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a great deal depends on how we regard the large subgerminal yolk- 
cavity and the breaking through of the invagination cavity or archen- 
teron. I agree with Van BENEDEN and WENCKEBACH in regarding the 
large subgerminal yolk-cavity as an intercellular space. Although 
it is a cavity continuous with the archenteron and should be regarded 
as a part of the latter, it is a fluid cavity arisen from physiological 
necessity (probably of having the nutritive material in a liquid form) 
and having a comparatively little morphological significance. Whether 
the lower half of the egg is formed by cells charged with nutritive 
material as in the Amphibia or by a large yolk-reservoir composed of 
cells, yolk-spheres, and nutritive fluid as in the Reptilia is of little 
moment, so far as morphology is concerned, and ought not to stand 
in the way of homologizing the two. The breaking through of the 
floor of the archenteron may be regarded in the following way: There 
is a large yolk reservoir which, let us suppose, is surrounded by a 
layer of cells (although in point of fact, the lower pole is not enclos- 
ed in until quite a late period). Then independently of it there 
arises an invagination cavity which judging from its destiny is the 
archenteron. Non from the nature of the thing, the yolk-bag is an 
appendage of the archenteron, as can be easily recognized in later 
stage and ought to be in close connection with latter. But from. 
its large size, and from the fact that it is present before the archen- 
teron is formed, the yolk-bag can come into connection with the 
archenteron only secondarily. This is done by a part of the floor 
of the archenteron splitting open, so to speak, and the split edges 
becoming continuous with the cell-layer surrounding the yolk - bag. 
When the definitive alimentary canal is formed, these open edges are 
again tucked in, until only the vitelline duct is left. The cell-layer. 
enclosing the yolk i. e. the primitive lower layer becomes continuous 
with the definitive hypoblast, and I regard it with HuBREcHT as the 
caenogenitic hypoblast arisen by precocious segregation. While 
my facts agree very closely with those brought out by VAN BENEDEN 
in Mammalia, I find myself unable to accept his theory of Lecitho- 
phor and Blastophor. 
According to the views set forth above, the accumulation of the 
yolk has profoundly affected the course of development especially in 
the precocious development of a part of the hypoblast and in the 
early spreading of the blastoderm over the surface of the yolk. But 
in the center of the blastoderm, a certain amount of undifferentiated 
materials is left in the shape of the primitive knob in order to per- 
form certain developmental processes of the palingenetic character: 
