other cell; but this forms first as an intranuclear structure which later moves out into the csrtoplasm aloogeide the nucleus. These conditiona indicate strongly the nuclear origin of the centrosome. And there ie some reason for believing the spindle also originally a nuclear structure, as it still is, in part at least. The spindle is a lees constant organ than the centrosome, compared with which it is of secondary importance. In several Protozoa and simple plants the spindle is entirely absent, usually where the centrosome is intranuclear, so that no definite mitotic figure is formed. In other fornos the spindle is intranuclear, and then the centrosomes or their equivalents may be absent, as in Opalina (Fig. 31). This form is further remarkable for the chromatic character of some of its spindle fibers, and in that, in the absence of centrosomes, the chromosomes separate by an active amoeboid movement, suggesting a possibly primitive behavior of the chromosomes in cell division (Metcalf).

Fig. 31. — Nuclear division in tbe Infusorian. OpaUna iratstinatii. After Metcalf. X 1200. A. Nucleus in anaphaae showing chromatic reticulum, which may be regarded as equivalent to the spindle, and branched, amoeboid chromoaomes. B. Late telophase.

It is of course impossible now to get definite information r^arding the evolution of the cell organs and the process of mitosis, and in these connections the conditions found at present among the Protozoa are only suggestive. Many of these conditions do suggest strongly, however, that the nucleus has been developed from the gradual aggregation of scattered, chemically differentiated particles; that within the nucleus certain chromatic elements became specialized into division centers which finally became extranuclear, achromatic bodies — the centrosomes; and that certain achromatic elements became specialized into a spindle, also at first intranuclear (linin), and at present in nearly all forms both intra- and extranuclear in origin. The chromosomes we must now consider more particularly.

Interest in the process of mitosis centers in the chromosomes and their behavior, for, as we have said, this whole process seems to be directed toward the equal distribution of the daughter chromosomes to the daughter cells, while the cytoplasm may or may not be equally divided at the same time. We do not know how the constituents of the nucleus other than the chromosomes are distributed in cell division. There is little reason for supposing that these are distributed with exact equality.

We may recognize two chief aspects of chromosomal behavior in mitosis. The first is the division of the chromatin granules or chromioles composing the chromosomes; these granules all divide in the same direction so that the total result appears as an exactly equal longitudinal division of each chromosome, or of the spireme in those cases where the division of the granules occurs very early. This is the essential act of chromosome reproduction and it is obviously a process concerning the chromatin alone, independent of the remainder of the mitotic mechanism. The second important fact is the distribution of the two chromosome halves or daughter chromosomes to the two daughter cells. This is accomplished by the extra-chromosomal elements of the mitotic figure, the chromosomes apparently taking a purely passive part in the process. We see in the mitotic figure, not a mechanism for cell division merely, for this is frequently accomplished in the absence of mitosis, nor for chromosome division, for this frequently precedes the formation of the mitotic figure; but essentially the mitotic figure is a mechanism effecting the equal distribution to the daughter cells of the products of chromosomal division within the nucleus of "the parent cell, so that each new cell has a complete group of chromosomes similar to those of the parent cell. The precision and wide occurrence of this equal distribution, through mitosis, of the chromosomes and of these cell organs alone, leads to the assumption that the chromosomes constitute the- essential physiological elements of the nucleus and therefore of the cell. There are many subsidiary facts indicating the great importance of these bodies. Consideration of the relation of the chromosomes to the special problems of heredity and sex will be deferred to a later chapter (Chapter VII) for fuller consideration, but we should mention here a few of the important facts and hypotheses regarding these bodies.

In the nuclei of many of the Protozoa definite chromosomes are already present, but in some unicellular organsims there are conditions suggesting a possible mode of evolution of these structures. We have mentioned the collection of the distributed chromatin granules into small groups through the cell; these groups have been regarded as the rudiments of chromosomes. After a definite nucleus is established the chromatin granules remain as definite bodies, and each divides in cell fission. Even when the chromatin granules merely become rearranged about a division center, as in Chilomonas and Trdchelomonas, although definite chromosomes may not be formed, the division of the granules occurs as the essential step in fission, just as later when the granules collect and fuse into chromosomes. In Paramcecium definite chromosomes are formed only in certain divisions, namely, those immediately preceding conjugation, that is during gametogenesis (Calkins and Cull, Fig. 82), and it is but a short step from this condition to the regular formation of chromosomes in all mitoses.

In the reconstruction of the daughter nucleus the chromosomes become vacuolated and finally break up into scattered granules whose distribution through the nucleus is so irregular that in nearly all cases no trace of chromosomal structure is apparent. The nucleus then grows rapidly, the chromatin content often increasing to many times that in the original chromosome group, until it soon reaches a quite definite size, varying widely in different cells, but fairly constant for cells of a single kind in. a given species. The nucleus and chromatin then remain without much further change, quantitative, at any rate, until toward the close of the vegetative or normally functional period of cell life. At the time of the next division much of the chromatin is usually eliminated from the nucleus, is cast out into the cytoplasm and disappears along with the nucleolus (Fig, 32); the chromosomes which then appear or reappear, are similar in every respect to those in the preceding division (Van Beneden). The chromosomes, therefore, show the same specific constancy as any other characteristic of the organism.

Fig. 32. — Eaiiy cleavages of the egg of tbe Nematode. Ascarii. Origin of primordial germ cells and casting out of chromatin in the somatic cella. From WUboh, "Cell," after Boveri. A. Two-cell stage dividing; polar view, a, atemcell, from which the germ cella are derived. B. Later stage of same division; lateral view, c, chromatin in somatic cell being thrown out into the cytoplasm. C. Completion of tout-cell stage showing eliminated chromatin. D. Division of four-cell stage showing continued chromatin elimination in the third somatic cell.

The most obvious chaxacteristic of the chromosomes is that of numerical constancy. In different species of organisms the number varies greatly but there is in general Uttle if any relation between the grade or relative complexity of the organism and the number of chromosomes in its nuclei; closely related species of a single genus may differ widely, e.g., Ascaris megcdocephala has four, A. lumbricoides forty-eight. In general the number seems highest in some of the Protozoa. Where there are very many minute chromosomes the difficulty of counting them exactly is very great and it cannot then be said precisely what or how constant the number is. In Mastigella there are about forty, in Actinosphoerium one hundred and thirty to one hundred and fifty, in Paramcedum about two hundred. Among the Metazoa the smallest number is two, in a variety of Ascaris, the largest known is one hundred and sixtyeight, in Artemia. Frequent numbers are twelve, sixteen, and twenty-four, but any number may be found within these known limits. The number is practically always an even one in somatic cells, even or odd in the germ cells or their inmiediate predecessors. The numbers found in the tissue cells of some of the familiar organisms are the following: rat, guinea-pig, ox, sixteen; Amphioxus, salmon, salamander, frog, mouse, man (female), twenty-four; earthworm, thirty-two; shark, thirty-six; sea-urchin, eighteen in one species, thirty-six or thirty-eight, in another; pine, onion, sixteen; lily, peony, twenty-four.

While the number of chromosomes is thus practically constant it is not absolutely invariable and deviations from the normal are now known to occur in several forms. Of course the most frequent variation is the typical reduction of the number to 5-half the somatic [^], during certain phases in the formation of the germ cells (Van Beneden), or throughout the gametophytic generation of many, perhaps most, plants. But as we shall see later, this should hardly be called a variation from normal. A deviation of an entirely different kind is seen in a few cases where the chromosome number is in cleavage or tissue cells of certain individuals; thus in the cleavage cells of Ascaris megcdocephala the number is two or four, in the tissues of Helix pomatia, twenty-four or forty-eight, in Strongylocentrotus, eighteen or thirty-six. In such cases each of the lesser number is said to be hivaUnt, and it is supposed, not without reason, that each is actually composed of two ordinary or univalent chromosomes. In a few instances the number may be even less than one-half the normal and each is then said to be plurivalent; thus in the formation of the embryo-sac in the lily a variation in certain nuclei has been found, the number varying by fours from twelve to twenty-four (s = 24). The significance of these unusual cases is varied and sometimes doubtful. Again, constant differences in chromosome number, in both somatic and germ cells, are associated with sex differences in a large number of species of several widely divergent phyla. In such cases the females have a somatic group from one to five, or even more, in excess of that of the male; in such cases the specific number is fixed, though some variable species are known.

None of these unusual deviations from the normal is of the character of a "normal variability." Indeed very few instances of this kind of variability in chromosome number are known. One instance is that of the salamander larva, in certain tissue cells of which the number is said to vary (Delia Valle) in different individuals from nineteen to forty, the normal being twenty-four, and in a single specimen limits of nineteen and twenty-seven have been described.

This very high degree of specific numerical constancy of the chromosomes indicates strongly that the appearance of a chromosome in mitosis is not determined by a large number of causes, but that it is the result of the operation of a single and simple factor; what this may be is a matter of conjecture only.

Another important characteristic of the specific chromosome group is the constancy of the form and size differentiations among the members of the group. In many organisms it can be seen clearly that the chromosomes of a single nucleus are not all of the same size and form; they may differ in shape, dimensions, and proportions (Fig. 33) (MontgomCTy). Moreover these differences are constant from one cell generation to the next, so that similar chromoBomes may be identified in successive mitoses. The form is somewhat more variable than the volume, which is remarkably uniform. It should be said that the size of a given chromoBome is not fixed throughout the entire cell history, for at certain periods, particularly in the germinal tissues, the chromosomes may be many times larger than at other periods (Fig. 34). But at corresponding cell ages the corresponding chromosomes are practically equal in volume, and in somatic cells such volume changes of single chromosomes are relatively infrequent.

Fig. 33. — Varioua chromoBome groups illuBtrBtinK variHtioii In dme and fonn. and coupling of chromoBomes. A.B, from Sutton, C. after Wilaon, D, after Agar. A.B. Spermatogonia of the graaBhopper, BTocktistola magna. C. Spermato-* goniuiD of the squash-bug, Ana»a triatie. D. Spermatogonium of the lung-fish, Lepidoairen.

One most significant and very important fact in this connection is that in the somatic cell the chromosomes are present in couples of similar elements; there are two of each size or form (Montgomery) (Figs. 33, 72, A; 142, A). The exeeption

are found chiefly in those forms where sex differences are found; in such cases one or more chromosomes are unpaired, or the members of the pair may be dissimilar in size. And further

in the germ cells with ^ chromosomes, none is paired — all are

single, each somatic pair is represented, and the groups in eggs and sperm are alike.

Fig. 34. — CbangeB in the volume of ohromosoiueB in the egg of the Elasmobranch. Prigtiunts. All drawn to same scale. From Wilson, "Cell," after Hfickert. A. In egg of 3 mm. diameter. Chromosoroea at maximal size and mioiinal atainiaR capadty. B. In egg of 13 mm. diameter. C. In fully grown ovarian egg. Minimal aiae and maximal staining capacity.

Such facts as those given above taken in connection with the precision with which each chromosome is halved in mitosis, lead almost irresistibly to the supposition that the chromosomes must be qualitatively unlike. Such qualitative differences cannot be observed directly, and can only be inferred, but as we shall see in connection with the relation of the chromosomes to heredity, this inference seems to be justified from the results of the experimental or accidental modification of the chromosomal content of the nuclei and the character of the resulting cells or cell groups (Chapter VII).

In connection with the chromosomes there remain to be mentioned two important hypotheses. The first is the hypothesis of the specificity of the chromosomes. Stated in its barest form the essence of this idea is that each chromosome functions, in cell life, in its own particular way, representing a center for reactions of a specific kind only; that the chromosomes are cell " organs," functionally differentiated and representing a division of labor roughly analogous to the functional differentiations of the whole animal body. It is impossible to discuss this hypothesis satisfactorily here and it is deferred to Chapter VII, where it occupies a natural place in our account of the mechanism of differentiation.

The second hypothesis is that of the genetic continuity of the chromosomes. The essential of this idea is that the chromosomes which appear during the preparation for a mitosis, are definitely related in a precise way, to the chromosomes entering that nucleus at the close of the preceding division. In its first form this hypothesis was called that of the individuality of the chromosomes (Rabl, Boveri), and it was held that the chromosomes actually, though not visibly, preserve their structural identity during the period of interkinesis, that the chromosomes of one mitosis are not related to those of the preceding division, but are actually the same chromosomes. The fact that little or no direct evidence of chromosomal identity during the interkinesis, is to be had, has led to the remodeling of the idea of individuality, into the hypothesis of genetic continuity.

The nature of the evidence bearing upon this hypothesis, while not scanty, is largely circumstantial, and hardly affords definite proof, either affirmatively or negatively. We may suggest some of this evidence without pretending to give a detailed account of the facts.

At the very beginning we must recognize that the chromosomes are actually visible, as differentiated structures, only during that comparatively brief period of cell life occupied by the process of mitosis. Considering first some of the facts opposed to this hypothesis, we should say that those who deny the fact of continuity, maintain that during the vegetative period of cell life the dissolution and disappearance of the chromosomes is not only apparent but real. There is truly little visible and direct evidence of chromosomes during interkinesis. And further, much new chromatin is formed during this period which cannot be distinguished from the chromosomal chromatin; in the early stages of mitosis much chromatin is again thrown out of the nucleus and takes no part in the formation of chromosomes. It is impossible to say whether the chromatin forming the chromosomes is or is not then, the same as that previously derived from them, or that resulting from growth. Many instances are known where the chromatin of the vegetative nucleus is nearly all contained within a single homogeneous chromatin nucleolus or karyosome (e.g., Asterias), and in preparation for mitosis some granules pass out of the karyosome and form into distinct chromosomes while the greater part of the karyosome diBaolves (Pig. 35) ; it is difficult to understand how the chromosomes could have preserved their integrity through such a history. In amitosis no chromosomes are visible yet the nuclei of cells thus formed seem to fiinction normally and such cells are capable of typical differentiations and in a few instances are said, with some question, however, to be subsequently capable of division by mitosis, and of normal chromosome formation.

Fig. 36. — FriiD&ry oocyte o( the Btar-fiah, Aaleriai forbeiU, at beEiDniDg of livision. From Dahlgren and Kepner (Jordan). ChromatiD leaving the " chro' " or chromatin □udeolus, and being added to the chromosomes.

Such an interruption of a series of mitotic divisions by a period of amitotic division would seem to exclude the possibility of both genetic relation and specificity of the chromosomes. In those forms where chromosomes are not normally formed, the chromatin granules are the units in nuclear division; and even when these are formed into chromosomes the essential step in nuclear division is the fission of these granules, which thus seem to be the real units of Hie chromatic substance. Yet one could hardly maintain the genetic continuity of these granules upon other than logical grounds, and to many there seems no stopping place short oi this, if the fact of continuity of organized chromatic structure be accepted to begin with.

On the other hand, those vho adhere to the continuity hypothesis find many supporting facte in the phenomena of fertilization and maturation, the importance of which can be appreciated more fully after our conaideration of these subjects. They assume, from the consideration of the behavior of the chromosomes, that they only apparently lose their structural and functional identity during the interkinesis, and that something directly representative of the chromosomal structure

Fig. 36. — Indications of the individu^ty of the cbromosomea in tbe oleBvaga of the egg of Aacaria. From Wilson, "Cell," after Boveri. E. Anaphaae of first cleavaae. F. Two-cell stage with lobed nuclei, the lobes formed by the ends of the cbromosomes. O. Eaily piophase of next division. Chromosomes reforrning. centroaoraes dividing. H. Late prophases of same division, the chromoaomeB lying with their ends in the same position as at the close of the preceding division.

is present in the resting nucleus, invisible directly and known only from its consequences. There are, it is true, a few instances in which the chromosomal arrangement b said really to be visible in the interkinesis (Figs. 36, 37), but these cases are not very clear, except in the maturation divisions leading directly to the formation of the specialized germ cells, where the chromosomes are definitely known to be directly continuous. The remarkable constancy of form, volume, and number of chromosomes throughout the cells of a given organism and species, is important evidence favoring the hypothesis under conaideration. The probability is so high as to amount almost to certainty, that if tbe chromosomes were new and independent formations in each mitosis, they would show & normal variability in number throughout long series of mitoses. However, numerical variability is so rare as to be practically absent, although the few exceptions known are emphasized by those who do not accept the idea of continuity. Constancy of form seems to be less precise than constancy of volume, but both are sulhciently marked to be noteworthy. It is difGcult to get precise observations here on account of the liability to shrinkage or deformation of the chromosome ia the preparation of the material for study. There is often undoubtedly a definite pairing of chromosomes in the somatic nuclei (Fig. 33), while in the germ nuclei, with ^ chromosomes, the same categories of chromosomal form and size found in somatic nuclei are distinguishable, but these are no longer paired — there are only single repreaentatives of each. This is one of the strongest points favoring this hypothesis. And no lees significant is the fact that the odd or unpaired chromosomes associated with sex differentiation, mentioned above, remfun constant in size and form and number, throughout the tissue cells of certiun individuals, and are easily recognizable, not only through their peculiar morphology but on account of their peculiar behavior as well. It is important in this connection, to no «« that the,e particular ch„mo»m» aSXl^f'to.lSLt may remain undissolved even in the early prophaBe at division of a sperreating nucleus, where they had often matogonium of Brachyatola magna. , J I ■ I ,. I ,. From Sutton. Spiremes fonnine in

been described as chromatm nucleoli lobes of the nucleus corresponding or other bodies, before their signifi- with the ohromoHoiiiea which eacance was appreciated or their history t^^d the nucleus at the close of the preceding division.

known.

Another group of facts of quite a different character has an important bearing upon these hypotheses. It sometimes happens in mitosis that one or more chromosomes belonging to one daughter group, accidentally become included with the other group so that one of the daughter nuclei has fewer, the other more, than the normal somatic number. In subsequent divisions of these cells the number of chromosomes appearing is not the normal, but the increased or diminished number, the sum of the two, however, always being 2s. Or in fertiliaation of the egg by the sperm, each of which has ^ chromosomes, various abnormalities occur id the distribution of the chromosomes, and it is always the eaee that the nwnfeer of chromosomes forming out of a nucUus is the same as the number passing into it, no matter how that deviates from the normal (Boveri) (Fig. 38) . There seems to be no regulation within the nucleus in this respect, such as would result were it a unified structure tending always to maintain its own normal. In many instances of alternation of generations, the sgametically produced generation is formed from a cell with ^ chromosomes; in all the cells of such an organism, the nuclei show the same reduced number, even in so complex an organism as the fern prothallus. In some forms, the chromosome groups derived from the egg and aperm nuclei, each of ^ chromosomes, remain distinguishable through a considerable series of the early divisions of the zygote; two separate spiremes, each forming ^ chromoeomee, may even be distinguished sometimes (RUckert). And in some hybrids where the chromosomes are unlike in number, or

Fig. 38. — Indications of the individuality of the cbromosomea in the fertilii». lion of Aacarit. From Wilson, "Cell," after Boveri, A. The two chromoeomea of the eeg-nucleus, accidentally separated, have given rise each to a reticular nucleus (9, 9); the sperm-nucleus below (d"). B. Later ataga of the same, a single chromosome in each egg-nucleuSi two in the apenn-nucleus, C An egg in which the second polar body has been retained; p.b.' the two chromosomes ariBiDg from it. 9 the egg-chromosomes, & the sperm-chromosomes. D. ResulUng equatorial plate with six chromosomes.

Fig 39. — The chromosome group in the hybrids of the Teleosts, Ftmdultu and Uenidia, showing the distinctnesa of the patera&l and maternal elementB. From Moenldiaua. A. Lat« anaphase of first cleavage of normally fertilized FuTidjilui. All chromoBOmee of the long type. B. Anaphase of first cleavage of normally tertiliied egg Ot Uenidia, All chromosomes of the short type. C, Anaphase of first cleavage of FvTidultiB egg fertilised with Menidia sperm. To the left long (Fundalui) chromosomes only, to the right short IMenidia) chromosomes only. D. Anaphase of first cleavage of Menidia egg fertiliaed with Fundvha sperm. B, Metaphase of ttrst cleavage of Fundtdue egg fertilized with Menidia speim. d^, chromosomee of paternal origin. 9 , chromosomes of maternal originform, or size, the two groups derived from the male and female parents remun distinct (Fig. 39), often for a very long time, perhaps even throughout life (Moenkbaus, Herbst, Baltzer).

While evidenoe of the kind suggested above does not constitute definite proof of the genetic continuity of the chromosomes, it is very difficult to explain the facts upon any other basis. In the absence of any other satisfactory hypothesis in this field it seems wise to accept a certain modification of the essential idea, as a working basis, while admitting the difficulties of demonstration and the existence of some {^parent contradictions. We may recognize in this difference of opinion regarding the continuity of the chromosomes, an outcropping of the opposed preformational and epigenetic conceptions, which pervade all descriptions of developmental phenomena. The hypothesis of chromosomal continuity is essentially a preformational view; those who deny continuity assume the epigenetic view.

Perhaps the immediate solution of the difficulty here may not be unlike the solution of the greater problem and more fundamental difference of opinion. It seems likely that what is directly continuous from nucleus to nucleus, i.e., what is preformed, is some sort of fundamental organization determining the chromosomal structure of the nucleus, just as the " organization " of the egg determines the structure of the embryo developed from it. Yet what appears is a new structure, formed epigenetically under or through the influence of the "organizational" factor, by the material present, and subject to the modifying influences of changing conditions external to the nucleus. That is to say, the formation of the chromosomes out of the chromatin of the vegetative nucleus is to be regarded as a true process of development. The reaction between the fundamental organized structure of the nucleus, and the stimuli acting upon it, consists in the formation of the chromosomes and other structures, not to be seen in the nucleus previous to this reaction. The chromosomes are thus no more genetically continuous than the organs of adults,, and yet there is a real continuity of organization underl3ring.

In many respects the nucleus is analogous to an organism, the chromosomes and other nuclear structures representing the organismal organs; both are functionally specific, are constant in number, form, and size throughout the species; both reproduce and exhibit development as a form of response. As Wilson points out, the analogy is far from complete — no complete analogy is known, but that there is an underlying organization of some kind, continuous and specific, seems clear although we remain entirely ignorant of what it really is, and just how it operates and is affected by new conditions.

Before leaving the subjects of the cell and cell division we must consider briefly two other questions which are of great importance, but which are also still in a hypothetical state. These are, the nature of the causes leading to cell division, and the nature of the fundamental mechanism of the process. The interactions between the nucleus and cytoplasm, and between these and the external medium, which constitute the life of the cell, are largely, probably wholly, of a physico-chemical nature. As such their normal procedure is dependent upon certain (nass relations of the interacting substances, and upon the maintenance of adequate pathways of interaction between them. For these reasons we look quite naturally, in searching for a possible cause of cell division, to the volumetric relations of the nucleus and cytoplasm, and to the extent of the surface of these parts of the cells in relation to the two masses.

Immediately after mitosis is completed the nucleus grows very rapidly for a brief period, and then much more slowly or not at all. The cytoplasm, however, does not show this rapid initial growth, but maintains a fairly uniform and continuous growth rate. As a result, in a newly formed cell the ratio of nuclear mass to cytoplasmic mass becomes quite high, but soon diminishes, and in an older cell diminishes rapidly, since the cytoplasm continues to grow after the nucleus nearly ceases. Considering first the relation of the surface to the mass, and assuming for illustration that the cell and nucleus are both spherical, we can see how this increase in size alters the relation of mass to path of interchange, since the area of the surface of an enlarging sphere increases relatively slower than the volume. Should the cell double its diameter through ' growth, it increases its volume eight times and its surface only four times. A cubical cell doubling the length of its sides reduces its ratio of area to volume as 2 : 1. Or expressing a similar relation somewhat differently, a spherical cell doubling its volume, lessens its ratio of surface to volume approximately as 5 : 4, while a cubical cell doubling its volume reduces the same relation approximately as 6 : 4.75.

These relations are probably important for both of the chief interactions of cell life, those between nucleus and cytoplasm, and between cytoplasm and surrounding medium. The only pathway between c)rtoplasm and nucleus is the nuclear membrane, while the surface of the cytoplasm or cell wall forms the pathway between cell and medium. There is of course a limit to the capacity, so to speak, of these surfaces, and as the cell increases in volume this limit tends to be reached. The ratios of these surfaces to the masses are raised by the division of the cell, which reduces volume more than surface, and thus restores the efficiency of the surfaces as pathways of interaction. The tendency for the cytoplasmic mass to increase more rapidly than the mass of the nucleus in older cells seems to be of even greater importance than these surface to mass relations. There seems to be a fairly definite specific limit to the ratio of nuclear mass to cytoplasmic mass, although it is difficult to say whether after all the nuclear surface relation is not even here an equally important factor. This mass relation is called the "kern-plasma" or nucleo-cytoplasmic relation, the importance of which is emphasized particularly by Richard Hertwig. There is a definite average cell size in a given tissue and species ; a large or a small organ or organism does not possess respectively larger or smaller cells, but larger or smaller numbers of cells of the same average dimensions. The limiting factor, however, seems to be not the actual bulk of the cell, but the proportion between the volume of the nucleus and that of the cytoplasm, i.e,, {frj • In many cells during, and immediately after, division the nucleus grows at the expense of the cytoplasm, and the ratio ly^j is raised (Fig. 40). The cell then enters upon its vegetative phase, during which the C3rtoplasm grows more rapidly than the nucleus, and the ratio diminishes toward the lower functional limit ; as the ratio approaches this limit the functional activities of the cell change, and the normal vegetative processes give place to that form of action which we call cell division, during which the ratio again rises to a value permitting normal vegetative functioning. It is not yet possible to state in precise quantitative terms what the limits of these ratios of volume and surface are in specific instances, nor even to say whether the volume or surface, or volume and surface relations are those essentially involved. But so far this nucleo-cytoplasmic hypothesis is the most plausible explanation of the nature of the immediate conditions of cell division. It should be said, however, that the applicability of Hertwig's "Kernplasma Relation" is still chiefly limited to Protozoan cells, and that even here there are many contradictions. Some of the more obvious exceptions to the definition of such a limiting ratio are to be explained as special adaptations. Such are the very great cytoplasmic bulk of many egg cells or the relatively large size of the nucleus in the sperm cell. Many other exceptions of special character are to be found, usually associated with reproductive proceeses, e.g., brood formation.

Fig. 40. — Curve showing the increase in volume of nucleus (a-d) and cytoplasm (h-c) during interkinesis, in the Infusorian; Frontonia leucas^ at temperature of 26° C. After Popoff. Ordinates, volume; abcissas, time in hours. Each curve shows a doubling of volume (1:2) during the seventeen hour period of the interkinesis. Each curve is based upon its own units of measurement, which are different for the two curves. The nucleo-cytoplasmic relation is identical at the beginning and end of the period. The size of the nucleus is relatively smallest at fifteen hours; then the nucleus begins to grow very rapidly, so that at the time of the division of the whole cell, the original relation is restored.

Fio, 41- — Diagrams of the arrangement of the Bpongioplasroic Detwork (mitome) in the cell. A, B, C, from Korschelt and Heider. after EeJdenhain. A, Schema of the arraDgement of the spongioplatim as it would appear in the alienee of a nucleus. Symmetrical monocentric system. B. Atrangement in the presence of a nucleus. Asymmetrical monocentric system. C. Arrangement at the beenning of mitoaia. Commencement of dicentric aystem. D. Arrangement during the process of mitosis. Symmetrical dicentric system. a,b, cell axis: k, nucleua.

Regarding the real nature of the mechanism of mitosis, even leas can be smd to be known than with respect to its causes. The arrangement of the achromatic amphiaster and the behavior of the chromosomes show that in the mitotically dividing cell the forces or tensions are arranged in a dicentric system, whereas in the vegetative cell the system is monocentric (Fig. 41). As the result of the action of the forces in this dicentric system the chromosome halves are separated and the cytoplasm divided. The problem here is to discover the nature of the forces and the cause .of the formation of the amphiaster in the form which it has. In explanation of the first part of the problem, i.e., the divergence of the chromosomes, it was formerly believed that the fibers in the amphiaster were the active elements and that different groups of these fibers had different functions. Thus the mantle fibers attached to the chromosomes were supposed to be contractile and by shortening to draw the chromosomes toward the ends of the spindle, the central spindle remaining rigid and resisting any tendency for the ends of the spindle to approach as the result of the contraction of the mantle fibers, and at the same time serving as a sort of track upon which the chromosomes would slide along. The asters then, either as anchors helped to fix the ends of the spindle, or by contraction served to draw the fully diverged chromosomes further into the daughter cells (Fol, Van Beneden, Heidenhain). This naive explanation cannot be applied in toto to any known instance of division, although certain features may be correct descriptions of the events in certain forms. And there are many facts opposing such an account of the action of the forces of mitosis, such as the absence of mantle fibers or of asters in many mitoses. Another hypothesis, in greater favor at present, and apparently well founded, is that the centrosomes and centrospheres rather than the achromatic fibers are the active elements, and further, that their activity is primarily of a chemical nature. Thus the chromosomes are believed to be chemically attracted toward the centrosomes, the achromatic fibers being passive and formed merely as the result of the rearrangement of cytoplasmic granules along the paths of the chemical transformations which have their seat in the centrosomes and extend thence through the cytoplasm (Strasburger). This hypothesis goes farther and makes it possible to explain the division of the C3rtoplasm on the same basis. For the chemical transformations centering in the centrosomes might easily influence the tensions of the comparatively impermeable surface film of the cell so that in the region near the centrosomes the tensions would be increased while that region farthest from the centrosomes and symmetrically related to them both, namely, the plane of the equatorial plate, would be a region of lower tension; the result of this would, of course, be a constriction in this plane. It is quite likely too that differences in electric tension accompany these chemical transformations, and these might assist in the alteration of the surface tensions in such a way as to contribute to the same end (R. S. Lillie). It is known that there are differences in the electric potential in different regions of the dividing cell, in some cases at least.

A further development of the chemical h3rpothesis attempts to explain the formation of the amphiaster itself. Thus, increase in the ratio of the volume of the cytoplasm to volume or surface of the nucleus beyond a certain point leads to a chemical alteration of the centrosomes such that they become the centers of two equivalent Series of chemical reactions with the cytoplasm, the result of which is the formation of the dicentric S3rstem. It is suggested, reasonably, that the chemical alteration is such that the centrosomes absorb or condense the more liquid parts of the cytoplasm, leaving this considerably more dense than in the vegetative condition (Biitschli, Rhumbler). The existence of two such centers withdrawing the more liquid parts of the cytoplasm would lead to the radiations seen in the asters and spindle, which would thus result from a physical alteration of the structure of the cytoplasm induced by the chemical changes within the centrosomes. This is all extremely hypothetical of course, but there are many inorganic phenomena as well as processes to be seen in the cleavage of the egg which lend considerable support here. At any rate these hypotheses represent the state of our present ignorance of the nature and origin of the mitotic figure and process. There are many reasons for believing that the chemical differentiations within the cell are of fundamental importance here, such as the fact that cells can be made to divide artificially by altering their chemical structure. And that interactions of the nucleus and cytoplasm are involved is indicated by the important observation that while the amphiaster may be formed in the absence of a nucleus, no real division of the cell may occur without the presence of nuclear material (Boveri, Ziegler).

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Kellicott WE. A Textbook of General Embryology (1913) Henry Holt and Co., New York.

Kellicott (1913): 1 Ontogeny | 2 The cell and cell division | 3 The germ cells and their formation | 4 Maturation | 5 Fertilization | 6 Cleavage | 7 The germ cells and the processes of differentiation, heredity, and sex determination | 8 The blastxtla, gastrula, and germ layers. Morphogenetic processes

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