260 EXPERIMENTS AND OBSERVATIONS 
ovary from an immature fish—for instance, a plaice 73 inches long 
killed in March—the youngest ova are seen beneath the surface of 
the germinal lamelle. The lamelle are covered by a very thin 
membrane containing nuclei. ‘This membrane represents what is 
known as the germinal epithelium, but it is so thin that it is diffi- 
cult to see much of it in transverse sections. The smallest ova are 
rounded, and their germinal vesicle contains one large nucleolus, 
whose diameter is usually equal to half the diameter of the whole 
vesicle. This nucleolus is solid and homogeneous, and is darkly 
stained by staining liquids. In the ova of many animals—for ex- 
ample, Ascidians, Hchinoderms, Molluscs—the germinal vesicle 
remains in this condition, having one large nucleolus until the 
egg is ripe. In the fish, as the egg gets larger, instead of one a 
number of nucleoli are developed, which, at first scattered through 
the reticulum of the vesicle, are subsequently arranged in a single 
layer in the external region just within the limiting membrane. 
None of the numerous nucleoli are as large as the original single one, 
and I believe that the latter subdivides. But I do not think all 
the new nucleoli are produced by subdivision of the original one, 
because in the younger and smaller ova there are to be seen very 
minute nucleolar granules together with a large undiminished 
nucleolus, so that it is to be inferred that many of the additional 
nucleoli are produced by the increase in size of these minute granules. 
In very young fish none of the ova contain a vitelline nucleus ; 
for instance, in sections from a plaice 3 inches long, killed in March 
and probably a year old, it is not to be seen. In these sections the 
largest ova have a diameter of ‘(07 mm. The production of young 
ova in this ovary was evidently going on rapidly ; nests of them 
are present at numerous spots in contact with the germinal epi- 
thelium. But even at this early stage nuclear division figures are 
not to be seen, nor have I detected such figures in sections of the 
ovary at any stage. It may be that even in the young ovary the 
youngest ova seen have passed beyond the division stage and 
entered upon the period of growth, and that division only takes 
place in the flat germ-cells of the epithelium, and cannot be seen in 
sections. But even when examining the germinal epithelium from 
the surface I have seen no division figures, nor am I aware that 
other observers have described any. It is a point which requires 
further investigation. 
In sections from a plaice 74 inches long, fixed with chromic and 
osmic acids, the vitelline nucleus is distinctly visible in the larger 
ova, whose diameter has a maximum of ‘14mm. The appearance 
of the nucleus is quite different from that of the nucleoli in the 
germinal vesicle ; its outline is not so definite, and it is seen to be a 
