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Show 276 MR. F. E. BEDDARD ON THE [May 4, whether it is really etrnctureless or radiately striate, are possibly to be explained by supposing that, as in Lepidosiren, it is structureless at one time and striate at another. So also with the observations of Brock upon the Teleostean ovum. This anatomist figures (loc. cit. pi. xxviii. tig. 7f, pi. xxix.fig. 6 Be) two ova, both of which are provided with two egg-membranes : in the one the outer egg-membrane is striate, while in the other it is unstriate ; in the latter case it is considerably thinner than in the former, and the processes of the follicular cells, whether by accident or design, are drawn as if stopping short at the outside of the membrane. Brock's figures seem to me to point to the conclusion that the outer egg-membrane (vitelline membrane) arises, as in Lepidosiren, as a differentiation of the outer layer of the egg-protoplasm, and is subsequently perforated by processes of the follicular cells as the ovum approaches maturity. On the other hand, the perforations in the zona radiata would seem in this case to be caused, as suggested above, by protoplasmic strands radiating outwards from the ovum. The observations of Owsiannikow appear to contradict this explanation. Owsiannikow has figured (loc. cit. pi. i. fig. 3) the branching of the processes of the follicular cells after they have passed through the outer membrane, in which the pores are wider apart, to fit into the more closely arranged pores of the inner membrane. III. History of the Germinal Vesicle. M y observations on the development of the germinal vesicle are unfortunately rather meagre. I have been unable to trace it continuously from the youngest to the most mature ova. The principal lacuna in the history of the germinal vesicle is the very interesting stage characterized by the immigration of the follicular cells into the substance of the ovum. In this stage I could observe no appearance whatever of a germinal vesicle, which is much to be regretted, as it would be interesting to know what are its relations to the immigrating follicular cells 1. 1 observed two distinct stages in the development of the germinal vesicle which will now be described. The first stage is found in young ova, in which the yolk-formation has not yet begun. The germinal vesicle is of an oval form, and lies excentrically within the ovum ; it is bounded externally by a very fine membrane in Ceratodus, according to Ayers's figure (loc. cit. pi. xviii. fig. 76). In my own preparations (Plate XXIX. figs. 3, 4) the limiting ' Dr. Balbiani has described in the egg of Geophilus (Zool. Anzeig. Nos. 155, 15G) a formation of cells from the germinal vesicle which pass through the substance of the ovum, and ultimately range themselves round its periphery to forma follicular epithelium. These stages are depicted in aseries of woodcuts, rome of which bear not a little resemblance to the stage in the maturation of the ovum of Lepidosiren described above. It is possible that the appearances described by Balbiani may ultimately be reconciled with those described by myself in the present paper. Balbiani's observations have not, however, been confirmed by the recent researches of Heathcote (Quart. Journ. Micr. Sci. 1886), though in m a n y insects' ova other observers have noted similar processes. |