GERMINAL MEMBRANE. 63 
as a portion of the germinal membrane separating from the 
whole by a constriction. Both layers contribute to its forma- 
tion, and it therefore consists of small transparent cells, some 
of which (probably those pertaining to the mucous layer) con- 
tain no nucleus, whilst others (those derived from the serous 
layer) exhibit the characteristic cell-nucleus with its nucleoli. 
In addition to these cells it contains a great many nuclei, 
around which no cells have as yet formed. Between the 
two layers of the germinal membrane other cells arise, which 
may be regarded as representing a third layer, the vascular, 
although they do not really form a connected independent 
layer; of these we shall treat hereafter. These three layers 
then, and pre-eminently the first two, form the mediate basis 
of all the subsequent tissues. 
The yelk is not a lifeless aliment for the embryo,—as it is 
when taken as food by the adult, to whose organism it is dead 
and must be chemically dissolved,—but the cells of the yelk 
take part in the vitality called forth by incubation. They 
effect an alteration in their contents, whereby the albumen 
which they contain loses its property of coagulating, and the 
granules become dissolved, in the same manner in which the 
granules of starch dissolve in the cells of the vegetable embryo. 
In short, the yelk bears the same relation to the embryo as 
regards its nutritive property, that the albumen bears to the 
vegetable embryo. 
In accordance with the analogy between the cells we are 
treating of and those of vegetables, all the changes in the egg, 
the growth of the germinal membrane, and even the first forma- 
tion of the embryo, proceed entirely without vessels. 
