276 MR. F, E. BEDDARD ON THE [May 4, 
whether it is really structureless 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 (Joc. cit. pl. xxviii. tig. 7 f, pl. 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 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 differentiation of the outer layer of the egg- 
protoplasm, and is subsequently perforated by processes of the 
follicular cells as the ovura 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 (Joc. cit. 
pl. 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. 
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 follicular cells *. 
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 (Joc. cit. pl. 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, 
156) a formation of cells from the germinal yesicle 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, 
some 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 many insects’ ova other observers have noted similar processes. 
