The Cell in Development and Inheritance

Van Beneden, E. — Recherches sur la maturation de I'oeuf. la fecondation et la division cellulaire : Arch, de Biol.., IV. 1883. Van Beneden and Neyt. — Nouvelles recherches sur la fecondation et la division mitosque chez TAscaride megalocephale : Bull. Acad. roy. de Belgigue, III. 14, III. Ibid. XXIV. 1890. Driiner, L. — Studien liber den Mechanismus der Zelltheilung. Jena. Zeitschr.^ Erlanger, R. von. — Die neuesten Ansichten liber die Zelltheilung und ihre Mechanik :

Zo'dl. Centra lb.. III. 2. 1896. Id. — Uber die Befruchtung und erste Teilung des Ascariseies : Arcli. mik. Anat., Flamming, W., '92. — Entwicklung und Stand der Kenntnisse liber Amitose : Merkel und Bofinefs Ergebnisse, II. 1892. Id. — Zelle. (See Introductory list. Also general list.) Fol, H. — (See List IV.) Heidenhain, M, — Cytomechanische Studien: Arch. f. Entiuickmcch., I. 4. 1895. Id. — Neue Erlauterungen zum Spannungsgesetz der centrirten Svsteme : Morph.

Arb., VII. 1897. Hermann, F. — Beitrag zur Lehre von der Entstehung der karyokinetischen Spindel : Phys. MiincJioi. 1^95' Heft I. Kostanecki and Siedlecki. — Uber das Verhalten der Centrosomen zum Protoplasma : Meves, Fr. — Zellteilung: Merkel und Bontiefs Ergebnisse, W. 1897. Reinke, F. — Zellstudien : I. Arch. mik. Afiat., XLIII. 1894 : II. /bid. WAV. 1S94. Strasburger, E. — Karyokinetische Probleme : Jahrb.f. l\ 'iss. Jiotan.. XX\'I II. i S95. Strasburger, Osterhout, Mottier, and Others. —Cytologische Studien aus dem Bonner

Institut: JaJirb. luiss. Bot.. XXX. 1897. Waldeyer. W. — iJber Karyokinese und ihre Beziehungen zu den Befruchtungsvor- " Not all the propjeny of the primary impregnated germ-cells are required for the forma- tion of the liudy in all animals; certain of the derivative germ-cells may remain unchanged and become included in that body which has been composed of their metamorphosed and diversely combined or confluent brethren; so included, any derivative germ-cell may com- mence and repeat the same processes of growth by imbibition and of propagation by spon- taneous fission as those to which itself owed its origin; followed by metamorjihoses and combinations of the germ-masses so produced, which concur to the development of another individual." RlCJiAKU UWEN.i

" Es theilt sich demgemass das befruchtete Ei in das Zellenmaterial des Individuums und in die Zellen fiir die Erhaltung der Art." M. NUbSBAUM.'^ The germ from which every living form arises is a single cell, de- rived by the division of a parent-cell of the preceding generation. In the unicellular plants and animals this fact appears in its simplest form as the fission of the entire parent-body to form two new and separate individuals like itself. In all the multicellular types the cells of the body sooner or later become differentiated into two groups, which as a matter of practical convenience may be sharply distin- guished from one another. These are, to use Wcismann's terms : (i) the somatic cells, which are differentiated into various tissues by which the functions of individual life are performed and which col- lectively form the ** body," and (2) \hQ (^ej'm-cells, which are of minor significance for the individual life and are destined to give rise to new individuals by detachment from the body. It must, however, be borne in mind that the distinction between germ-cells and somatic cells is not absolute, as some naturalists have maintained, but only relative. The cells of both groups have a common origin in the parent germ-cell ; both arise through mitotic cell-division during the cleavage of the ovum or in the later stages of development ; both have essentially the same structure and both may have the same power of development, for there are many cases in which a small fragment of the body consisting of only a few somatic cells, perhaps only of one, may give rise by regeneration to a complete body. The dis- tinction between somatic and germ-cells is an expression of the

physiological division of labour; and while it is no doubt the most fundamental and important differentiation in the multicellular body, it is nevertheless to be regarded as differing only in degree, not in kind, from the distinctions between the various kinds of somatic cells. In the lowest multicellular forms, such as Volvox (Fig. 57), the differentiation appears in a very clear form. Here the body consists of a hollow sphere, the walls of which consist of two kinds of cells. The very numerous smaller cells are devoted to the functions of nutri-

Fig. 57. — Volvox, showing the small ciliated somatic cells and eight large germ-cells (drawn from life by J. H. Emerton). tion and locomotion, and sooner or later die. A number, usually eight, of larger cells are set aside as germ-cells, each of which by progressive fission may form a new individual like the parent. In this case the germ-cells are simply scattered about among the somatic cells, and no special sexual organs exist. In all the higher types the germ-cells are more or less definitely aggregated in groups, supported and nour- ished by somatic cells specially set apart for that purpose and forming distinct sexual organs, the ovaj^es and spcrmarics or their equivalents. Within these organs the germ-cells are carried, protected, and nour- ished; and here they undergo various differentiations to prepare them for their future functions.

In the earlier stages of embryological development the progenitors of the germ-cells are exactly alike in the two sexes and are indistin- guishable from the surrounding somatic cells. As development pro- ceeds, they are first differentiated from the somatic cells and then diverge very widely in the two sexes, undergoing remarkable trans- formations of structure to fit them for their specific functions. The structural difference thus brought about between the germ-cells is, however, only the result of physiological division of labour. The female germ-cell, or ovum, supplies most of the material for the body of the embryo and stores the food by which it is nourished. It is therefore very large, contains a great amount of cytoplasm more or less laden with food-matter (j7^/X' or dciitoplasui), and in many cases becomes surrounded by membranes or other envelopes for the pro- tection of the developing embryo. On the whole, therefore, the early life of the ovum is devoted to the accumulation of cytoplasm and the storage of potential energy, and its nutritive processes are largely constructive or anabolic. On the other hand, the male germ-cell or spermatozoon contributes to the mass of the embryo only a very small amount of substance, comprising as a rule only a single nucleus and a very small quantity of cytoplasm. It is thus relieved from the drudgery of making and storing food and providing protection for the embryo, and is provided with only sufficient cytoplasm to form a locomotor apparatus, usually in the form of one or more cilia, by which it seeks the ovum.

It is therefore very small, performs active movements, and its metabolism is characterized by the predominance of the destructive or katabolic processes by which the energy neces- sary for these movements is set free.^ When finally matured, there- fore, the ovum and spermatozoon have no external resemblance ; and while Schwann recognized, though somewhat doubtfully, the fact that the ovum is a cell, it was not until many years afterward that the spermatozoon was proved to be of the same nature.

The animal (i^^^^ (Figs. 5CS-59) is a huge spheroidal cell, sometimes naked, but more commonly surrounded by one or more membranes which may be perforated by a minute opening, the inicropylc, through which the spermatozoon enters (Fig. 63). It contains an enormous nucleus known as the germinal vesicle, within which is a very con- spicuous nucleolus known to the earlier observers as the germinal spot. In many eggs the latter is single, but in other forms many

1 The metaliolic contrast between the germ-cells has been fully discussed in a most sug- gestive manner by Geddes and Thompson in their work on the Evolution of Sex ; and these authors regard this contrast as but a particular manifestation of a metabolic contrast charac- teristic of the sexes in general. nucleoli are present, and they are sometimes of more than one kind as in tissue-cells.i In many forms no centrosome or attraction-sphere is found in the ^gg until the initial stages in the formation of the polar bodies, though Mertens ('93) describes a centrosome and attrac- tion-sphere in the young ovarian eggs of a number of vertebrates (Fig. 79), while Platner ('89) and Stauffacher ('93) find what they believe to be centrosomes in much later stages of AulostoDiuvi and Cyclas, lying outside the nuclear membrane. Beside these cases should be placed those described by Balbiani, Munson, Nemec, and others in which a body closely resembling an attraction-sphere is identified as a "yolk-nucleus" or "vitelline body," as described at page 158. In none of these cases is the identification of this body wholly satisfactory, nor is it known to have any connection with the polar mitoses. Most observers find no centrosome until the prophases of the first polar mitosis. Its origin is still problematical, some observers believing it to arise de novo in the cytoplasm (Mead), others concluding that it is of nuclear origin (Mathews, Van der Stricht, Riickert), still others that it persists in the cytoplasm hidden among the granules. In any case it is again lost to view after forma- tion of the polar bodies, to be replaced by the cleavage-centrosomes which arise in connection with the spermatozoon (p. 187).

The egg-cytoplasm almost always contains a certain amount of nutritive matto^, the yoik or deiUoplasin, in the form of liquid drops, solid spheres or other bodies suspended in the meshwork and varying greatly in different cases in respect to amount, distribution, form, and chemical composition. The nucleus or germinal vesicle occupies at first a central or nearly central position, though it shows in some cases a distinct eccentricity even in its earliest stages. As the growth of the agg proceeds, the eccentricity often becomes more marked, and the nucleus may thus come to lie very near the periphery. In some cases, however, the peripheral movement of the germinal vesicle occurs only a very short time before the final stages of maturation, which may coincide with the time of fertilization. Its form is typically that of a spherical sac, surrounded by a very distinct membrane (Fig. 58); but during the growth of the Q.gg it may become irregular or even amceboid (Fig. j'j), and, as Korschelt has shown in the case of insect-eggs, may move through the cytoplasm toward the source of food. Its structure is

1 Hacker ('95, p. 249) has called attention to the fact that the nucleolus is as a rule single in small eggs containing relatively little deutoplasm (crulenterates, echinodernis, many annelids, and some copepods), while it is multiple in large eggs heavily laden with deutoplasm (lower vertebrates, insects, many Crustacea). on the whole that of a typical cell-nucleus, but is subject to very great variation, not only in different animals, but also in different stages of ovarian growth. Sometimes, as in the echinoderm ovum, the chro- matin forms a beautiful and regular reticulum consisting of numer- ous chromatin-granules suspended in a network of linin (Fig. 58). In other cases, no true reticular stage exists, the nucleus containing throughout the whole period of its growth the separate daughter-chro- mosomes of the preceding division (copepods, selachians, Amphibia),^

g.v. Nucleus or germinal vesicle, containing an irregular discontinuous network of chromatin; g.i. nucleolus or germinal spot, intensely stained with haematoxylin. The naked cell-body con- sists of a very regular alveolar meshwork, scattered through which are numerous minute granules or microsomes. {Cf. Figs. 11, 12.) Below, at s, is an entire spermatozoon shown at the same enlargement (both middle-piece and fiagellum are slightly exaggerated in size).

and these chromosomes may undergo the most extraordinary changes of form, bulk, and staining-reaction during the growth of the Q^gg^ It is a very interesting and important fact that during the growth and maturation of the ovum a large part of the chromatin of the germinal vesicle may be lost, either by passing out bodily into the cytoplasm, by conversion into supernumerary or accessory nucleoli which finally degenerate, or by being cast out and degenerating at the time the polar bodies are formed (Figs. 97, 128).

The nucleolus of the egg-cell is, as elsewhere, a variable quantity and is still imperfectly understood. It often attains an enormous development, forming the " Keimfleck " or "germinal spot" of the early observers. There are some cases {e.g. echinoderm eggs) in which it is always a single large spherical body (Fig. 58), and this condition appears to be characteristic of the very young ovarian e^'-f^s of most animals. As a rule, however, the number of nucleoli ^in- creases with the growth of the ovum, until, in such forms as Amphibia and reptiles, they may be numbered by hundreds.

In a large number of cases the nucleoli are of two quite distinct types, which Flemming has distinguished as the *' principal nucleolus " Fig. 59. — Ovum of the cat, within the ovary, directly reproduced from a photograph of a preparation by Dahlgren. [Enlarged 235 diameters.] The ovum lies in the Graafian follicle within the discus proligerus, the latter forming the immediate follicular investment {corona radiata) of the Qg^. Within the torona is the clear zona pellucida or egg-membrane. {Cf. Fig. 92.)

{Hajiptmicleolus) and ''accessory nucleoli" {Ncbennuclcoli). These differ widely in staining-reaction ; but it does not yet clearly appear whether they definitely correspond to the plasmosomes and karyo- somes of tissue-cells (p. 34). The principal nucleolus, which alone is present in such eggs as those of echinoderms, often stains deeply with chromatin-stains, yet differs more or less widely from the chromatin-network,! and in some cases at least it does not contribute

to the formation of chromosomes. It cannot therefore be directly com- pared to the net-knots or karyosomcs of tissue-cells. This nucleolus is often vacuolated and sometimes assumes the form of a hollow vesicle. It is rarely double or multiple. The accessory nucleoli, on the other hand, are in general coloured by plasma-stains, thus resembling the plasmosomes of tissue-cells ; they arc often multiple, and as a rule they arise secondarily during the growth of the (t^g (Fig. 6i). The accessory nucleoli often have no connection with the principal ; but in some mollusks and annelids an accessory and a principal nucleolus are closely united to form a single compound body (Figs. 60, 61). The numerous nucleoli of the amphibian or reptilian ^gg appear to be of the "accessory" type. The singular inconstancy of the nucleolus is evidenced by the fact that even closely related species may differ in this regard. Thus, in Cyclops brcviconiis, according to Hacker, the very young ovum contains a single intensely chromatic nucleolus ; at a later period a number of paler accessory nucleoli appear ; and still later the principal nucleolus disappears, leaving only the accessory ones. In C. strenuus, on the other hand, there is throughout but a single nucleolus.

The physiological meaning of the nucleoli is still involved in doubt. Many cases are, however, certainly known in which the nucleolus plays no part in the later development of the nucleus, being cast out or degenerating /;/ situ at the time the polar bodies are formed. It is, for example, cast out bodily in the medusa Alquorca (Hacker) and in various annelids and echinodcrms, afterward lying for some time as a '' metanucleus " in the egg-cytoplasm before degenerating. In these cases the chromosomes are formed in the germinal vesicle inde- pendently of the nucleoli (Fig. 125), which degenerate in j//;/ when the membrane of the germinal vesicle disappears. In such cases it seems quite certain that the nucleoli do not contribute to the forma- tion of the chromosomes, and that their substance represents passive material which is of no further direct use. Hence we can hardly doubt the conclusion of Hacker, that the nucleoli of the germ-cells are, in some cases at least, accumulations of by-products of the nuclear action, derived from the chromatin cither by direct transformation of its substance, or as chemical cleavage-products or secretions. It will be shown in Chapter V. that in some cases a large part of the chromatic reticulum is cast out, and degenerates at the time the polar bodies are formed. The immense growth of the chromatin during the ovarian development is probably correlated in some way with the intense constructive activity of the cytoplasm (p. 339); and when this latter process has ceased a large part of the chromatin-substance, having fulfilled its functions, is cast aside. It seems not improbable that the nucleoli are tributary to the same general process, perhaps

A. Before fertilization. The large germinal vesicle occupies a nearly central position. It con- tains a network of chromatin in which are seen five small darker bodies; these are the quadruple chromosome-groups, or tetrads, in process of formation (not all of them are shown) ; those alone persist in later stages, the principal mass of the network being lost; g.s. double germinal spot. consisting of a chromatic and an achromatic sphere. This egg is heavily laden with yolk, in the form of clear deutoplasm-spheres {d) and fat-drops (/). uniformly distributed through the cyto- plasm. The peripheral layer of cytoplasm (peri-vitelline layer) is free from deutoplasni. Outside this the membrane. B. The egg some time after fertilization and about to divide. The deuto- plasm is now concentrated in the lower hemisphere, and the peri-vitelline layer has disappeared. Above are the two polar bodies {p.b.). Below them lies the mitotic figure, the chromosomes dividing.

serving as storehouses of material formed incidentally to the general nuclear activity, but not of further direct use. Carnoy and Le Brun ('97, '99) reach, however, the conclusion that in the germinal vesicle of Amphibia the chromosomes are derived not from the chromatin-network, but solely from the nucleoli. The apparent contradiction of this result with that of other observers is, Fig. 61. — Germinal vesicles of growing ovarian eggs ut the iaiiiLininiincli, L^f/io {A-D), and the spider, Epeira {E-F). [Ohst.]

A. Youngest stage with single (principal) nucleolus. B. Older egg, showing accessory nucle- olus attached to the principal. C. The two nucleoli separated. D. Much older stage, showing the two nucleoli united. E. (ierminal vesicle of Epeira, showing one accessory nucleolus at- tached to the principal, and one free. /•"■. Later stage ; several accessory nucleoli attached to the principal. perhaps, only a verbal one; for the ''nucleoli" are here evidently chromatin-masses, and the disappearance of the chromatic network is comparable with what occurs at a later period in the annelid Qgg (Figs. 97, 128).

The egg-cytoplasm varies greatly in appearance with the varia- tions of the deutoplasm. In such eggs as those of the echinoderm (Fig. 58), which have Httle or no deutoplasm, the cytoplasm forms a regular meshwork, which is in this case an undoubted alveolar struc- ture, the structure of which has already been described at p. 28. In eggs containing yolk the deutoplasm-spheres or granules are laid down in the spaces of the meshwork and appear to correspond to the alveolar spheres of the echinoderm ^^^g (p. 50). If they are of large size the cytoplasm assumes a *' pseudo-alveolar" structure (Fig. 60), much as in plant-cells laden with reserve starch; but reasons have already been given (p. 50) for regarding this as only a modification of the "primary" alveolar structure of Butschli. There is good reason to believe, however, that the egg-cytoplasm may in some cases form a true reticular structure with the yolk-granules lying in its interstices, as many observers have described. In many cases a pe- ripheral layer of the ovum, known as the cortical or peri-vitelline layer, is free from deutoplasm-spheres, though it is continuous with the protoplasmic meshwork in which the latter lie (Fig. 60). Upon fertilization, or sometimes before, this layer may disappear by a peripheral movement of the yolk, as appears to be the case in Nereis. In other cases the peri-vitelline substance rapidly flows toward the point at which the spermatozoon enters, where a proto- plasmic germinal disc is then formed; for example, in many fish-eggs.

The character of the yolk varies so widely that it can here be con- sidered only in very general terms. The deutoplasm-bodies are com- monly spherical, but often show a more or less distinctly rhomb(^idal or crystalloid form as in Amphibia and some fishes, and in such cases they may sometimes be split up into parallel lamellae known as yolk- plates. Their chemical composition varies widely, judging by the staining-reactions; but we have very little definite knowledge on this subject, and have to rely mainly on the results of analysis of the total yolk, which in the hen's ^gg is thus shown to consist largely of pro- teids, nucleo-albumins, and a variety of related substances which are often associated with fatty substances and small quantities of car- bohydrates (glucose, etc.). In some cases the deutoplasm-spheres stain intensely with nuclear dyes, such as hsematoxylin ; e.i:^. in many worms and mollusks; in other cases they show a greater affinity for plasma-stains, as in many fishes and Amphibia and annelids ( Fig. 60). Often associated with the proper deutoplasm-spheres are drops of oil, either scattered through the yolk (Fig. 60) or united to form a .jingle large drop, as in many pelagic fish-eggs.

The deutoplasm is as a rule heavier than the protoplasm ; and in such cases, if the yolk is accumulated in one hemisphere, the ^gg assumes a constant position with respect to gravity, the egg-axis standing vertically with the animal pole turned upward, as in the frog, the bird, and many other cases. There are, however, many cases in which the o.^^ may lie in any position. When fat-drops are present thev usually He in the vegetative hemisphere, and since they

are lighter than the other constituents they usually cause the egg to lie with the animal pole turned downwards, as is the case with some annelids ( .AV/'^/.v) and many pelagic fish-eggs. Fig. 62. — Schematic figure of a median longitudinal section of the egg of a fly ^.Musca), showing axes of the bilateral egg and the membranes. [From KORSCHEI.T and Hkidkr, after Hf:nkin(; and Blochmann.] e.n. The germ-nuclei uniting; VI. micropyle; p.b. the polar bodies. The fiat side of the egg is the dorsal, the convex side the ventral, and the micropyle is at the anterior end. The deutoplasm (small circles) lies in the centre surrounded by a periph- eral or peri-vitelline layer of proto- plasm. The outer heavy line is the chorion, the inner lighter line the vitelline membrane, both being per- forated by the micropyle, from which exudes a mass of jelly-like substance.

The egg-envelopes fall under three catetcories. These are : — bv the ovum itself. {b) The chorion, formed outside the ovum bv the activity of the maternal follicle-cells. {c) Accessory envelopes, secreted by the walls of the oviduct or other maternal structures after the ovum has left the ovary. Only the first of these properly be- lonirs to the ovum, the second and third being purely maternal products. There are some e^^i^s, such as those of certain coelenterates {e.g: Rcnilla\ that are naked throughout their whole develop- ment. In many others, of which the sea-urchin is a type, the fresh-laid ^<^^ is naked but forms a vitelline membrane almost instantaneously after the sperma- tozocin touches it.^ In other forms (in- sects, birds) the vitelline membrane may be present before fertilization, and in such cases the Q.^^g is often surrounded by a chorion as well. The latter is usually very thick and firm and may have a shell-like consistency, its surface sometimes showing various peculiar markings, prominences, or sculptured patterns characteristic of the species (insects).^

1 That the vitelline membrane does not preexist seems to be estal)lished by the fact that egg-frai^ments likewise surround themselves vlth a membrane when fertilized. [Hertwig.] - In some cases, according to Wheeler, the insect-egg has only a chorion, the vitelline membrane being absent. The accessory envelopes are too varied to be more than touched upon here. They inchide not only the products of the oviduct or uterus, such as the albumin, shell-membrane, and shell of birds and reptiles, the gelatinous mass investing amphibian ova, the capsules of molluscan ova and the like, but also nutritive fluids and capsules secreted by the external surface of the body, as in leeches and earth- worms.

When the ^gg is surrounded by a membrane before fertiUzation it is often perforated by one or more openings known as micropylcs, through which the spermatozoa make their entrance (Figs. 62, 63). Where there is but one micropyle, ^ it is usually situated very near the upper or anterior pole (fishes, many insects), but it may be at the opposite pole (some insects and mollusks), or even on the side (insects). In many insects there is a group of half a dozen or more pig. 63.- Upper pole of the egg of Ar^o- micropyles near the upper pole of nanta. [Ussow.]