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 alreadv -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 contains 

 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. If a 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 velk-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 smallest of them, 

 which lie 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- 



