Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/433

ANIMAL.] KEPKODUCTION 415 pseudopodial origin and the nutritive function are equally obvious. The resemblance of such stages to definite Protozoan types is not a little remarkable; thus, while, in fig. 8, b is perfectly amoeboid, the resemblance of c to a Heliozoon or of d to a Gregarine is almost equally striking. The ovum is rarely destitute of egg-membranes, but these may be of very various kinds : thus we may have a vitelline membrane proper, formed by the protoplasm of the ovum, or a chorion formed by the cells of the follicle ; or secondary membranes may be present (alone or in addi- tion to the primary membrane) like the shell of a Bird's egg, which is formed by the walls of the oviduct, or the shell of many Trematode ova, which is secreted by a special gland. Any or all of these membranes may be provided with a special aperture, the micropyle, first discovered by Keber, but this is by no means universally present, as he supposed. This may correspond to the point of attachment of the immature ovum, or may arise elsewhere ; in the first case its function is obviously nutritive, though later it may also serve for the entrance of the spermatozoon. The identification of the ovum as a cell, and of its germinal vesicle and spot as nucleus and nucleolus, although a result only established after prolonged contro- versy, and of capital importance, is sufficiently familiar. The invaluable labours of the older generation of embryo- logists from Von Baer to Allen Thomson, and even the still classical monographs of such recent workers as Waldeyer or Ludwig (1874), can only be alluded to; this department of the subject is, however, of peculiarly easy access, thanks to the exceptionally excellent state of its bibliography, and to the recent discussions of Balfour, Hensen, and others. Much, however, remains to be ascer- tained respecting the finer histology of the ovum, and many investigations are at present in active progress, along the lines of that more general inquiry into the minute structure of cells in general which has of recent years been again becoming of paramount interest in morpho- logical research. The protoplasm of the ovum may, as has been said, acquire a very varying quantity of food yolk, may become, that is to say, more or less closely packed with highly refracting spherules of modified protoplasm (see egg of Hydra, fig. 8, &), which may again present various morphological differentiations, as in Ascaris (fig. 11). And, as explained below under segmentation, it is with regard to the presence, amount, and position of the food- yolk that the important varieties of that process are to be understood. The great differentiation both of protoplasm and of yolk in the animal series, as might be expected, appears to exhibit all gradations from the most simple amoeboid state of a more or less granulated or semi-fluid mass to the most problematical complexity. Of this the best known instance is probably that of the egg of Ascaris, described by Van Beneden (vide infra, pp. 416 sq.). Not only has a protoplasmic network been frequently described" in both holoblastic and meroblastic ova, but a radiate struc- ture as well,' the former evidently corresponding to the stroma first described by Frommann and Heitzmann and subsequently by so many authors in both animal and vegetable cells, while the latter appearance recalls the striated appearance of the ectoplasm of certain amoeboid organisms described by Strasburger. The concentric differentiation of the ovum has also often been described, and is lately well discussed by Flemming, Brass, and others. Thus Pfliiger, in the half-ripe ovum of the Cat, describes the central mass as clear, the cortical layer as rich in granules; Van Beneden in the egg of the Bat enumerates two similar layers surrounded by an almost granular cortical layer ; while Flemming himself, in the ovum of the Rabbit, describes a coarsely granular region around the nucleus, a clearer central region finely granular and with a reticulated structure, and a coarsely granular cortical region. We have still, however, to learn how far such differentiations in structure reticulate, radial, and concentric are constant for individual or general cases, and how far they may be permanent or merely incidental to certain phases of development. In these regards the recent publications of Carnoy and Brass are of special interest the former on account of its minuteness of micrographic detail, the latter in its attempt at physiological interpretation. The results of the former will be best understood from his own figures (fig. 9). FIG. 9 (after Carnoy). A, two cells from the leaf parenchyma of Allium Cepa, showing on the right the nucleus uninjured, with its nuclear network and two nucleoli, and on the left a nucleus in which the razor has unrolled or dragged away the coiled filament of nuclein, but left a protoplasmic reticulum with nucleolus; B, intestinal epithelium cell of an Insect, showing in the proto- plasm a radiating reticulum with granular semifluid contents, and in the nucleus a similar structure, but with a convoluted nuclein filament which has shrunk together ; C, cell of the larval genital gland of a parasitic Fly, show- ing various forms of protoplasmic network formed in different cells, and in the nucleus the same distinctness of its finely granular protoplasmic reticulum and its contained striated nuclein filament; D, young ovum of beetle, in which nucltin filament has broken down into spherules. Brass, starting from the familiar structure of an amoeba, with its clear and granular ectoplasm, from which the pseudopodia are emitted, its semi-fluid and highly granular endoplasm around the nucleus, and the less granular intermediate zone, assigns to these definite physiological functions, to the first that of contractility (Bewegungs- plasma), to the second that of nutrition (Emdhrungs- plasma), and to the third that mainly of respiration (Athmungsplasma, Nahrungsschichf). He holds that some such concentric disposition of the protoplasm is a normal and constant fact of cell-structure, and insists upon it with special reference to the ovum. From his somewhat vague and diffuse development of these views, it must suffice here to note his opinion that the chromatin of the nucleus, as well as the protoplasmic reticulum, and in fact all the former constituents of the cell, are of quite secondary importance to the colourless protoplasm. The former is to him in fact no more than reserve material, while to the latter he assigns all active functions, thus' substantially reviving the view so long and energetically maintained by Beale. The recent observations of Wielowiejski may also be noted in this connexion, as he not only brings his con- tribution to the ontogeny of the ovum, but to some extent also distinguishes a regional adaptation of its protoplasmic structure -to its functions. From the rapid succession of new contributions to the solution of the problem of egg and cell structure might be gathered many other points of interest, morphological and physiological, empirical and speculative. Thus, for in- stance, Sabatier describes senile degeneration in Ascidian ova, an observation of wide suggestiveness ; while, again, on the important problem of the relation of nucleus to protoplasm there are many recent discussions, e.g., from