62 THE OVUM AND 



which are the subsequent cells of the proper yelk-substance. 

 They are formed round about the •neighbourhood of the vitelline 

 membrane, with the exception of that spot where the germ- 

 vesicle and the rudiments of the germinal membrane lie. These 

 cells expand very rapidly, while at the same time a new layer 

 is formed on the outside of them, and so on successively. In 

 this manner they surround the white cells of the yelk-cavity 

 with a layer of yellow cells, which is constantly increasing in 

 thickness ; as, however, a vacant space remains at the spot where 

 the germinal vesicle and germinal membrane are situated, by 

 the increasing thickness of the yelk-substance, the space be- 

 comes converted into a canal. The development of the vitelline 

 membrane proceeds continuously with these changes, in pro- 

 portion as the increasing contents require. When the yelk- 

 cell has attained its due size and the egg leaves the ovary, the 

 germ-vesicle, like most other cell-nuclei, disappears, and the now 

 more fully developed germinal membrane remains. It is made 

 up of globules, probably cells, having coarsely-granulated con- 

 tents. It grows during the process of incubation by the con- 

 tinual development of new cells. After sixteen hourV incuba- 

 tion, a distinction may be observed in the cells composing the 

 membrane. The more external ones form a layer, in which the 

 cells exhibit a nucleus of the characteristic form, and contain 

 a quantity of transparent fluid and minute isolated granules. 

 These cells are therefore clear, and firmly united together, and 

 have only a minimum of intercellular substance between them ; 

 they represent the serous layer of the germinal membrane. The 

 under stratum of the germinal membrane or mucous layer con- 

 tains cells of another kind ; they have no nucleus of the cha- 

 racteristic form, but contain one or more dark globules, and 

 frequently also some minutely granulous substance. These cells 

 lie loosely together in a larger quantity of intercellular sub- 

 stance, which contains smaller granules of different kinds, in 

 addition. When this division of the membrane into the 'two 

 layers is completed, and its superficies has become considerably 

 extended, and after a transparent spot, the area pellucida, has 

 formed in its centre— (the cells of the mucous layer in this 

 area being much smaller, but of pretty equal size, as com- 

 pared with one another, and having transparent contents with 

 very minute isolated granules),— the embryo is developed, 



