OCR Text |
Show 1886.] STRUCTURE OF THE OVUM IN THE DIPNOI. 523 one of the cells, which compose the central mass of the follicle, acquiring a predominance in size over its neighbours or being differentiated in any other way. The fact of there being two kinds of ova with a different mode of development is not new to the Vertebrata. In m y former paper I have compared the follicle and its contents in Protopterus to the " egg-nest" of Elasmobranchs, the points of difference being perhaps on the whole greater than the points of resemblance. But, since the formation of " egg-nests " is so general among the Vertebrata, it seems to m e that there is probably some genetic connection between these structures and the "egg-nest" of Protopterus. It has been shown that in Mammals, Elasmobranchs, and Reptiles the permanent ova are formed in two ways:-either (1) by the direct development of one of the primitive ova, which surrounds itself with a follicular layer derived from the ordinary undifferentiated germinal cells; (2) a number of primitive ova coalesce together to form a nest; their nuclei multiply, and some atrophy, serving as pabulum for a limited number which subsequently separate off, accompanied by some of the undifferentiated germinal cells, to form as many ova. These two modes of development are not regarded by Balfour as morphologically very different; the latter mode of development has been brought about to secure the adequate nourishment of a certain number of cells which form the permanent ova. Balfour's l observations certainly do not show any morphological difference between the ova produced in these two different ways; in both cases the ovum is the equivalent of a single cell; but the physiological difference is considerable. I have referred above (p. 512) to the presence of two kinds of cells among the central cells, distinguishable by the characters of their nuclei; in some the nucleus was rather larger and more oval in form than in others, where it was smaller and rounder, and, in fact, exactly like the nucleus of a follicular cell. It is possible that the cells with larger nuclei correspond to primitive ova and the smaller to the ordinary germinal cells ; in this case the resemblance of the central mass of cells to the egg-nest of the Elasmobranch will be diminished ; against this supposition is the fact that there are nuclei of intermediate size, but these may have been produced by a recent division of some of the other nuclei. Judging from analogy, however, it is probable that some of the cells of the germinal epithelium are specialized into primitive ova from undifferentiated germinal cells, although in patches of germinal epithelium covering the ovary I failed to detect any such specialization in the nuclei. The important facts to be borne in mind in comparing the egg-nest of the Elasmobranch with that of the Dipnoi2 appear to m e to be the early formation of the complicated follicular layers in the latter and the early commencement of yolk-secretion. The germinal cells being 1 Balfour, loc. cit. 2 It is important to remember that the formation of "nests" is not confined in Elasmobranchs to tbe embryonic period (see Balfour, loc. cit. p. 415). |