54 THE OVUM AND 
by a very transparent, perfectly structureless membrane, which 
represents a closed cell-membrane, having as little connexion 
with the ovary as with the layers of cells, and which is deno- 
minated vitelline membrane. It is as readily separated from 
the ovary as from the layer of cells, the latter, therefore, cannot 
be merely its epithelium. 
If we now proceed to examine larger eggs from the ovary, 
such, for instance, as have attained a diameter of half an inch 
or more, and are already yellow-coloured, on their being divided 
across the centre under water, a white substance, the yelk- 
cavity, will be found in their interior. This cavity contaims 
those cells, now in a higher stage of development, which in the 
first instance alone formed the contents of the egg. Around 
these a stratum of yellow substance, the proper yelk-substance, 
appears, and round this again lies the layer of cells. Globules 
may be recognised in the proper yelk-substance with the aid of 
the microscope, as in the same substance of the mature yelk. 
These globules, then, have been formed between the yelk-cavity 
and the layer of cells. The question, however, arises how 
this has been effected? The following may be supposed to be 
the mode of their production :—the innermost portion of the 
yelk, the yelk-cavity, is the part which is first formed, the 
innermost yelk-globules are therefore also the oldest, and the 
formation of the new yelk-globules takes place externally upon 
the internal surface of the layer of cells. Ifa small portion 
of the layer of cells be so placed under the microscope that the 
inner surface becomes turned towards the eye, and a spot be 
sought for at which a thin layer of yelk-substance is attached 
to it, it will be seen that the yelk-globules do actually become 
smaller in the proximity of the layer of cells, whilst in other re- 
spects they retain their general appearance. The smallestof them, 
which le immediately upon the inner surface of the layer of 
cells, are even smaller than the cells of the layer itself. It is 
therefore extremely probable, that the formation of new yelk- 
globules takes place on the inner surface of the layer of cells, 
and that the globules then expand to their normal size some- 
what quickly, for the stratum of small ones is but thin. Mean- 
while new ones continue to form externally, until the yelk has 
reached its normal size. The formation of the canal leading 
from the yelk-cavity to the germinal vesicle may also be ex- 
