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 hours’ 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 imtercellular 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, 
