632 H. GRAHAM CANNON 
cannot be stated with certainty, but it is thought very probable 
that they arise from cells originally lying round the genital 
rudiment which pass inwards on the inside of the blastoderm, 
as will be described below. 
5. ForRMATION OF THE GERM-LAYERS. 
The first sign of differentiation of the blastoderm is the 
appearance of a group of cells—more vacuolated than the 
rest—on one side of the embryo which subsequently proves 
to be the ventral side. These cells contain a large amount of 
yolk, and in their earliest stages their nuclei are very obscure. 
They will be called collectively the ‘ Ventral Mass’ (fig. 1). 
When cleavage is complete each blastomere consists of an 
inner yolky part and an outer non-yolky part. In their very 
earliest stages the cells of the ventral mass are completely 
pervaded by yolk and so are conspicuous by not showing the 
outer non-yolky zone. Soon a few of these cells pass inwards, 
so that the ventral mass becomes a small heap of vacuolated 
yolky cells on one side of the embryo, but as yet shows no 
further sign of differentiation. 
The cells of the ventral mass on one side, which is seen later 
to be the anterior side, now proliferate and form a mass of 
yolky cells whose protoplasm stains comparatively deeply 
(fig. 2). The compactness of these cells and the distinct manner 
in which they are marked off from each other indicate that 
their protoplasm has a greater surface-tension than that of 
the cells of the remainder of the ventral mass. The nuclei 
of these cells, which are now becoming distinct, are large 
compared with those of the blastoderm cells—approximately 
twice as large. Their nucleoli are distinct and stain deeply. 
Behind these cells, that is at the posterior part of the ventral 
mass, are a few cells which still form a single layer. They are 
very much vacuolated and contain a large amount of yolk. 
Their protoplasm does not stain at all deeply and the cells are 
not at all compact. At first their nuclei are not distinct, as 
with the remainder of the cells of the ventral mass, but soon 
these become quite clear and show very marked characteristics. 
