Fig. 73. Diagrammatic longitudinal section of a
hen's egg before incubation, after Allen Thomson.
Bl, germinal disk. GD, yellow yolk. WD, white yolk.
DM, vitelline membrane. EW, albumen. Ch, chalaza.
S, shell membrane. KS, shell. LR, air chamber.
We now understand why the eggs of many animals should be of such enormous size and often of such complex structure. The eggs of birds are especially remarkable in this respect, and it has till recently been disputed whether they are really morphologically equivalent to a single cell. But this is undoubtedly the case, and though only the small thin germinal disk (Fig. 73, Bl) with its nucleus is the active part of this cell—the cell-body proper—yet all the rest—the enormous sphere of yolk with its regular layers of yellow (GD) and white (WD) yolk, the concentric layers of fluid albumen (EW) round about this, the chalazæ (Ch), and finally, the delicate shell membrane (S) and the limy shell (KS)—belong to this cell, and have arisen in connexion with it (Fig. 73).
LECTURE XV
THE PROCESS OF FERTILIZATION
Cell-division and nuclear division—The chromatin as the material basis of inheritance—The rôle of the centrosphere in the mechanism of division—The Chromosomes—Fertilization of the egg of the sea-urchin according to Hertwig—Of the egg of Ascaris according to Van Beneden—The directive divisions, or the extrusion of the polar bodies—Halving of the number of chromosomes—The same in the sperm-cell—Reducing division in parthenogenetic eggs—In the bee—Exceptional and artificial parthenogenesis—Rôle of the centrosphere in fertilization and in parthenogenesis.
Now that we have made ourselves acquainted with the two kinds of germ-cells on the union of which 'sexual reproduction' depends, we may proceed to a more detailed discussion of the process of fertilization itself. But it is indispensable that we should take account of the processes of nuclear and cell-division, as these have been gradually recognized and understood in the course of the last decade. It may appear strange that the processes of division should throw light on the apparently opposite processes of cell-union, but it is the case, and no understanding of the latter is possible without a knowledge of the former.
From the time of the discovery of the cell until well on in the sixties the process of cell-division was looked on as a perfectly simple process, as a mere constriction in the middle of the cell. It was observed that a cell in the act of dividing ([Fig. 59], A) stretched itself out, that its nucleus also became longer, became thinner in the middle, assumed a dumb-bell form, and was then gradually constricted, giving rise to two nuclei ([B]), whereupon the body of the cell also constricted and the two daughter-cells were formed ([C]). In certain worn-out or highly differentiated cells a cell-division of this kind really seems to occur—the so-called 'direct' division—but in young cells, and indeed in all vigorous cells, the process, which looks simple, is, in reality, exceedingly complex. Not only is the structure of the nucleus incomparably more complex than was recognized a quarter of a century ago, but nature has placed within the cell a special and marvellously intricate apparatus, by means of which the component parts of the nucleus are divided between the two daughter-nuclei.
For a long time all that was distinguished in the cell-nucleus was the nuclear membrane and a fluid content in which one or more nuclear bodies or nucleoli float. But this does not by any means exhaust what can now be recognized in the structure of the nucleus, and the most important constituents are not even among these, for recent researches, especially those of Häcker, have shown that the nucleolus or the nucleoli, to which there was formerly an inclination to attach a very high importance, must be regarded as only transient formations and not living elements—in fact, as mere collections of organic substance—'bye-products of the metabolism,' which at a definite time, that is just before the division of the nucleus, disappear from the nuclear space and are used up. We now know that in the resting cell, that is, in the cell which is not in the act of dividing ([Fig. 74], A), a very fine network of pale threads, often very difficult to make visible, fills the whole nuclear cavity, like a spider's web or the finest soap bubbles, and that in this so-called nuclear framework there are embedded granules of rounded or angular form (A, chr) which consist of a substance which stains deeply with such pigments as carmine, hæmatoxylin, all aniline dyes, &c., and which has therefore received the name of chromatin. Often, indeed generally, these granules are exceedingly small, but sometimes they are bigger, and in that case they are less numerous and more easily made visible; in all cases, however, they are in a certain sense the most important part of the nucleus, for we must assume that it is their influence which determines the nature of the cell, which, so to speak, impresses it with the specific stamp, and makes the young cell a muscle-cell or a nerve-cell, which even gives the germ-cell the power of producing, by continued multiplication through division, a whole multicellular organism of a particular structure and definite differentiation, in short, a new individual of the particular species to which the parents belong. We call the substance of which these chromatin granules consist by the name first introduced into science by Nägeli, though only to designate a postulated substance which had not at that time been observed, but which he imagined to be contained within the cell-body—by the name Idioplasm, that is to say, a living substance determining the individual nature (εἶδος = form). I am anticipating here, and I reserve a more detailed explanation until I can gradually bring together all the facts which justify the conception I have just indicated of the 'chromatin grains' as an 'idioplasm,' or, as we may also call it, a 'hereditary substance.'