276 MR. F. E. BEDDARD ON THE [May 4, 



whether it is really Btrnctnreless or radiately striate, are possibly 

 to be explained by supposing that, as in Lepidosiren, it is structure- 

 less at one time and striate at another. 



So also with the observations of Brock upon the Teleostean ovum. 

 This anatomist figures (be. cit. pi. xxviii. tig. 7/, pi. xxix. fig. 6 B e) 

 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 acci- 

 dent or design, are drawn as if stopping short at the outside of 

 the membrane. Brock's figures seem to me to point to the con- 

 clusion that the outer egg- membrane (vitelline membrane) arises, as 

 in Lepidosiren, as a diflerintiation of the outer layer of the egg- 

 protoplasm, and is subst'Cjaently 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. 1 he 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 Germijial Vesicle. 



My observations on the development of the germinal vesicle are 

 unfortunately rather meagre. I have been unable to trace it con- 

 tinuously 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 foUicular cells \ 



1 observed two distinct stages in the development of the germiniil 

 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 clescribed in the egg of Gcophibis (Zool. Auzeig. Nos. 155, 

 156) a formation of cells from the germinal vesicle which pa^s 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, 

 pome of which bear not a little resemblance to the stage in the maturation of 

 the ovum of Lepidosiren described above. It is po.ssible that the appearances 

 described by Balbiani may ultimately be reconciled with those described by 

 myself in the present paper. Balbiaui's observations have not, however, been 

 confirmed by the recent researches of Heathcote (Quart. Journ. Micr. Sci. 1886), 

 tliQUgh in many insects' ova other observers have noted similar processes. 



