The cell in development and inheritance (1900) 3
Embryology - 25 Apr 2024    Expand to Translate
|
|---|
| Google Translate - select your language from the list shown below (this will open a new external page) |
|
العربية | català | 中文 | 中國傳統的 | français | Deutsche | עִברִית | हिंदी | bahasa Indonesia | italiano | 日本語 | 한국어 | မြန်မာ | Pilipino | Polskie | português | ਪੰਜਾਬੀ ਦੇ | Română | русский | Español | Swahili | Svensk | ไทย | Türkçe | اردو | ייִדיש | Tiếng Việt These external translations are automated and may not be accurate. (More? About Translations) |
Wilson EB. The Cell in Development and Inheritance. Second edition (1900) New York, 1900.
| Historic Disclaimer - information about historic embryology pages |
|---|
 |
=Chapter III The Germ-Cells
"Not all the progeny of the primary impregnated germ-cells are required for the formation of the body 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 commence and repeat the same processes of growth by imbibition and of propagation by spontaneous fission as those to which itself owed its origin; followed by metamorphoses and combinations of the germ-masses so produced, which concur to the development of another individual."
Richard Owen.
Parthenogenesis, p. 3, 1849.
"Es theilt sich demgemass das befruchtete Ei in das Zellenmaterial des Individuums und in die Zellen fiir die Erhaltung der Art."
M. Nussbaum.
Arch, Mik. Anat,, XVIIL, p. 112, 1880.
The germ from which every living form arises is a single cell, derived 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 distinguished from one another. These are, to use Weismann's terms : (i) the somatic cells, which are differentiated into various tissues by which the functions of individual life are performed and which collectively form the " body," and (2) the germ-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 distinction 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 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 nourished by somatic cells specially set apart for that purpose and forming distinct sexual organs, the ovaries and spennaries or their equivalents. Within these organs the germ-cells are carried, protected, and nourished ; 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 indistinguishable from the surrounding somatic cells. As development proceeds, they are first differentiated from the somatic cells and then diverge very widely in the two sexes, undergoing remarkable transformations 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 {yolk or deutoplasm\ and in many cases becomes surrounded by membranes or other envelopes for the protection 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 necessary for these movements is set free. 1 When finally matured, therefore, 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.
A. The Ovum
The animal egg (Figs. 58-59) 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 micropyle, through which the spermatozoon enters (Fig. 63). It contains an enormous nucleus known as the germinal vesiele, within which is a very conspicuous nucleolus known to the earlier observers as the gertninal spot. In many eggs the latter is single, but in other forms many nucleoli are present, and they are sometimes of more than one kind, as in tissue-cells. 1 In many forms no centrosome or attraction-sphere is found in the egg until the initial stages in the formation of the polar bodies, though Mertens ('93) describes a centrosome and attraction-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 Aulostomum 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. Iji 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, Ruckert), still others that it persists in the cytoplasm hidden among the granules. In any case it is again lost to view after formation of the polar bodies, to be replaced by the cleavage-centrosomes which arise in connection with the spermatozoon (p. 1 87).
1 The metabolic contrast between the germ-cells has been fully discussed in a most suggestive 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 characteristic of the sexes in general.
The egg-cytoplasm almost always contains a certain amount of nutritive matter, the yolk or deutoplasm, 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.
1. The Nucleus
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 egg 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 egg it may become irregular or even amoeboid (Fig. jy\ and, as Korschelt has shown in the case of insect-eggs, may move through the cytoplasm toward the source of food. Its structure is 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 chromatin forms a beautiful and regular reticulum consisting of numerous 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-chromosomes of the preceding division (copepods, selachians, Amphibia), 1
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 (co^lenterates, echinoderms, many annelids, and some copepods), while it is multiple in large eggs heavily laden with deutoplasm (lower vertebrates, insects, many Crustacea).
Fig. 58. — Ovarian egg of Ihe sea-urchin, ToxopniMtts (X750). g.v. Nucleus or germinal vesicle, containing an irregular discontinuous network of chromatin ; gj. nucleolus or germinal spot, intensely stained with hematoxylin. The naked cell-body conlists of a very regular alveolar meshwork, scattered through which are numerous minute granule* or microsomes. (Cf. Figs. 11. la.) Below, at 1, is an entire spermatoio6n shown at the same enlargement (both middle-piece and flagellum 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 egg. 8 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 eggs of most animals. As a rule, however, the number of nucleoli increases 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. 50. — Ovum of (he cal, wi
hin the ovary, directly reproduced from a photograph of
preparation by DaHLGKEN. [Enl
rged S3S diameters.] The ovum lies in the Graafian (bilk
within the discus prohgtrus, the
atler forming the immediate follicular investment (torei
radiala) ol the egg. Within the
arena is the clear tana pillucida or egg-membrane. (C
Fig. oa-)
{Hauptnucleolus) and "accessory nucleoli" {Nebennucleolt}. These differ widely in staining-reaction ; but it does not yet clearly appear whether they definitely correspond to the plasmosomes and karyosomes 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-nerwork, 1 and in some cases at least it does not contribute to the formation of chromosomes. It cannot therefore be directly compared to the net-knots or karyosomes 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 are often multiple, and as a rule they arise secondarily during the growth of the egg (Fig. 61). 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 egg 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 brevicomis, 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.
1 Cf. List, '96, Montgomery, '98, 2, and Obst., '99.
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 in situ at the time the polar bodies are formed. It is, for example, cast out bodily in the medusa Aiquorea (Hacker) and in various annelids and echinoderms, 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 independently of the nucleoli (Fig. 125), which degenerate in situ when the membrane of the germinal vesicle disappears. In such cases it seems quite certain that the nucleoli do not contribute to the formation 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 either 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
Fig. &>.— Eggs of the annelid Ntrtis, before and after fertilization, X 400 (for intermediate stages see Fig. 95).
A. Before fertiljiation. The large germinal vesicle occupies a nearly central position. It eontains a network of chromatin in which are seen five small darker bodies; these are the quadruple chromosome-groups, or tetrads, in process ol formation (not all of (hem are shown) ; these alone persist in later stages, the principal mass of the network being lost; gj. double germinal spot, consisting of achromatic and an achromatic sphere. This egg is heavily laden with yolk, in the torn of clear demoplasm-sphcres (d) and fat-drops (/). uniformly distributed through Ihe cytoplasm. The peripheral layer of cytoplasm (peri -vitelline layer) is free from deutoplasm. Outside this the membrane. B, The egg some time after fertilisation and about to divide. The deutoplasm is now concentrated in the lower hemisphere, and the peri -vitelline layer has disappeared. Above are the two polar bodies (pi.). Below them lies the mitotic figure, 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- Si. — Germinal vesicles of growing ovarian eggs ol the lamellibcanch, Unto {A-D), and the spider, Efieira [E-fi). [OUST.]
A. Youngest stage with single (principal) nucleolus. B. Older egg. showing accessory nucleolus allachcd to the principal. C. The two nucleoli separated. D. Much older stage, showing the two nucleoli united. E. Germinal vesicle of Epeira, showing one accessory nucleolus at. tached to the principal, and one free. F. 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 egg (Figs. 97, 128).
2. The Cytoplasm
The egg-cytoplasm varies greatly in appearance with the variations of the deutoplasm. In such eggs as those of the echinoderm (Fig. 58), which have little or no deutoplasm, the cytoplasm forms a regular meshwork, which is in this case an undoubted alveolar structure, 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 egg (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 Biitschli. 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 peripheral 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 protoplasmic germinal disc is then formed; for example, in many fish-eggs.
The character of the yolk varies so widely that it can here be considered only in very general terms. The deutoplasm-bodies are commonly spherical, but often show a more or less distinctly rhomboidal or crystalloid form as in Amphibia and some fishes, and in such cases they may sometimes be split up into parallel lamellae known as yolkplates. 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 egg is thus shown to consist largely of proteids, nucleo-albumins, and a variety of related substances which are often associated with fatty substances and small quantities of carbohydrates (glucose, etc.). In some cases the deutoplasm-spheres stain intensely with nuclear dyes, such as hematoxylin ; e.g. 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 single 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 egg 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 egg may lie in any position. When fat-drops are present they usually lie in the vegetative hemisphere, and since they are lighter than the other constituents they usually cause the egg to lie with fe% the animal pole turned downwards, as
is the case with some annelids {Nereis) and many pelagic fish-eggs.
3. The Egg-envelopes
The egg-envelopes fall under three categories. These are : —
(a) The vitelline membrane, secreted by the ovum itself.
(b) The chorion, formed outside the ovum by 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 belongs to the ovum, the second and third being purely maternal products. There are some eggs, such as those of certain ccelenterates (e.g. Rcnilla), that are naked throughout their whole development. In many others, of which the sea-urchin is a type, the fresh-laid egg is naked but forms a vitelline membrane almost instantaneously after the spermatozoon touches it. 1 In other forms (insects, birds) the vitelline membrane may be present before fertilization, and in such cases the egg 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). 3
' That the vitelline membrane doe* not preexist seems to be established by the fact that egg fragments likewise surround themselves with a membrane when fertilized. [Hertwig.]
1 In tome cases, according to Wheeler, the insect-egg has only a chorion, (he vitelline membrane being absent.
Fig. 62. — Schematic figure of a median longitudinal section of theegg of a fly (A/UCd), showing axes of the bilateral egg and the membranes. [From KORSCHELT and HEIDF.K, after HK.NK1NG and Bl.OCHMANN.]
ui. The germ-nuclei uniting ; m. micropyle; p.i. the polar bodies. The flat side ol the egg is the dorsal, the convex side the ventral, and the micropyle is al the anterior end. The deutoplasm (small circles) lies in the centre surrounded by a peripheral or peri-vile lline layer ol protoplasm. The outer heavy line is Ihe chorion, the inner lighter line the vitelline membrane, both being perforated by the micropyle. from which exudes a mass of jelly-like substance.
The accessory envelopes are too varied to be more than touched upon here. They include 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 earthworms.
When the egg is surrounded by a membrane before fertilization it is often perforated by one or more openings known as micropyles, through which the spermatozoa make their entrance (Figs. 62, 63). Where there is but one micropyle,
Fig. 63. — Upper pole of the egg of Argonauta. [USSOW.]
The egg is surrounded by a very thick membrane, perforated at m by the funnelshaped micropyle; below the latter lies the egg-nucleus in the peri-vitelline layer of protoplasm ; p.b. the polar bodies.
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 micropylcs near the upper pole of the egg, and perhaps correlated with this is the fact that several spermatozoa enter the egg t though only one is concerned with the actual process of fertilization.
The plant-ovum, which is usually known as the oospJicre (Figs. 64, 107), shows the same general features as that of animals, being a relatively large, quiescent, rounded cell containing a large nucleus. It never, however, attains the dimensions or the complexity of structure shown in many animal eggs, since it always remains attached to the maternal structures, by which it is provided with food and invested with protective envelopes. It is therefore naked, as a rule, and is not heavily laden with reserve food-matters such as the deutoplasm of animal ova. A vitelline membrane is, however, often formed soon after fertilization, as in echinoderms. The most interesting feature of the plant-ovum is the fact that it often contains plastids (leucoplasts or chromatophores) which, by their division, give rise to those of the embryonic cells. These sometimes have the form of typical chromatophores containing pyrenoids, as in Volvox and many other Algae (Fig. 64). In the higher forms (archegoniate plants), according to the researches of Schmitz and Schimper, the egg contains numerous minute colourless "leucoplasts," which afterward develop into green chromatophores or into the starch-building amyloplasts. This is a point of great theoretical interest ; for the researches of Schmitz, Schimper, and others have rendered it highly probable that these plastids are persistent morphological bodies that arise only by the division of preexisting bodies of the same kind, and hence may be traced continuously from one generation to another through the
Pig. 6+. — Germ-cells of Vahiex. [Overton.] A. Ovum (oosphere) containing a large central nucleus and a peripheral layer of chromatcphores. /. pyrcnoid. B. Spermatozoid ; c.v. contractile vacuoles; e. "eye-spot" (chromoplastid) ; f. pyrenoid. C. Spermatoioid stained to show the nucleus ().
germ-cells. In the lower plants (Algas) they may occur in both germcells ; in the higher forms they are found in the female alone, and in such cases the plastids of the embryonic body are of purely maternal origin.
B. The Spermatozoon
Although spermatozoa were among the first of animal cells observed by the microscope, their real nature was not determined for more than two hundred years after their discovery. Our modern knowledge of the subject may be dated from the year 1841, when Kolliker proved that they were not parasitic animalcules, as the early observers supposed, but the products of cells preexisting in the parent body. Kolliker, however, did not identify them as cells, but believed them to be of purely nuclear origin. We owe to SchweiggerSeidel and La Valette St. George the proof, simultaneously brought forward by these authors in 1865, 1 that the spermatozoon is a complete cell, consisting of nucleus and cytoplasm, and hence of the same morphological nature as the ovum. It is of extraordinary minuteness, being in many cases less than -rjrAin^ * ne bulk of the ovum. a
1 Arch. Mik. Ana/., I. '65.
In the sea-urchin. Toxofntust/s, I estimate its hulk as being between usVuTl and rnAnnr the volume of the ovum. The inequality is in many cases very much greater.
Its precise study is therefore difficult, and it is not surprising that our knowledge of its structure and origin is still far from complete.
Apical body or acrosome.
Nucleus.
End-knob.
Middle-piece.
Envelope of the tail.
.Axial filament
1. Flagellate Spermatozoa
In its more usual form the animal spermatozoon resembles a minute, elongated tadpole, which swims very actively about by the vibrations of a long, slender tail morphologically comparable with a single cilium or flagellum. Such a spermatozoon consists typically of four parts, as shown in
Fig. 65: —
1. The nucleus, which forms the main portion of the "head," and consists of a very dense and usually homogeneous mass of chromatin staining with great intensity with the so-called "nuclear dyes" {e.g. hematoxylin or the basic tar-colours such as methyl-green). It is surrounded by a very thin cytoplasmic envelope.
2. An apical body, or acrosome, lying at the front end of the head, sometimes very minute, sometimes almost as large as the nucleus, and in some cases terminating in a sharp spur by means of which the spermatozoon bores its way into the ovum.
3. The middle-piece, or connecting piece, a larger cytoplasmic body lying behind the head and giving attachment to the tail, from which it is not always distinctly marked off. This body shows the same staining-reactions as the acrosome, having an especial affinity for " plasmastains" (acid fuchsin* etc.). At its front end it is in some forms (mammals) separated from the nucleus by a short clear region, the neck. Like the acrosome, the middle-piece is in some cases derived from an " archoplasmic " mass, representing an attraction-sphere (Lumbricus) or a portion of the Nebenkern (insects), and it contains, or according to some authors actually arises from, the centrosome (salamander, mammals, insects, etc.).
4. The tail, or flagellum, in part, at least, a cytoplasmic product developed in connection with the centrosome and " archoplasm "
Fig. 6$. — Diagram of the flagellate spermatozoon.
(attraction-sphere or " Nebenkern") of the mother-cell. It consists of a fibrillated axial filament surrounded by a cytoplasmic envelope, and in certain cases (Amphibia) bears on one side a fin-like undulating membrane (Fig. 66). Toward the tip of the flagellum the envelope suddenly disappears or becomes very thin, leaving a short end-piece which by some authors is considered to consist of the naked axial filament. The axial filament may be traced through the middle-piece up to the head, at the base of which it usually terminates in a minute body, single or double, known as the end-knob. Recent research has proved that the axial filament grows out from the spermatid-centrosome, the latter in some cases persisting as the end-knob (insects, mollusks, mammals), in other cases apparently enlarging to form the main body of the middle-piece (salamander). The tail-envelopes, on the other hand, arise either from the " archoplasm " of the Nebenkern (insects) together with a small amount of unmodified cytoplasm, or from the latter alone (salamander, rat).
Fig. (56. — Spermatozoa of fishes and Amphibia. [Ballowitz.]
A. Sturgeon. //. Pike. C. D. teuciscus. E. Triton (anterior part). F. Triton (posterior part of flagellum). G. Raja (anterior part), a. apical body; e. end-piece; f. flagellum; k. endknob ; m. middle-piece ; n. nucleus ; s. apical spur.
From a physiological point of view we may arrange the parts of the spermatozoon under two categories as follows : —
1. The essential structures which play a direct part in fertilization. These are : —
(a) The nucleus, which contains the chromatin.
(b) The middle-piece, which either contains a formed centrosome
or pair of centrosomes (end-knob), or is itself a metamorphosed centrosome. This is probably to be regarded as the fertilizing element par excellence, since there is reason to believe that when introduced into the egg it gives the stimulus to division. 2. The accessory structures, which play no direct part in fertilization, viz. : —
(a) The apex or spur, by which the spermatozoon attaches itself
to the egg or bores its way into it, and which also serves for the attachment of the spermatozoon to the nurse-cells or supporting cells of the testis.
(b) The tail, a locomotor organ which carries the nucleus and
centrosome, and, as it were, deposits them in the egg at the time of fertilization. There can be little doubt that the substance of the flagellum is contractile, and that its movements are of the same nature as those of ordinary cilia. Ballowitz's discovery of its fibrillated structure is therefore of great interest, as indicating its structural as well as physiological similarity to a muscle-fibre. The outgrowth of the axial filament from the centrosome is probably comparable to the formation of spindle-fibres or astral rays, a conclusion of especial interest in its relation to Van Beneden's theory of mitosis (p. 100). Tailed spermatozoa conforming more or less nearly to the type just described are with few exceptions found throughout the Metazoa from the ccelenterates up to man ; but they show a most surprising diversity in form and structure in different groups of animals, and the homologies between the different forms have not yet been fully determined. The simpler forms, for example, those of echinoderms and some of the fishes (Figs. 66 and 100), conform very nearly to the foregoing description. Every part of the spermatozoon may, however, vary more or less widely from it (Figs. 66-68). The head (nucleus) may be spherical, lance-shaped, rod-shaped, spirally twisted, hook-shaped, hood-shaped, or drawn out into a long filament; and it is often divided into an anterior and a posterior piece of different staining-capacity, as is the case with many birds and mammals, but it is probable that the anterior of these may represent the acrosome. An interesting form of head is described by Wheeler ('97) in the spermatozoon of Myzostoma, where it is a greatly elongated fusiform body, passing insensibly into the tail without distinct rtriddlepiece and containing a single series of chromatin-discs. The number of these in M. glabrum is 24, which is the somatic number of chromosomes in this species. In M. cirrifenim the number of chromatin-discs is more than 60. Somewhat similar spermatozoa occur in the accelous Turbellaria. 1 The acrosome sometimes appears to be wanting, e.g. in some fishes (Fig. 66). When present, it is sometimes a minute rounded knob, sometimes a sharp stylet, and in some cases terminates in a sharp barbed spur by which the spermatozoon appears to penetrate the ovum {Triton). In the mammals it is sometimes very small (rat), sometimes very large (guinea-pig), and in some forms is surrounded by a cytoplasmic layer forming the " head-cap " (Figs. 68, 86). It is sometimes divided into two distinct parts, a longer posterior piece and a knob-like anterior piece (insects, according to Ballowitz).
Fig. 67. — Spermatozoa of various animals. \A-I % L, from Ballowitz ; J t K, from von Brunn.]
A (At the left). Beetle (Ccpris), partly macerated to show structure of flagellum; it consists of a supporting fibre (s.f.) and a fin-like envelope (/) ; n. nucleus; a. a. apical body divided into two parts (the posterior of these is perhaps a part of the nucleus). B. Insect (Calathus), with barbed head and fin-membrane. C. Bird {Phyllopneustc). D. Bird {Muscicapa), showing spiral structure ; nucleus divided into two parts (h 1 , » 2 ) ; no distinct middle-piece. E. Bulfinch ; spiral membrane of head. F. Gull (Larus) with spiral middle-piece and apical knob. G. H. Giant spermatozoon and ordinary form of Tadorna. /. Ordinary form of the same stained, showing apex, nucleus, middle-piece and flagellum. J. " Vermiform spermatozoon " and, A', ordinary spermatozoon of the snail Paludirta. L. Snake {Coluber), showing apical body (a), nucleus, greatly elongated middle-piece (m), and flagellum (/).
The middle-piece or connecting-piece shows a like diversity (Figs. 66-68). In many cases it is sharply differentiated from the flagellum, being sometimes nearly spherical, sometimes flattened like a cap against the nucleus, and sometimes forming a short cylinder of the same diameter as the nucleus, and hardly distinguishable from the latter until after staining (newt, earthworm). In other cases it is very long (reptiles, some mammals), and is scarcely distinguishable from the flagellum. In still others (birds, some mammals) it passes insensibly into the flagellum, and no sharply marked limit between them can be seen. In many of the mammals the long connectingpiece is separated from the head by a narrow " neck " in which the end-knobs' lie, as described below.
Internally, the middle-piece consists of an axial filament and an envelope, both of which are continuous with those of the flagellum. In some cases the envelope shows a distinctly spiral structure, like that of the tail-envelope ; but this is not always visible. The most interesting part of the middle-piece is the " end-knob " in which the axial filament terminates, at the base of the nucleus. In some cases this appears to be single. More commonly it consists of two or more minute bodies lying side by side (Fig. 68, B t D).
The flagellum or tail is merely a locomotor organ which plays no part in fertilization. It is, however, the most complex part of the spermatozoon, and shows a very great diversity in structure. Its most characteristic feature is the axial filament, which, as Ballowitz has shown, is composed of a large number of parallel fibrillae, like a muscle-fibre. This is surrounded by a cytoplasmic envelope, which sometimes shows a striated or spiral structure, and in which, or in connection with which, may be developed secondary or accessory filaments* and other structures. At the tip the axial filament may lose its envelope and thus give rise to the so-called "end-piece" (Retzius). In Triton, for example (Fig. 66, F), the envelope of the axial filament (*' principal filament ") gives attachment to a remarkable fin-like membrane, having a frilled or undulating free margin along which is developed a "marginal filament." Toward the tip of the tail the fin, and finally the entire envelope, disappears, leaving only the axial filament to form the endpiece. After maceration the envelope shows a conspicuous cross-striation, which perhaps indicates a spiral structure such as occurs in the mammals. The marginal filament, on the other hand, breaks up into numerous parallel nbrillae, while the axial filament remains unaltered ( Ballowitz).
1 Cf. Wheeler, p. 7.
A fin-membrane has also been observed in some insects and fishes, and has been asserted to occur in mammals (man included). Later observers have, however, failed to find the fin in mammals, and their observations indicate that the axial filament is merely surrounded by an envelope which sometimes [A-Fbam shows traces of the same ir M>1 . spiral arrangement as that and which is so conspicuous in the connecting- piece. In the
A. Badger (living). B. The same after C. Bai (t'tjperitgQ). D. The same, flage middle-piece or connecting-piece. Showing el
J£. Head of the spermatoioiin of Ihe bai \nnino- , ., . _.
hphiu) snowing details. /•. Hod of ipernuMozoon Skate the tail has two filaoflhepig. a. Opossum (after staining). H. Double mentS, both Composed of
TZ?™ "° m ' he ^ J ' /er "" ° f "" ° POSSl " n " P arallel fibrilllE . connected by
h.c. head-cap (acrosome) ; *. end-knob: m. mid- a membrane and spirally
SfiTiir*" (i " * E - F ' C ° nsiS,inE °" W ° twisted about each other; a
different parts;. *
somewhat similar structure occurs in the toad. In some beetles there is a fin-membrane attached to a stiff axial "supporting fibre" (Fig. 67, A). The membrane itself is here composed of four parallel fibres, which differ entirely from the supporting fibre in staining-capacity and in the fact that each of them may be further resolved into a large number of more elementary fibrilla;.
A. B. C. Living amceboid spermatozoa of the ci D. £. Spermatozoa of crab, Dromia. F. Of Eth»i. I. Spermatozoon of lobster. Hmana. [Herrick J. Spermatozoon of crab, Porctllaxa. [GroBBEN.
Many interesting details have necessarily been passed over in the foregoing account. One of these is the occurrence, in some mammals, birds, Amphibia (frog), and mollusks, of two kinds of spermatozoa in the same animal. In the birds and Amphibia the spermatozoa are of two sizes, but of the same form, the larger being known as "giant spermatozoa" (Fig. 67, G, H). In the gasteropod Palndina the two kinds differ entirely in structure, the smaller form being of the usual type and not unlike those of birds, while the larger, or "vermiform," spermatozoa have a worm-like shape and bear a tuft of cilia at one end, somewhat like the spermatozoids of plants (Fig. 67, % A"). In this case only the smaller spermatozoa are functional (von Brunn).
No less remarkable is the conjugation of spermatozoa in pairs (Fig. 68,//), which takes place in the va: defcrctts in the opossum (Selenka) and in some insects (liallowitz, Auerbach). Ballowitz's researches ('95) on the double spermatozoa of beetles (Dytueida) prove that the union is not primary, but is the result of an actual conjugation of previously separate spermatozoa. Not merely two, but three or more spermatozoa may thus unite to form a " spermatozeugma," which swims like a single spermatozoon. Whether the spermatozoa of such a group separate before fertilization is unknown ; but Ballowitz has found the groups, after copulation, in the female receptaculum, and he believes that they may enter the egg in this form. The physiological meaning of the process is unknown.
2. Other Forms of Spermatozoa
The principal deviations from the flagellate type of spermatozoon occur among the arthropods and nematodes (Fig. 69). In many of these forms the spermatozoa have no flagellum, and in some cases they are actively amoeboid; for example, in the daphnid Polyphemus (Fig. 69, A, B, C) as described by Leydig and Zacharias. More commonly they are motionless like the ovum. In the chilognathous myriapods the spermatozoon has sometimes the form of a bi-convcx lens (Polydesmus), sometimes the form of a hat or helmet having a double brim (Julus). In the latter case the nucleus is a solid disc at the base of the hat In many decapod Crustacea the spermatozoon consists of a cylindrical or conical body from one end of which radiate a number of stiff spine- like processes. The nucleus lies near the base. In none of these cases has the centrosome been identified.
3. Paternal Germ-eel Is of
Plants In most of the flowering plants the male germ-cells are represented by two " generative nuclei," lying at the tip of the pollen tube (Fig. 106). On the other hand, in the cycads( Figs. 87, 108) and in a large number of the lower plants (pteridoB. Later phytes, Muscineae, and many others), the male germ-cell is a minute actively swimming cell, known as the spermatozoid, which is closely analogous to the spermatozoon. The spermatozoids are in general less highly differentiated than spermatozoa, and often show a distinct resemblance to the
Fig. 70. — Spermatotoids of JEFF.]
A. Mother-cells with reticular 1 stage, with spermatotoids funning. C. Mature spermatozoid (the elongate nucleus black).
asexual swarmers or zoospores so common in the lower plants (Figs. 70, ji). They differ in two respects from animal spermatozoa: first in possessing not one but two or several flagella ; second, in the fact that these are attached as a rule not to the end of the cell, but on the side. In the lower forms plastids are present in the form of chromatophores, one of which may be differentiated into a red " eye-spot," as in Volvox and Fuats (Figs. 57, 71, A\ and they may even contain contractile vacuoles ( Volvox) ; but both these structures are wanting in the higher forms. These consist only of a nucleus with a very small amount of cytoplasm, and have typically a spiral form. In Chara, where their structure and development have recently been carefully studied by Belajeff, the spermatozoids have an elongated spiral form with two long flagella attached near the pointed end, which is directed forward in swimming (Fig. 70). The main body of the spermatozoid is occupied by a dense, apparently homogeneous nucleus surrounded by a very delicate layer of cytoplasm. Behind the nucleus lies a granular mass of cytoplasm, forming one end of the cell, while in front is a slender cytoplasmic tip to which the flagella are attached. Nearly similar spermatozoids occur in the liverworts and mosses. In the ferns and other pteridophytes a somewhat different type occurs
4. Of an alga (Fvcm) ; a red cbrc-matophore al Ihe right ie nucleus. B. Liverwort [fillia). C. Moss (Sptegnum) . Manilla. E. Vera, {Aitgiifteris). f. Fern, J-htgopltrii
nucleus dark). (Cf. Figs. 3?, 88.)
144 THE GERM-CELLS
(Figs. 71, 88). Here the spcrmatozoid is twisted into a conical spiral and bears numerous cilia attached along the upper turns of the spire. The nucleus occupies the lower turns, and attached to them is a large spheroidal cytoplasmic mass, which is cast off when the spermatozoid is set free or at the time it enters the archegonium. This, according to Strasburger, probably corresponds to the basal cytoplasmic mass of Chara. The upper portion of the spire to which the cilia are attached is composed of cytoplasm alone, as in Chara. Ciliated spermatozoids, nearly similar in type to those of the higher cryptogams, have recently been discovered in the cycads by Hirase (Gingko\ Ikeno (Gycas), and Webber (Zatnia). They are here hemispherical or pear-shaped bodies of relatively huge size (in Zamia upward of 250 p in length), with a large nucleus filling most of the cell and a spiral band of cilia making from two to six turns about the smaller end (Figs. 87, 108).
As will be shown farther on (p. 173), the "anterior" cytoplasmic region of the spermatozoid, to which the cilia are attached, is probably the analogue of the middle-piece of the animal spermatozoon ; and the work of Belajeff, Strasburger, Ikeno, Hirase, Webber, and Shaw gives good ground for the conclusion that it has an essentially similar mode of origin, though we are still unable to say exactly how far the comparison can be carried. The " posterior " region of the spermatozoid appears to correspond, broadly speaking, to the acrosome.
C. Origin of the Germ-cells
Both ova and spermatozoa take their origin from cells known as primordial germ-cells, which become clearly distinguishable from the somatic cells at an early period of development, and are at first exactly alike in the two sexes. What determines their subsequent sexual differentiation is unknown save in a few special cases. From such data as we possess, there is very strong reason to believe that, with a few exceptions, the primordial germ-cells are sexually indifferent, i.e. neither male nor female, and that their transformation into ova or spermatozoa is not due to an inherent predisposition, but is a reaction to external stimulus. Most of the observations thus far made indicate that this stimulus is given by the character of the food, and that the determination of sex is therefore in the last analysis a problem of nutrition. Thus Mrs. Treat (*73) found that if caterpillars were starved before entering the chrysalis state they gave rise to a preponderance of male imagoes, while conversely those of the same brood that were highly fed produced an excess of females. Yung ('8 1 ) reached the same result in the case of Amphibia, highly fed tadpoles producing a great excess of females (in some cases as high as g:>%) and underfed ones an excess of males. The same result, again, is
ORIGIN OF THE GERM-CELLS 145
given by the interesting experiments of Nussbaum ('97) on the rotifer Hydatina, which is an especially favourable case since sex is here determined at a very early period, before the egg is laid, the eggs being of two sizes, of which the smaller give rise only to males, and the larger only to females. The earlier experiments of Maupas ('91) on this form seemed to show conclusively that the decisive factor was temperature acting on the parent organism, since in a high temperature an excess of females produced male eggs, and in a low temperature the reverse result ensued. Nussbaum shows, however, that this is not a direct effect of temperature, but an indirect one due to the greater birth-rate and the greater activity of the animals under a higher temperature, which result in a speedier exhaustion of food. Direct experiment shows that, under equal temperature-conditions, well-fed females produce a preponderance of female offspring, and vice versa, precisely as in the Lepidoptera and Amphibia. These cases show that sex may be determined by conditions of nutrition either affecting the embryo itself (Lepidoptera, Amphibia) long after the egg is laid, or by similar conditions affecting the parent-organism and through it the ovarian egg.
Nutrition is, however, not the only determining cause of sex, as is shown by the long-known case of the honey-bee. Here sex is determined by fertilization, the males arising only from unfertilized eggs by parthenogenesis, while the fertilized eggs give rise exclusively to females, which develop into fertile forms (queens) or sterile forms (workers), according to the nature of the food. This is a very exceptional case, yet here too it is the more highly fed larvae that produce fertile females. It is interesting to compare with this case that of the plant-lice or aphides. In these forms the summer broods, living under favourable conditions of nutrition, produce only females the eggs of which develop parthenogenetically. In the autumn, under less favourable conditions, males as well as females are produced ; and that this is due to the external conditions and not to a fixed cyclical change of the organism is proved by the fact that in the favourable environment of a greenhouse the production of females alone may continue for years. 1
We are not yet able to state whether there is any one causal element common to all known cases of sex-determination. The observations cited above, as well as a multitude of others that cannot here be reviewed, render it certain, however, that sex as such is not inherited. What is inherited is the capacity to develop into either male or female, the actual result being determined by the combined effect of conditions external to the primordial germ-cell.
1 See Geddes, Sex, in Encyclopedia Britannica ; Geddes and Thompson, The Evolution of Sex, 1889; Brooks, The Law of Heredity, 1883; Watase ('92), The Phenomena of Sex-differentiation.
146 THE GERM-CELLS
In the greater number of cases the primordial germ-cells arise in a germinal epithelium which, in the coelenterates (Fig. 72), may be a part of either the ectoderm or entoderm, and, in the higher types, is a modified region of the peritoneal epithelium lining the body-cavity. In such cases the primordial germ-cells may be scarcely distinguishable at first from the somatic cells of the epithelium. But in other cases the germ-cells may be traced much farther back in the development, and they or their progenitors may sometimes be identified in the gastrula or blastula stage, or even in the early cleavage-stages Thus in the worm Sagitta, Hertwig has traced the germ-cells back to
B
Fig. 71. — Origin of (he germ-cells in a hydro-medusa, Oadonima. [WF.1SMANN.] A. Young stage ; section through wall of manubrium of Ihe medusa ; ova developing in Ihe ectoderm (tc). B, Laler stage, showing older ova (a) and "nutritive cells" (ir). The ova contain small nuclei prohably derived from engulfed nutritive cells.
two primordial germ-cells lying at the apex of the archenteron. In some of the insects they appear still earlier as the products of a large "pole-cell" lying at one end of the segmenting ovum, which divides into two and finally gives rise to two symmetrical groups of germcells. Hacker has recently traced very carefully the origin of the primordial germ-cells in Cyclops from a "stem-cell " (Fig. 74) clearly distinguishable from surrounding cells in the early blastula stage, not only by its size, but also by its large nuclei rich in chromatin, and by its peculiar mode of mitosis, as described beyond.
The most beautiful and remarkable known case of early differentiation of the germ-cells is that of Ascaris, where Boveri was able to trace them back continuously thiough all the cleavage-stages to the
ORIGIN OF THE GERM-CELLS
14?
two-cell stage ! Moreover, from the outset the progenitor of the germcells differs from the somatic cells not only in the greater size and richness of chromatin of its nuclei, but also in its mode of mitosis; for in all those blastomeres destined to produce somatic cells a portion of
Pig. 73. — Origin of the primordial germ-cells and cells of Aicaris. [BOVER1.]
A. Two-cell stage dividing; 1, stem-cell, from which the side, later in the second cleavage, showing the twt chromatin (c) in the somatic cell. C. Resulting 4-cf D. The third cleavage, repeating the foregoing process i
irisethe germ-cells. B. Th types of mitosis and the o I singe; the eliminated chi
the chromatin is cast out into the cytoplasm, where it degenerates, and only in the germ-cells is the sum-total of the chromatin retained. In Ascaris megalocephala univalens the process is as follows (Fig. 73): Each of the first two cells receives two elongated chromosomes. As
148 THE GERM-CELLS
the ovum prepares for the second cleavage, the two chromosomes reappear in each, but differ in their behaviour (Fig. 73, A, £). In one of them, which is destined to produce only somatic cells, the thickened ends of each chromosome are cast off into the cytoplasm and degenerate. Only the thinner central part is retained and distributed to the daughter-cells, breaking up meanwhile into a large number of segments which split lengthwise in the usual manner. In the other cell, which may be called the stem-cell (Fig. 73, s), all the chromatin is preserved and the chromosomes do not segment into smaller pieces. The results are plainly apparent in the four-cell stage, the two somatic nuclei, which contain the reduced amount of chromatin, being small and pale, while those of the two stem-cells are far larger and richer in chromatin (Fig. 73, C). At the ensuing division (Fig. 73, D) the numerous minute segments reappear in the two somatic cells, divide, and are distributed like ordinary chromosomes ; and the same is true of all their descendants thenceforward. The other two cells (containing the large nuclei) exactly repeat the history of the two-cell stage, the two long chromosomes reappearing in each of them, becoming segmented and casting off their ends in one, but remaining intact in the other, which gives rise to two cells with large nuclei as before. This process is repeated five times (Boveri) or six (Zur Strassen), after which the chromatinelimination ceases, and the two stem-cells or primordial germ-cells thenceforward give rise only to other germ-cells and the entire chromatin is preserved. Through this remarkable process it comes to pass that in this animal only the germ-cells receive the sumtotal of the egg-chromatin handed down from the parent. All of the somatic cells contain only a portion of the original germ-substance. " The original nuclear constitution of the fertilized egg is transmitted, as if by a law of primogeniture, only to one daughter-cell, and by this again to one, and so on ; while in the other daughter-cells the chromatin in part degenerates, in part is transformed, so that all of the descendants of these side-branches receive small reduced nuclei/' *
It would be difficult to overestimate the importance of this discovery ; for although it stands at present an almost isolated case, yet it gives us, as I believe, the key to a true theory of differentiation development, 2 and may in the end prove the means of explaining many phenomena that are now among the unsolved riddles of the cell.
Hacker ('95) has shown that the nuclear changes in the stemcells and primordial eggs of Cyclops show some analogy to those of Ascaris, though no casting out of chromatin occurs. The nuclei are very large and rich in chromatin as compared with the somatic cells, and the number of chromosomes, though not precisely determined,
1 Boveri, '91, p. 437. 2 Cf. p. 426.
ORIGIN OF THE GERM-CELLS
1 49
is less than in the somatic cells (Fig. 74). Vom Rath, working in the same direction, believes that in the salamander also the number of chromosomes in the early progenitors of the germ-cells is one-half that characteristic of the somatic cells. 1 In both these cases, the chromosomes are doubtless bivalent, representing two
A. Young embryo, showing stem-cell (il). B. The stem-cell has divided inro f rise to the primordial germ-cell (g). C. Later stage, in section ; the primordial gen migrated into the interior and divided into two ; two groups of chromosomes in each.
chromosomes joined together. In Ascaris, in like manner, each of the two chromosomes of the stem-cell or primordial germ-cells is probably plurivalcnt, and represents a combination of several units of a lower order which separate during the segmentation of the thread when the somatic mitosis occurs.
1 C/.p. 256, Chapter V.
150 THE GERM-CELLS
D. Growth and Differentiation of the Germ-cells
I. The Ovum
(a) Growth and Nutrition. — Aside from the transformations of the nucleus, which are considered elsewhere, the story of the ovarian history of the egg is largely a record of the changes involved in nutrition and the storage of material. As the primordial germ-cells enlarge to form the mother-cells of the eggs, they almost invariably become intimately associated with neighbouring cells which not only support and protect them, but also serve as a means for the elaboration of food for the growing egg-cell. One of the simplest arrangements is that occurring in coelenterates, where the egg lies loose either in one of the general layers or in a mass of germinal tissue, and may crawl actively about among the surrounding cells like an Amceba. In such cases (hydroids) the egg may actually feed upon the surrounding cells, taking them bodily into its substance or fusing with them 1 and assimilating their substance with its own. In such cases (Tubularia, Hydra) the nuclei of the food-cells long persist in the egg-cytoplasm, forming the so-called " pseudo-cells," but finally degenerate and are absorbed by the egg. It would here seem as if a struggle for existence took place among the young ovarian cells, the victorious individuals persisting as the eggs; and this view is probably applicable also to the more usual case where the egg is only indirectly nourished by its brethren.
In most cases, as ovarian development proceeds, a definite association is established between the egg and the surrounding cells. In one of the most frequent arrangements the ovarian cells form a regular layer or follicle about the ovum (Figs. 59, 79), and there is very strong reason to believe that the follicle-cells are immediately concerned with the conveyance of nutriment to the ovum. A number of observers have maintained that the follicle-cells may actually migrate into the interior of the egg, and this seems to be definitely established in the case of the tunicates and mollusks (Fig. 75). 2 Such cases are, however, extremely rare ; and, as a rule, the material elaborated by the nutritive cells is passed into the egg either in solution or in the form of granular or protoplasmic substance. 8 An interesting case of this kind occurs in the cycads, where, according to Ikcno C98), the egg-cell is connected with the surrounding cells by broad protoplasmic bridges through which cytoplasmic material flows directly into the egg-cell.
Very curious and suggestive conditions occur among the annelids and insects. In the annelids the nutritive cells often do not form
1 Cf. Doflein, '97. " See Floderus, '95, and Obst, '99. 8 C/p. 349
GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 151
a follicle, but in some forms each egg is accompanied by a single nurse-cell, attached to its side, with which it floats free in the bodycavity. In Ophryotrocha, where it has been carefully described by Korschelt, the nurse-cell is at first much larger than the egg itself, and contains a large, irregular nucleus, rich in chromatin (Fig. 76). The egg-cell rapidly grows, apparently at the expense of the nursecell, which becomes reduced to a mere rudiment attached to one side of the egg and finally disappears. There can hardly be a doubt, as Korschelt maintains, that the nurse-cell is in some manner connected with the elaboration of food for the growing egg-cell ; and the intensely chromatic character of the nucleus is well worthy of note in this connection. Still more interesting are the conditions observed by Wheeler ('96, '97) in Mysostoma, where the young egg is accompanied by two nurse-cells, one at either end. These cells fuse bodily with the egg, one having " something to do in forming the vacuolated cytoplasm at the animal pole, . . . the other in forming the granular cytoplasm at the vegetative pole" ('97, p. 42). The polar axis thus determined persists as that of
the ripe ovum. This \
seems one of the clearest
cases showing the establishment of the egg-polarity through the
relation of the egg to its environment. 1
Somewhat similar nurse-cells occur in the insects, where they have been carefully described by Korschelt. The eggs here lie in a series in the ovarian "egg-tubes" alternating with nutritive cells variously arranged in different cases. In the butterfly Vanessa, each egg is surrounded by a regular follicular layer of cells, a few of which at one end are differentiated into nurse-cells. These cells are very large and have huge amoeboid nuclei, rich in chromatin (Fig. 77, A). In the ear-wig, Forfiada, the arrangement is still more remarkable, and recalls that occurring in Ophryotrocha. Here each 1 Cf. p. 386.
F'8 75-
ieg B s of Hilix. [Or
I S 2
THE GERM-CELLS
egg lies in the egg-tube just below a very large nurse-cell, which, when fully developed, has an enormous branching nucleus as shown in Fig. 163. In these two cases, again, the nurse-cell Is characterized by the extraordinary development of its nucleus — a fact which points to an intimate relation between the nucleus and the metabolic activity of the cell. 1
In all these cases it is doubtful whether the nurse-cells are sistercells of the egg which have sacrificed their own development for the sake of their companions, or whether they have had a distinct origin from a very early period. That the former alternative is possible is shown by the fact that such a sacrifice occurs in some animals after the eggs have been laid. Thus in the earthworm, Lumbricus terrcs
Fig. 76. — Egg and nurse-cell in [he annelid, Oplayotriicha, [KORSUIELT.] A. Young stage, The nurse-cell (u) larger than [he egg (?). B. Growth of Ibe ovum. C. Late stage, the nurse-cell degenerating.
tris, several eggs are laid, but only one develops into an embryo, and the latter devours the undeveloped eggs. A similar process occurs in the marine gasteropods, where the eggs thus sacrificed may undergo certain stages of development before their dissolution. 1
ifi) Differentiation of the Cytoplasm and Deposit of Deutoplasm. — In the very young ovum the cytoplasm is small in amount and free from deutoplasm. As the egg enlarges, the cytoplasm increases enormously, a process which involves both the growth of the protoplasm and the formation of passive deutoplasm-bodies suspended in the protoplasmic network. During the growth-period a peculiar body known as the yolk-nucleus appears in the cytoplasm of many ova, and this is probably concerned in some manner with the growth 1 See p. 338. * See McMurrich, '96.
GROWTH AND DIFFERENTIATION OF THE GERM-CELLS
153
of the cytoplasm and the formation of the yolk. Both its origin and its physiological rite are, however, still involved in doubt.
The deutoplasm first appears, while the eggs are still very small, in the form of granules which seem to have at first no constant position with reference to the egg-nucleus, even in the same species. Thus Jordan ('93) states that in the newt {pieinyctylus) the yolk may be first formed at one side of the egg and afterward spread to other parts, or it may appear in more or less irregular separate patches which finally form an irregular ring about the nucleus, which at this period has an approximately central position. In some Amphibia
Fig. 77. — Ovarian eggs of insects. [Korschki.t.] A. Egg of the butterfly. Vaxeua, surrounded by its follicle; above. Ihiee nurse-cells (a.c.) with branching nuclei; g,v, germinal vesicle. B. Egg of water- beetle. Tlylticuj, living; the egg (ff.tf.) lies between two groups of nutritive cells ; the germinal vesicle sends amoeboid processes into the dark mass of food-granules.
the deutoplasm appears near the periphery and advances inward toward the nucleus. More commonly it first appears in a zone surrounding the nucleus (Fig. 78, C, D) and advances thence toward the periphery (trout, Henneguy ; cephalopods, Ussow). In still others {e.g. in myriapods, Balbiani) it appears in irregular patches scattered quite irregularly through the ovum (Fig. 78, A). In Branchipus the yolk is laid down at the centre of the egg, while the nucleus lies at the extreme periphery (Brauer). These variations show in general no definite relation to the ultimate arrangement — a fact which proves that the eccentricity of the nucleus and the polarity of the
154
THE GERM-CELLS
egg cannot be explained as the result of a simple mechanical displacement of the germinal vesicle by the yolk, as some authors have maintained.
The primary origin of the deutoplasm-grains is a question that involves the whole theory of cell-action and the relation of nucleus
Kg. 78. — Young ovarian eggs, showing yolk-nuclei and deposit of deuloplasm.
A. Myriapod (Gteftilui) with single "yolk-nucleus" (perhaps an attraction-sphere) and scattered deuloplasm. [BAUtlANI.]
B. The same with several yolk-nuclei, and " attraction-sphere," 1. [BaLBIanI.]
C. Kish (Siorpicia), iiilh deutoplasm forming a ring about the nucleus, and an irregular mass of "eliminated chromatin" (? yolk-nucleus). [Van liAMHEKE.]
D. Ovarian egg of young duck (three months) surrounded l>y a follicle, and containing a " yolknucleus." y.n. [Mertkns.]
and cytoplasm in metabolism. The evidence seems perfectly clear that in many cases the deutoplasm arises in situ in the cytoplasm like the zymogen-granules in gland-cells. But there is now also a very considerable body of evidence indicating that a part of the egg-cytoplasm is directly or indirectly derived from the nucleus through the agency of the yolk-nucleus or otherwise ; and the
GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 155
subject can best be considered after an account of that body. It may be mentioned here, however, that a large number of observers have maintained a giving off of nuclear substance to the cytoplasm, in the form of actual buds from the nucleus (Blochmann, Scharff, Balbiani, etc.) as separate chro matin-rods or portions of the chromatin network (Fol, Blochmann, Van Bambeke, Erlanger, Mertens, Calkins, Nemec, etc.) or as nucleolar substance (Leydig, Balbiani, Will, Leydig, Henneguy), but nearly all of these cases demand reexamination.
Pig. 79. — Young ovari A. Egg or young magpie (eight • yesiele and '■ attraction-sphere." B. Pi of new-bom cal containing " attraction-spnere (jj rounded by follicle and containing besides [he nuc and a yolk-nucleus (>.*.). E. Of young chick ci deutoplasm- spheres (black). F. Egg o( new-born
eggs ol birds and mammals. [Mertens.]
s(, surrounded by the follicle and containing germinal lordial egg (oogonium) of new-born cat, dividing. C. Egg sphere" (j) and centrosome. D. Of young thrush sur
dby f,
(c) Yolk-nucleus. — The term yolk-nucleus or vitelline body (Dotterkern, corps vitelliri) has been applied to various bodies or masses that appear in the cytoplasm of the growing ovarian egg ; and it must be said that the word has at present no well-defined meaning. As originally described by von Wittich ('45) in the eggs of spiders, and later by Balbiani ('93) in those of certain myriapods, the yolk-nucleus has the form of a single well-defined spheroidal
156 THE GERM-CELLS
mass which appears at a very early period and persists throughout the later ovarian history. In other forms there are several so-called " yolk-nuclei," sometimes of fairly definite form as described in the Amphibia by Jordan ('93) and in some of the myriapods by Balbiani ('93). In some forms the numerous "yolk-nuclei" are irregular, illdefined granular masses scattered through the cytoplasm, as described by Stuhlman ('86) in the eggs of insects. In still others the " yolknucleus " or " vitelline body " closely simulates an attraction-sphere, being surrounded by distinct astral radiations and enclosing one or more central granules like centrosomes (Geopkilus, Balbiani, '93, and Limulus, Munson, '98). Balbiani is thus led to regard the yolknucleus in general as being a metamorphosed attraction-sphere. Miss Foot C96) has brought forward evidence to show that the polar rings, observed in the eggs of certain leeches and earthworms, are also to be regarded as "yolk-nuclei" (Fig. 102). Henneguy ('93, '96) finally compares the yolk-nucleus to the macronucleus of the Infusoria (!).
In the present state of the subject it is quite impossible to reconcile the discordant accounts that have been given regarding the structure, origin, and fate of the " yolk-nuclei " , and from the facts thus far determined we can only conclude that the various forms of " yolknuclei " hjive little more in common than the name. It is, in the first place, doubtful whether the " yolk T nuclei " simulating an attraction-sphere have anything in common with the other forms ; and Mertens ('93), Munson ('98), have shown that the young ovarian ova of various birds and mammals (including man) and of Limulus contain one or more "yolk-nuclei" in addition to the "attractionsphere" ("vitelline body" of Munson). In the second place there seem to be two well-defined modes of origin of the yolk-nucleus. In one type, illustrated by Jordan's observations on the newt ('93), the " yolk-nuclei " arise separately in situ in the cytoplasm without direct relation to the nucleus. The same is true of the small peripheral " yolk-nuclei " of Limulus (Munson). In a second and more frequent type the " yolk-nucleus " first appears very near to or in contact with the nucleus, suggesting that the latter is directly concerned in its formation. The latter is the case, for example, in the eggs of Cymatogasttr (Hubbard, '94) Syngnathus (Henneguy, '96), the earthworm (Calkins, '95, Foot, '96), Polyzoninm and other myriapods (Nemec, '97, Van Bambeke, '98), Limulus (Munson, '98), Cypris (Woltereck, '98), and Molgula (Crampton, '99). In nearly all of these forms the yolk-nucleus first appears in the form of a cap closely applied to one side of the nucleus (Figs. 80, 81), sometimes so closely united to the latter that it is difficult to trace a boundary between them. At a later period the yolk-nucleus moves away from the nucleus and in
GROWTH AND DIFFERENTIATION OF THE GERM-CELLS
157
most, if not in all, cases breaks up into smaller and smaller fragments which contribute, directly or indirectly, to the cytoplasmic growth. In all these cases the history of the yolk-nucleus is such as to indicate the participation of the nucleus in its formation. Calkins ('95) endeavours to show that the yolk-nucleus in Lumbricus is directly derived from the nucleus by a casting out of a portion of the chrc
Fig. 8o.-Yolk-n BaMBCKe; F-!, CR:
A. Early ovarian egg ei Lumtrieia. B. Later rian egg of Pholius. D. Laler stage; disintegration of yolk-nucleus. E. Remains ot tne yi nucleus scattered through the cytoplasm. F. Early stage of yolk-nucleus in Me/gula. G-/. I integration of the yolk-nucleus and enlargement of the products to form deutopl asm -spheres.
matin-reticulum — a result agreeing in principle with earlier observations on other eggs by Halbiani, Henneguy, Leydig, Will, and other observers. This conclusion rests partly on the apparent direct continuity of yolk-nucleus and chromatin, partly on the stainingreactions. Thus when treated with the Biondi-Ehrlich mixture (basic methyl-green, acid red fuchsin), the yolk-nucleus at first stains green like the chromatin, while the cytoplasm is red, and this is the case
158 THE GERM-CELLS
even after the yolk-nucleus has quite separated from the nuclear membrane. Later, however, as the yolk-nucleus breaks up, it changes its staining power, and stains red like the cytoplasm. The later observations of Miss Foot ('96) give ground to doubt the conclusion that the yolk-nucleus is here actually metamorphosed chromatin, for by the combined action of lithium carmine and Lyons blue its substance is sharply differentiated from the chromatin. Still later studies by Crampton C99) on Molgiila demonstrate that in this case the yolk-nucleus is not directly derived from chromatin, but they nevertheless indicate clearly the formation of the yolk-nucleus by or under the immediate influence of the nucleus — a conclusion also reached on less satisfactory evidence by Hubbard, Van Bambeke, Woltereck, and Nemec. The general morphological history of the yolk-nucleus is here closely similar to that of Lumbricns (Fig. 80), except that no direct continuity between it and the nuclear substance was observed. Stained with methyl-green-fuchsin the yolk-nucleus and major part of the nuclear substance stain red, while the scattered nuclear chromatin-granules and the cytoplasm stain green. Millon's test, combined with digestion-experiments and the foregoing stainingreactions, proves that the yolk-nucleus and the red staining nuclear substance consist of albuminous substance and differ widely from the general cytoplasm, which probably consists largely of nucleoalbumins (cf. p. 331). These reactions give strong ground for the conclusion that the substance of the yolk-nucleus, which progressively accumulates just outside the egg-nucleus, is formed through the direct activity of the latter, perhaps arising within the nucleus and passing out into the cytoplasm. It is possible, further, that even the scattered " yolk-nuclei " that seem to be of purely cytoplasmic origin may arise in a similar manner, either, as Crampton suggests, through the early formation and breaking up of a single yolk-nucleus, or in some less obvious way.
Interesting questions are suggested by those " yolk-nuclei," such as occur in Gcophilus and Limn/us, that so closely simulate an attraction-sphere. Munson's observations show that this body ("vitelline body ") first appears in the very young ova as a crescent applied to the nucleus precisely as in Molgiila or Lumbricns, but containing one or more central granules (Fig. 81). In later stages it becomes spherical, moves away from the nucleus, and assumes the form of a typical radial attraction-sphere with concentric microsomecircles and astral rays. It is hardly possible to doubt that this body in Limulus is of the same general nature as the yolk-nucleus of Lumbricns, Molgiila, Cypris, Cymatogastcr, or P hole us ; and if it be a true attraction-sphere in the one case we must probably so regard it in all. This identification is, however, by no means complete;
GROWTH AND DIFFERENTIATION OF THE GERM-CELLS
159
and even Munson's careful studies do not seem definitely to establish its connection with the attraction-sphere or centrosome of the last oogonium-division. That a body simulating an at traction- sphere and containing a central granule may arise de novo in the cytoplasm is shown by Lenhossek's observations on the spermatids of the rat (p. 170); and the central granule is in this case certainly not a centrosome, since the true centrosomes are found in another part of the cell. It is quite possible that the "vitelline body" of Limulus may have a similar origin. Nemec ('97) finds in Polyzonium in the earliest stages a single body applied to the nucleus and later two bodies, one of which enlarges to form a cap-shaped yolk
Plg. 81. — Forms of yolk-ni
A. Very young ovarian eggs of Limulus ; on Ihe nucleus; ai Ihe right older egg shot "vitelline body" in Ihe form of an aliractio: nuclei"' O-H.) in Limului. D. Very early nucleus. E. Older egg with yolk-nucleus i disintegration ol the yolk-nucleus.
Petytoaium. [A-C, MUNSON; D-F, N EM BC] left " vitelline body " ft) in Ihe form of a cap tral formation. B. Older stage of the same; ev/ith central granule. C. Peripheral "yolk1 egg of a myriapod. Polytonium, with yolk's! body ya). F. Still later stage, beginning
nucleus like those described above, while the other assumes the structure of a radiating attraction-sphere containing a central granule (centrosome?), and his observations suggest that the two bodies in question may have a common origin (Fig. 81). In none of these cases do the astral radiations, surrounding this body, seem to have any connection with cell-division, and it is probable that a careful comparison of their physiological significance here, in leucocytes, and in mitotic division, may give us a better understanding of the general significance of astral formations in protoplasm.
The fate and physiological significance of the yolk-nucleus are still to a considerable extent involved in doubt. In many cases it
l6o THE GERM-CELLS
breaks up into smaller and smaller granules {Lumbricus, Molgu/a, Pholais, some myriapods, Antedon\ which scatter through the cytoplasm and are believed by many observers (Balbiani, Mertens, Will, Calkins, Crampton, Nemec), following the earlier views of Allen Thomson, to become directly converted into deutoplasm-spheres (Fig. 80). Other observers (Van Bambeke, Foot, Stuhlman, and others) adopt the original view of Siebold, that the fragments of the yolk-nucleus are absorbed or converted into protoplasmic elements and thus only indirectly contribute to the yolk. In still other cases {e.g. the "vitelline body" of Litnulus) the yolk-nucleus does not fragment, but seems to serve as a centre about which new deutoplasmic material is formed. A review of the general subject shows that we are justified only in the somewhat vague conclusion that the yolk-nucleus is probably involved in some manner in the general cytoplasmic growth ; and that the facts strongly suggest, though they hardly yet prove, that at least some forms of yolk-nuclei are products of the nuclear activity and form -a, connecting link between that activity and the constructive processes^ of the cytoplasm. That the yolk-nuclei have no very definite ntaqrphological value, and that they are not necessary to growth, seems to^be shown by Henneguy's observation, that in the eggs of vertebrates it is in some forms invariably present, in others only rarely, and 1% still others is quite wanting ('96, p. 162). If this be the case, we Tiust conclude that the yolk-nucleus consists of material that contribute? to the constructive process, but is not necessarily localized in a definite body. As to its exact role we are, as Henneguy has said, reduceJ to mere hypotheses. 1 The facts indicate that this material is a prod uct of the nuclear activity, and that it may in some cases contribute directly to formed elements of the cytoplasm. It is probable, however, that beyond this the yolk-nucleus may supply materials, perhaps ferments, that play a more subtle part in the constructive process, and of whose physiological significance we are quite ignorant. The whole subject seems a most interesting and important one for further study of the actions of the cell in constructive metabolism, and it is to be hoped that further research will place the facts in a clearer light.
2. Origin of the Spermatozoon
(a) General. — The relation of the various parts of the spermatozoon to the structures of the spermatid is one of the most interesting questions in cytology, since it is here that we must look for a basis of interpretation of the part played by the sperm a J, 96, p. 170.
GROWTH AND DIFFERENTIATION OF THE GERM-CELLS l6l
tozoon in fertilization. Obviously the most important of the questions, thus suggested, is the source of the sperm-nucleus and centrosome, though the relation of the other parts to the spermatidcytoplasm involves some interesting problems.
Owing to the extreme minuteness of the spermatozoon, the changes involved in the differentiation of its various parts have always been, and in some respects still remain, among the most vexed of cytological questions. The earlier observations of Kolliker, Schweigger-Seidel, and La Valette St. George, already mentioned, established the fact that the spermatozoon is a cell ; but it required a long series of subsequent researches by many observers, foremost among them La Valette St. George himself, to make known the general course of spermatogenesis. This is, briefly, as follows : From the primordial germ-cells arise cells known as spermatogonia} which at a certain period pause in their divisions and undergo a considerable growth. Each spermatogonium is thus converted into a spermatocyte, which by two rapidly succeeding divisions gives rise to four spermatozoa, as follows. 2 The primary spermatocyte first divides to form two daughter-cells known as spermatocytes of the second order or sperm-mother-cells. Each of these divides again — as a rule, without pausing, and without the reconstruction of the daughter-nuclei — to form two spermatids or sperm-cells. Each of the four spermatids is then directly transformed into a single spermatozoon, its nucleus becoming very small and compact, its cytoplasm giving rise to the tail and to certain other structures. The number of chromosomes entering into the nucleus of each spermatid and spermatozoon is always one-half that characteristic of the tissue-cells, and this reduction in number is in most, if not in all, cases effected during the two divisions of the primary spermatocyte. The reduction of the chromosomes, which is the most interesting and significant feature of the process, will be considered in the following chapter, and we are here only concerned with the transformation of the spermatid into the spermatozoon.
All observers are now agreed that the nucleus of the spermatid is directly transformed into that of the spermatozoon, the chromatin becoming extremely compact and losing, as a rule, all trace of its reticular structure. It is further certain that in some cases at least the spermatid-centrosome passes into, or gives rise to, a part of the middle-piece, and that from it the axial filament grows out into the tail. The remaining structures arise, as a rule, from the cytoplasm, and both the acrosome and the envelope of the axial filament often show a direct relation to the remains of the achromatic figure (" ar 1 The terminology, now almost universally adopted, is due to La Valette St. George. Cf. Fig. 1 1 8. 2 See Fig. 1x9.
M
162
THE GERM-CELLS
choplasm " or " kinoplasm ") which is found in the spermatid in the form of a sphere (sometimes an attraction-sphere) or " Nebenkem " or both. Apart from the nuclear history, these facts have been definitely determined in only a few cases, and much confusion still exists in the accounts of different observers. Thus a number of investigators (e.g. Platner, Field, Benda, Julin, Prenant, Niessing) have asserted that the centrosome passes into the acrosome, instead of
[P*U I.MIR
dyad of
in of the spermatoioftn in an inset
Fig. 137) below, fi. Reconsliiulion of the nuclei. C. Spermatid wiih Nebenltem (<V) a acrosome (a). D. Nebenkem double, with cemrosome between the two halves. E. F. O. Elongation of the spermatid, outgrowth of axial filament, migration of acrosome. //. Giant spermatid (douhfe sin I with two centrosomes and axial filaments. /. Giant spermatid (quadruple size)
the middle-piece — a result which stands in contradiction with the fact that during fertilization in a large number of accurately known cases the centrosome arises from or in immediate relation to the middlepiece (Amphibia, echinodcrms, tunicates, annelids, mollusks, insects, etc.; see p. 212). The clearest and most positive evidence on this question, afforded by recent observations on the spermatogenesis of insects, annelids, mollusks, Amphibia, and mammals, leaves, however, little doubt that the former result was an error and that, as the facts
GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 1 63
of fertilization would lead us to expect, the centrosome of the spermatid passes into the middle-piece.
Accounts vary considerably regarding the origin of the acrosome, which according to most authors is of cytoplasmic origin, while a few describe it as arising inside or from the anterior part of the nucleus.
(b) Composition of the Spermatid. — The confusion that has arisen in this difficult subject is owing to the fact that the spermatid may contain, besides the nucleus and centrosome, no less than three additional bodies, which were endlessly confused in the earlier studies on the subject. These are the Nebenkern} the attraction-sphere or idiozome (Meves), and the chromatoid Nebenkdrper (Benda).
The Nebenkern (Fig. 82), first described by Butschli ('71) in the spermatids of butterflies, was afterward shown by La Valette (*86), Platner ('86, '89), and many later investigators to arise wholly or in part from the remains of the spindle of the second spermatocyte division. Its origin is thus related to that of an attraction-sphere (which it often closely simulates), since the latter likewise arises from the achromatic figure. To the remains of the spindle, however, may be added granular elements, probably forming reserve-material ("centro-deutoplasm of Erlanger), that are scattered through the cytoplasm or aggregated about the equator of the spindle (Fig. 126). Thus the Nebenkern may have a double origin, though its basis is formed by the spindle-remains. The Nebenkern sometimes takes a definite part in the formation of the tail-envelopes and of the acrosome (insects), but in many cases it seems to be wholly wanting. 2 The idiozome is in some cases an undoubted attraction-sphere derived from the aster of the last division and at first containing the centrosome, e.g. in the earthworm as shown by Calkins C95) and Erlanger ('96, 4), in the salamander and guinea-pig, Meves ('96, '99), and in Helix according to Korff ('99), though in later stages the centrosomes usually pass out of the body of the idiozome. In some cases, however (in the rat, according to Lenhoss^k, '99), the idiozome seems to arise independently through condensation of the cytoplasmic substance into a sphere having no relation to the centrosomes. In some cases the idiozomes of adjoining cells remain for a time connected by bridges of material (Fig. 7) representing the remains of the spindle, and hence corresponding to a Nebenkern (e.g. salamander, Meves, '96), and the distinction between Nebenkern and idiozome here fades away. The idiozome is usually concerned in the formation of the acrosome (Amphibia, mammals), but sometimes seems
1 The English equivalent of this should be paranucleus^ but the latter word has already been used in various other senses, and it seems preferable to retain Biitschli's original German word.
3 For critical discussion, see Erlanger, '97, x.
[64 THE GERM-CELLS
to degenerate without contributing directly to the sperm-formation {Helix). The chromatoid Ncbenkbrper, finally, is a small rounded body, staining with plasma-stains, which appear always to degenerate without taking direct part in the formation of the spermatozoon. It is possibly an extruded nucleolus (Lcnhossek), but its origin and meaning are not definitely known.
(c) Transformation of the Spermatid into the Spermatozoon. — In the works of earlier authors it is often impossible to distinguish
Kg. tj. — Formation of Ihe spermatozoon from the spermatid in the
.4. Young spermatid, showing the nucleus above, and below the colourle and Ihe chromalic sphere. />. Later stage, showing the chromatic sphere ai of the nucleus. C. D. E.F. Later stages, showing the transformation of Ihe chi the middle-piece («).
spher.
which of the various achromatic elements mentioned above have been under observation. We may therefore confine ourselves mainly to the latest works, in which these distinctions are clearly recognized. Owing to their great size, the spermatozoa of Amphibia have been the subject of most careful study; yet a clearer view of the subject
GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 165
may, perhaps, be obtained by taking the spermatogenesis of annelids and insects as a basis of comparison. In the insects (butterflies), Butschli showed, in 1871, that the tail is formed by an elongation of the cell-body, into which extends the elongated Nebenkern, now divided into two longitudinal halves (Fig. 82). Platner ('89), confirming this observation, further showed that the Nebenkern (in Pygcera) consisted of two parts, stating that one (" large mitosome") gives rise to the investment of the axial filament, the other (" small mitosome ") to the middle-piece ; while a third still smaller body, described as a " centrosome," passes to the apex. The later works of Henking('9i) and Wilcox ('95, '96) render it nearly certain that Platner confused the acrosome with the centrosome, the first-named observer finding in Pyrrhocoris and the second in Caloptenus that Platner's "centrosome" is derived from the Nebenkern, while Wilcox traced the centrosome directly into the middle-piece. Paulmier, finally, has shown in Anasa that the axial filament grows out from the centrosome, 1 proving that such is the case by the highly interesting observation that in giant spermatozoa, arising by the non-division of the primary or secondary spermatocytes, either two or four centrosomes are present, each of which gives rise to a single axial filament, though only one Nebenkern is present (Fig. 82). (The bearing of this important fact on the centrosome-question is indicated elsewhere.) These observations, made on three widely different orders of insects, seem to leave no doubt that in insects the centrosome lies in the middle-piece (i.e. at the base of the nucleus), while both the acrosome and the inner tailenveloges are derived from the Nebenkern. The outer envelope of the tail is derived from unmodified cytoplasm.
In the earthworm the phenomena are slightly different, the middlepiece arising from an idiozome or attraction-sphere (Calkins, '95), in which lies the centrosome (Erlanger, '96), while the Nebenkern seems to have no part in the formation of either acrosome or tail-envelopes. 2
We turn now to the Amphibia, elasmobranchs, and mammals, in which the same general result has been attained, though there is still some divergence of opinion regarding the exact history of the centrosome. Working on the basis laid by Flemming ('87, '88), Hermann ('89) traced the middle-piece in the salamander to a " Nebenkorper," which he believed to be not a Nebenkern but an attraction-sphere,
1 Moore ('95) seems to have been the first actually to describe the outgrowth of the axial filament from the centrosome, in the elasmobranchs. It has since been described by Meves ('97, 2) and Hermann ('97) in the salamander, by Lenhossek C97), Meves ('98, '99), and Bardeleben ('97) in the rat, guinea-pig, and man; by Godlewski ('97) and Korff ('99) in Helix, and by several others.
a Calkins's preparations, which I have carefully examined, seem to leave no doubt that the middle-piece arises from a true attraction-sphere derived from the spindle-poles; but Erlanger believes that the granular " centrodeutoplasm " also contributes to the sphere.
166
THE GERM-CELLS
consisting of three parts, lying side by side in the cytoplasm (Fig. 83). These are (a) a colourless sphere, shown by Meves's later researches to be probably an attraction-sphere ; (6) a minute, intensely staining corpuscle, and (c) a small, deeply staining ring. The concurrent results of Hermann ('89, '92, '97), Benda ('93), and Meves ('96, '97, 2) have shown that the small corpuscle (e) is one of the centrosomes of the spermatid, and all these observers agree that it passes into or gives
H. Microti forms the t middle-pi ci
— Formation of Ihe sperm atoiofln in Amphibia. [A-E. Salamimdrj ,
iuma. MCGRFJJOR.]
in.itid with peripheral pair of centrosomes lying outsidi: the sphere, and a
ames near Ihc nucleus, outer one ring-shaped. C. Inner ccntrosome inside Ihe
arging to form middle-piece. D. Itortionof much older spermatid, showing divergence
5 of the ring (r). E. Portion ol mature sptrmato/oiin, showing upper half of ring al
i;il filarm-ril proceeding from it. oatid of .-trnptimiiii, showing sphere-bridges and ring-shaped mid-bodies. G. Later
t ccniriisurnc ring -shaped, inner one double; sphere (j| convened inlo the acrosome.
>n of the centrosomes. /. Middle-piece al bascof nucleus, y. The inner cenlrosome
id-knob within the middle-piece, which is now inside the nucleus. A'. Enlargement of
e, end-knob within it; elongation of Ihe ring.
GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 1 67
rise to the middle-piece. According to Meves, who has most thoroughly studied the entire formation of the spermatozoon, the history of these parts is as follows : In the young spermatids the two centrosomes lie quite at the periphery of the cell (Fig. 84), 1 and from the outer one grows out the axial filament. The two centrosomes, leaving the idiozome by which they are first surrounded, now pass inwards toward the nucleus, the outer one meanwhile becoming transformed into the ring mentioned above, while the axial filament passes through it to become attached to the inner centrosome. The latter pushes into the base of the nucleus and enlarges enormously to form a cylindrical body constituting the main body of the middle-piece. The ring meanwhile divides into two parts, the anterior of which gives rise to a small, deeply staining body at the posterior end of the middle-piece identical with the "end-knob." The other half of the ring wanders out along the tail, finally lying at the limit between the main part of the latter and the end-piece. The envelope of the axial filament, here confined to that side opposite the marginal fin (i.e. the "ventral " side of Czermak), is formed by an outgrowth of the general cytoplasm along the axial filament. The fin and marginal filament are believed by Meves, as I understand him, to be formed from the axial filament ('97, 2, p. 127). 2 The acrosome, finally, is formed from the idiozome which wanders around the nucleus to its anterior pole. McGregor's results on Amphinma ('99) agree in their broader features with those of Meves, but differ on two points, one of which is of great importance. The acrosome here arises from only a part of the sphere (idiozome), while a second smaller part passes to the base of the nucleus and forms the main part of the middle-piece. The inner centrosome passes into the middle-piece to persist as the endknob from which the axial filament passes out into the tail (Fig. 84). The history of the sphere thus recalls the phenomena seen in the Nebenkern of the insect-spermatid ; though the posterior moiety does not contribute to the tail-envelope, while the history of the inner centrosome is somewhat like that observed in the mammals, as described beyond. In the elasmobranchs Moore ('95), Hermann ('98), Suzuki ('98), and Benda ('98) likewise traced the spermatid-centrosome into the middlepiece (Fig. 85), and Moore first showed that from it the axial filament grows out. 8 Moore derived both middle-piece and acrosome from the
1 Cf. their position in epithelial cells, p. 57.
2 Hermann C97) gives a somewhat different account of the process, believing that the ring is derived from the mid-body of the last mitosis. Meves and McGregor have, however, shown that the ring and mitt-body coexist in the early spermatids (Fig. 84), which seems decisive against Hermann's conclusion.
8 Hermann finds also the ring observed in the salamander, and believes it to be the midbody. The middle-piece is regarded by him as a product of the spindle-remains, but on both these points he is contradicted by Suzuki.
i68
THE GERM-CELLS
" archoplasm " of the spermatid. Suzuki's studies clearly show, however, that the entire axial filament of the long middle-piece arises by the elongation of the inner centrosomc, while the outer centrosome, from which the axial filament of the tail grows out, lies at the posterior limit of the middle-piece (Fig. 85). A nearly similar result is reached by Korff ('99) in the case of Helix. It was shown by Godlewski ('97) that in this form the axial filament likewise grows out
P'6 85 — Formation ol ihe sperroaiotoon to clasmobraochs. [M-C. Suzuki. D, Moore; and tn JM.x. t-G, Ku«1<F.]
A-D Ouinroiuh of anal filamrni (mm peripheral ceolrotome {<>). which penltt* at Ihe posterior limit n| ihe middle-piece, 01 connecting-piece (»). K!ongation ol mnti cenitosome (f*) a] filament of Ihe latlcr. E-G show similar phenomena in Helix, with casting off
■ spher
K peripheral, and c
/. flagellum; t. end-knob, derived
from the centrosome. Korff's later studies show that here, exactly as in the elasmobranch, the axial filament grows out from the peripheral centrosome and is afterward transformed into a ring (Fig. 85). The inner centrosome elongates to form a rod, which afterward becomes a long filament traversing the elongated middle-piece and terminating in front in an end-knob at the base of the nucleus, while the ring lies at its posterior limit. The idiozome (a true attractionsphere) degenerates without taking part in the formation of an aero
GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 169
some. The envelope of the middle-piece is here formed out of the general cytoplasm.
In the mammals the recent work of Lenhossek on the rat ('98) and Meves on the rat, guinea-pig, and man ('98, '99) gives a result agreeing in its broader features with the forms already considered. In all these mammals the young spermatids are closely similar to those of the salamander, containing two peripherally placed centrosomes, from the outer one of which the axial filament grows out(Fig. 86). Meves
Fig. W. — Formation of the spermatozoon in mammals. [Meves.] lalid of man, showing centrosomes and axial filament. B. Spermatid of guinea-pig, me. C. Nearly mature spermatozoon, showing backward migration of the ring.
attire spermatoiodn
r. final position
of the
ing.
ed by
cytoplas
n of th
st of which is afterward thrown
es; c,f
ecting-picce ; /
ftagcllum ;
ning
nd- knobs
(idiozo
me).
and Lenhossek differ somewhat in their accounts of the later history of these centrosomes, though agreeing that both contribute to the formation of the middle-piece. Lenhossek states that in the rat both centrosomes persist at the base of the nucleus to form the end-knob, which, as Jensen showed ('87), is double in this animal. Meves finds the process to be more complicated, agreeing in the main with that observed by him in the salamander. In man and the rat the inner centrosome passes to the base of the nucleus and flattens against it to form a small disc-shaped body. The posterior centrosome divides
170 THE GERM-CELLS
into two parts, of which the anterior gives rise to the end-knob, while the posterior is transformed into a ring, which wanders back to its final position at the posterior end of the so-called "connecting-piece." From this it follows that the latter body ( Verbindungsstiick) does not correspond to the middle-piece of the salamander (here represented by the small disc-shaped body at the base of the nucleus), but belongs to the flagellum proper. The origin of the axial filament and endknob is, however, nearly the same in the two cases. In the guineapig the process is somewhat more complicated and is not quite cleared up by Meves ; but the origin and fate of the ring is the same, and the end-knob passes into the neck of the spermatozoon as in the rat. Taken together, these observations conclusively show that in mammals and Amphibia the end-knob is a derivative of the centrosome, thus sustaining, though with some modifications, Hermann's earlier conjecture ('92) as to the nature of this body; and they overturn Niessing's result ('96) that the centrosome passes into the acrosome. As in the salamander, the acrosome is formed from an idiozome derived in the guinea-pig from the remains of the attraction-sphere (Meves), while in the rat, according to Lenhoss£k, it is independently formed in the cytoplasm without relation to the preceding mitotic figure or the centrosomes. Within the sphere appears a small, deeply staining body, resembling a centrosome, yet staining differently from the true centrosome, which enlarges to form the acrosome, while about it is formed a clear substance forming the " head-cap " (p. 1 39). In the rat the acrosome remains small (" Spitzenknopfchen " of Merkel); in the guinea-pig it becomes nearly as large as the nucleus itself (Fig. 86). An interesting feature in the formation of the mammalian spermatozoon is the casting off of a portion of the spermatid-cytoplasm in the form of a "cytoplasmic vesicle" or "tailvesicle," which degenerates without further use (Fig. 86). This process, described by Meves ('99) in the guinea-pig, is closely similar to that which occurs in the spermatozoid- formation in ferns (p. 144).
Rc'sume'. In reviewing the foregoing facts we find, despite many variations in detail, three points of fundamental agreement, namely : (1 )the origin of the sperm-nucleus from that of the spermatid ; (2) the origin of a part at least of the " middle-piece " from the spermatidcentrosomes; and (3) the outgrowth of the axial filament from one of the spermatid-centrosomes. It is clear, however, that the term middlepiece has been applied to structures of quite different morphological nature, which agree only in lying behind the nucleus. Thus in the salamander the inner centrosome gives rise to the main body of the middle-piece ; in the rat or in man it gives rise only to the small discshaped body lying in the " neck " in front of the so-called middle
GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 171
piece ; while in Helix or the elasmobranch it is transformed into a long filament traversing a cytoplasmic " middle-piece " which forms a considerable part of the flagellum. The term middle-piece has thus become highly ambiguous and should only be employed, if at all, as a convenient descriptive term which has no definite morphological meaning.
A very striking fact in the origin of the spermatozoon is the prominent part played by the " archoplasm," i.e. substance in the form of idiozome or Nebenkern derived from the mitotic figure. Both the source and the fate of this material seem, however, to vary in different cases, the acrosome now arising from the Nebenkern (insects), now from the idiozome (salamander), the envelope of the flagellum being formed in some cases from the Nebenkern (insects), in others from unmodified cytoplasm (salamander, snail), while the idiozome may form the acrosome (salamander, mammal) or degenerate without apparent use (snail). We find here, I think, additional reason for regarding "archoplasm" not as a distinct and permanent form of protoplasm, but only as a phase in the general metabolic transformation of the cell-substance, which may or may not persist and play a definite morphological rdle in the cell according to circumstances. The close relation of this substance to the motor phenomena of the cell cannot, however, be overlooked. 1
The outgrowth of the axial filament from the centrosome is a highly interesting fact, whether we compare it with the analogous phenomena in plants (p. 172) or with the facts observed in ordinary ciliated cells. In the latter case (Fig. 17), as has long been known, each cilium is attached to a small, highly refracting body known as the "basal knob " lying near the cell-periphery. These bodies stain intensely in iron hematoxylin, and it has been recently suggested by Henneguy ('98) and Lenhoss^k ('98) that they are of the same nature as centrosomes. The truth of this surmise must be tested by further study ; but it seems highly probable that they are at least analogous to the spermatid-centrosome. Ishikawa ('99) has clearly shown that in the formation of the swarm-spores of Noctiluca the flagellum grows out from that end of the cell at which the centrosome lies, its substance apparently arising from the central spindle, while the centrosome lies at its base. A very interesting fact discovered by Moore ('95) in elasmobranchs, and confirmed by Meves ('97, 5) and Henneguy ('98) in the insects, is a more or less abortive attempt to form a flagellum by the spermatocytes, £# one or two generations before the spermatozoon. In the insects (Fig. 166) Henneguy has found the cilia actually attached to the centrosomes of the mitotic figure, thus removing every doubt as to their nature. 2
1 Cf. 323. 2 Cf. Paulmier on giant spermatozoa, p. 165.
172 THE GERM-CELLS
It is an important question whether the axial filament actually arises from the substance of the centrosome or is formed by differentiation from the cytoplasmic substance, after the fashion of an astral ray or spindle-fibre. M eves ('97, p. 117) accepts the latter alternative ; but the observations of Korff on Helix and of Suzuki on elasmobranchs seem to show clearly that, in these cases at least, the inner centrosome elongates bodily to form an extremely long filament traversing the greater part of the flagellum, and apparently of the same nature as the true axial filament developed from the outer or distal centrosome. This seems to establish a probability in favour of the first of the above alternatives, and to show that contractile elements may be directly derived from the centrosome-substance. If this be true, this substance is itself nearly related with " archoplasm " ; and the origin of a centrosome de novo may be brought under the same category with the formation of archoplasm. 1
3. Formation of the Spermatozoids in Plants
While the origin of the spermatozoids has not yet been as fully investigated as that of the spermatozoa, recent researches have given good ground for the conclusion that essentially similar phenomena are involved in the two cases. All recent observers are agreed that the nucleus of the spermatozoid is directly derived from that of the spermatid, while the cytoplasm of the latter gives rise to the cilia and to certain other structures. The principal interest of the subject now lies in the origin of the cilia and their relation to the " archoplasmic " or " kinoplasmic " structures of the mother-cell. Belajeff ('92, '94) found that in Chara the cilia grow forth from a small, highly refracting body, taking an intense plasma-stain, that lies in the cytoplasma beside the nucleus. He afterward found the same body "which reminds one of a centrosome " in the developing spermatozoids of ferns and Equisetaceae (Fig. 88), where it grows out into a band, lying in the anterior part of the spermatozoid, from which the cilia grow forth. Comparing these results with those of Hermann, Belajeff concluded " that the deeply staining corpuscle " {i.e. the centrosome) " in the spermatids of the salamander and the mouse corresponds completely to the deeply staining corpuscle in the spermatogenic cells of the Characeae, ferns, and Equisetaceae " ; that, furthermore, " the middle-piece of the spermatozoon represents the band which bears the cilia of the plant spermatozoid, while the taillike flagella 2 of the salamander • or mouse represents the cilia." 8
1 ty P- 3 21 * For the function of the centrosome in fertilization, see p. 208. 8 In the original " Faden " perhaps meant to designate the axial filament.
8 '97> 3
GROWTH AND DIFFERENTIATION OF THE GERM-CELLS
173
This tallies with Strasburger's earlier conclusion that the cilia-bearing region consists of "kinoplasm " and corresponds to the middle-piece ('92, p. 139), but gives a stiil more definite basis of comparison. 1
The history of the centrosome-like bodies {blepharoplasts of Webber, '97, 3) has been carefully followed out in Zamia and Gingko by Webber ('97), and in Cycas by Ikeno ('97, '98) with nearly similar results. In all these forms (Fig. 87) the blepharoplasts appear in the
1 [he cycads. {A, Gingko ; B-D, Zamia,
Pig. &!■ — Formation of the sperm Webber; £-/, tycos, Ikeno.]
A. Developing pollen-lube, showing stalk-cell (r). vegetative cell (») and generative cell (f), the latter with two blepharoplasts. B. Generative cell, somewhat later, with blepharoplasts and ■Item. C. The same in the prophases of division, showing breaking up of blepharoplasts. D. The two spermatids formed by division of the generative cell ; blepharoplasts fragmented ; from these fragments arises the cilia-bearing band. E. Blepharoplasl of Cyias, at a stage somewhat later than Fig. C\ cilia developing. F. Later stage: ciliated band {derived from the last stage) attached to a prolongation from the nucleus. G. Cilia-bearing band continuous. H. Nearly ripe spermatoioid with nucleus in the centre; ciliated band, shown in section, forming a spiral. /. Slightly later stage, viewed from above, showing the spiral course of the band (cilia omitted).
penultimate cell-generation lying one on either side the nucleus, and in earlier stages surrounded by astral radiations very closely resembling those of a typical mitotic aster, and they lie opposite the poles
1 The " anterior " region of the spermatoioid thus corresponds to the " posterior " region of the spermatoioBn, the confusion of terms having arisen from the fact that the former swims with the cilia-bearing region in front, the latter with the flagellum directed backward.
THE GERM-CELLS
of the ensuing division-spindle. They seem, however, to have no part in the formation of the mitotic figure or in division, and both
Pig. 88.B-G, HELAJEF1>; U, C. O, SHAW), Cymmegrammt (Il-K, I1ei.ajf.KF). and Equiitlum (L-JV.
Bklajuffj.
A. Primary spermatogonium (two generations before the primary spermatocytes) in division, showing cent rosomes. //. Primary spermatocyte with pair of " lilepharoplasioids" (centrosomes). C. Spindle of primary spermatocyte (first mntu rat ion -division). /). Four of the eight secondary spermatocytes with bltphnroplast. E-G. Prophase o! second maturation jli vision. H. Pair of spermatids ( Gymnogrammc) with blepliaroplasts. I~y. Formation of the ciliated band Irom the blephamplasl. A'. Nearly ripe spermatoioid, showing ciliated band (*). nucleus, and " cytoplasmic vesicle" (the latter is ultimately cast off). L, M. Spermatids of HamtMtMM. If. Rip* spermaloioid from above, showing spiral ciliated band. U. Ripe spermaioioid of Manilla with very long spiral ciliated band.
STAINING-REACTIONS OF THE GERM-NUCLEI 175
Webber and Ikeno have produced apparently strong evidence * that they arise separately and de novo in the cytoplasm. After the ensuing division (by which the two spermatids are formed) the astral rays disappear, and the blepharoplast gives rise by a peculiar process to a long, spiral, deeply staining band, from which the cilia grow forth. The later studies of Shaw ('98, 1) and Belajeff ('99) on the blepharoplasts in Onoclea and Marsilia leave no doubt that these bodies are to be identified with centrosomes. In Marsilia Shaw first found the blepharoplasts lying at the poles of the spindle during the anaphase of the first maturation-division and very closely resembling centrosomes. Each blepharoplast, at first single, divides into two during the late telophase, and during the prophases of the second division the halves diverge to opposite poles of the nucleus and lie at the respective spindle-poles. This account is confirmed by Belajeff, who shows further that during the prophases astral rays surround the blepharoplasts, and a central spindle is formed between them (Fig. %%), Belajeff also finds centrosomes in all of the earlier spermatogenic divisions. The blepharoplasts are thus proved to be, in one case at least, dividing organs which in every way correspond to the centrosomes of the animal spermatocytes ; and the justice of BelajefFs comparison is demonstrated. Shaw believed that the primary blepharoplast, which by division gives rise to those of the two spermatids, arose de novo. He made, however, the significant observation that in Marsilia " blepharoplastoids," exactly like the blepharoplasts, appear at the spindle-poles of the preceding (antepenultimate) division, and that each of these divides into two in the late telophase. These are said to disappear, without relation to the blepharoplasts which at a slightly later period are found at the spindle-poles of the first maturation division ; but in view of the demonstrated continuity of the blepharoplasts during the second division we may well hesitate to accept this result, as well as Webber's conclusion regarding the independent and separate origin of the blepharoplasts in Zamia. In any case the facts give the strongest ground for the conclusion that the formation of the spermatozoids agrees in its essential features with that of the spermatozoa, and for the expectation that the history of the achromatic structures in fertilization will yet be found to show an essential agreement in plants and animals.
E. Staining-reactions of the Germ-nuclei
It was pointed out by Ryder in 1883 that in the oyster the germnuclei stain differently in the two sexes ; for if the hermaphrodite
1 Dr. Webber has kindly given me an opportunity to look through his beautiful preparations.
176 THE GERM- CELLS
gland of this animal be treated with a mixture of saffranin and methylgreen, the egg-nuclei are coloured red, the sperm-nuclei bluish green. A similar difference was afterward observed by Auerbach ('91) in the case of many vertebrate germ-cells, where the egg-nucleus was shown to have a special affinity for various red and yellow dyes (eosin, fuchsin, aurantia, carmine), while the sperm-nuclei were especially stained with blue and green dyes (methyl-green, aniline-blue, haematoxylin). He was thus led to regard the chromatin of the egg as especially " erythrophilous," and that of the sperm as " cyanophilous." That the distinction as regards colour is of no value has been shown by Zacharias, Heidenhain, and others ; for staining-agents cannot be logically classed according to colour, but according to their chemical composition ; and a red dye, such as saffranin, may in a given cell show the same affinity for the chromatin as a green or blue dye of different chemical nature, such as methyl-green or haematoxylin. Thus Field has shown that the sperm-nucleus of Asterias may be stained green (methyl-green), blue (haematoxylin, gentian violet), red (saffranin), or yellow (iodine), and it is here a manifest absurdity to speak of " cyanophilous " chromatin^, p. 335). It is certainly a very interesting fact that a difference of staining-reaction exists between the two sexes, as indicating a corresponding difference of chemical composition in the chromatin ; but even this has been shown to be of a transitory character, for the staining-reactions of the germ-nuclei vary at different periods and are exactly alike at the time of their union in fertilization. Thus Hermann has shown that when the spermatids and immature spermatozoa of the salamander are treated with saffranin (red) and gentian violet (blue), 1 the chromatic network is stained blue, the nucleoli and the middle-piece red ; while in the mature spermatozoon the reverse effect is produced, the nuclei being clear red, the middle-piece blue. A similar change of stainingcapacity occurs in the mammals. The great changes in the stainingcapacity of the egg-nucleus at different periods of its history are described at pages 338-340. Again, Watas£ has observed in the newt that the germ-nuclei, which stain differently throughout the whole period of their maturation, and even during the earlier phases of fertilization, become more and more alike in the later phases, and at the time of their union show identical staining-reactions. 2 A very similar series of facts has been observed in the germ-nuclei of plants by Strasburger (p. 220). These and many other facts of like import demonstrate that the chemical differences between the germ-nuclei are not of a fundamental but only of a secondary character. They are doubtless connected with the very different character of the metabolic processes that occur in the history of the two germ-cells ; and the difference of the staining-reactioh is probably due to the fact that the sperm-chromatin contains a higher percentage of nucleinic acid, while the egg-chromatin is a nuclein containing a much higher percentage of albumin.
1 By Flemming's triple method. * '92, p. 492.
Literature
Ballowitz, S. — Untersuchungen iiber die Struktur der Spermatozoen : i. (birds)
Arch. mik. Ana/., XXXII. 1888; 2. (insects) Zeitschr. wiss. Zool., L. 1890;
3. (fishes , amphibia, reptiles) Arch. mik. Anat., XXXVI. 1890; 4. (mam mals) Zeit. wiss. Zool., LI I. 1891. Belajeff, W. — Uber die Centrosomen in den spermatogenen Zellen: Ber. d.
deutsch. bot. Ges., XVII., 6. 1899. Boveri, Th. — ITber Differenzierung der Zellkerne wahrend der Furchung des Eies
von Ascaris meg.: Anat. Anz. 1887. Id. — Die Entwicklung von Ascaris megalocephala mit besonderer Riicksicht auf die
Kernverhaltnisse : Festschr. fur C. v. Kupffer. Jena* 1899. Brnnn, M. von. — Beitrage zur Kenntniss der Samenkorper und ihrer Entwickelung
bei Vogeln und Saugethieren : Arch. mik. Anat., XXXIII. 1889. Hacker, V. — Die Eibildung bei Cyclops und Camptocanthus : Zool. Jahrb., V.
1892. (See also List V.) Hermann, F. — Urogenitalsystem : Struktur und Histiogenese der Spermatozoen :
Merkel und Bonnets Ergebnisse, II. 1 892 . Ikeno, S. — Untersuchungen liber die Entwickelung der Geschechtsorgane, etc., bei
Cycas : Jahrb. wiss. Bot., XXXII., 4. 1898. Kdlliker, A. — Beitrage zur Kenntniss der Geschlechtsverhaltnisse und der Samen fliissigkeit wirbelloser Tiere. Berlin, 1841. Leydig, Fr. — Beitrage zur Kenntniss des thierischen Eies im unbefruchteten Zu stande: Zool. Jahrb., III. 1889. Meves, F. — ITber die Entwicklung der mannlichen Geschechtszellen von Salaman dra : Arch. mik. Anat., XLVIII. 1896. Id. — tfber Struktur und Histogenese der Samenfaden des Meerschweinchens :
Arch. mik. Anat., LIV. 1899. Schweigger-Seidel, F. — Uber die Samenkorperchen und ihre Entwicklung : Arch.
mik. Anat., I. 1865. Straaburger, E. — Histologische Beitrage; Heft IV.: Das Verhalten des Pollens
und die Befruchtungsvorgange bei den Gymnospermen, Schwarmsporen, pflanz liche Spermatozoiden und das Wesen der Befruchtung. Fischer, Jena, 1892. Thomson, Allen. — Article " Ovum, 1 ' in Todd's Cyclopedia of Anatomy and Physiology. 1859. Van Beneden, E. — Recherches sur la composition et la signification de Toeuf : Mem.
cour. de VAcad. roy. de Belgique. 1870. Waldeyer, W. — Eierstock und Ei. Leipzig, 1870. Id. — Bau und Entwickelung der Samenfaden : Verh. d. Anat. Ges. Leipzig, 1887.
1 See also Literature, V., p. 287.
| Historic Disclaimer - information about historic embryology pages |
|---|
|
Wilson EB. The Cell in Development and Inheritance. Second edition (1900) New York, 1900.
Cite this page: Hill, M.A. (2024, April 25) Embryology The cell in development and inheritance (1900) 3. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/The_cell_in_development_and_inheritance_(1900)_3
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
