OCR Text |
Show 280 MR. F. E. BEDDARD ON THE [May 4, It seems to me to be fairly certain that the ova represented in Plate XXVIII. figs. 1, 2, 4, are progressive stages in egg-development: there is a gradual increase of size accompanied by various changes in the egg-contents, the germinal vesicle, and the egg-membranes, which all tend to prove the truth of this supposition. All these different stages were frequently to be observed in a single section. In the same sections were a number of very peculiar ova, displayed in Plate X X V I I I . fig. 3 ; these were for the most part intermediate in size between ova of the third stage (fig. 2) and the more mature ova (fig. 4 ) ; in a few cases, however, they were distinctly smaller than ova of the third stage. This last fact would be hardly sufficient of itself to prove that these ova do not form a stage intercalated between Stages 3 and 4, because there is often some irregularity in size ; larger ova, particularly if they are not much larger, could not be positively regarded as more mature than smaller ova. In the first place, however, the structure of these ova and the conditions of the follicular epithelium are so markedly different from ova of any other stage, that it stems difficult to assign them to the same series ; and, in the second place, I have been able to observe no transitional stages between these ova and those belonging to Stages 2 or 3. On the other hand, these ova eventually acquire the same appearance as ova that have evidently passed through Stages 2 and 3 ; to this point I shall return later. I must for the present leave it an open question as to whether these ova form part of the same series as those displayed in Plate XXVIII. figs. 1, 2, 4, or whether the ovary of Protopterus contains ova of two kinds which follow a different course of development though they ultimately come to be identical in appearance. These ova (Plate XXVIII. fig. 3) are surrounded by a follicular epithelium which, like that of the other ova, consists of a single layer of cells; these cells are long and somewhat columnar in form (Plate X X I X . fig. 7 ) ; they are limited by a distinct membrane and contain a darkly stained nucleus as well as a number of hiyhly refracting round particles which resemble in every respect the yolk-spherules of the contained ovum. These follicular cells rest upon the surface of the ovum, and there is no trace whatever of any membrane separating them from the ovum. The connective-tissue sheath of the follicle was very distinctly marked off from the surrounding ovarian stroma ; near to the periphery of the ovum were an immense number of blood-capillaries of large size and gorged with blood. These capillaries, although probably belonging to the stroma-sheath, were in many cases pressed down among the epithelial cells of the follicle, appearing as if they were actually lying between the cells; this remarkable appearance of the blood-vessels is illustrated in Plate X X I X . fig. 7, which represents a portion of the periphery of such an ovum very highly magnified. The interior of the ovum contained abundant yolk ; but in no case did I succeed in finding any trace of a germinal vesicle. The yolk consisted of highly refracting spherules which were for the most part of small size, but here and there were very much larger spherules. |