DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 415 
of epiblast formation over the surface of the ventral hypoblast 
is in some way associated with the increased amount of food 
material deposited in the ovum. 
The archenteron in the Cephalochorda, Elasmobranchs, 
Cyclostomes, Teleosteans, Ganoids, and Amphibians is from 
the first in direct connection with the exterior by an opening, 
at the margins of which the epiblast and hypoblast are con- 
tinuous; this is the true blastopore, which becomes occluded 
by the fusion of its margins, either from before backwards or 
from behind forwards, giving rise to a primitive streak in all 
the orders mentioned except the Cephalochords (40). Cun- 
ningham (8) and Rabl (87) have already drawn attention to 
the fact that increase in the posterior part of the ventral 
hypoblast splits the ventral lip of the blastopore, and carries 
the two halves outwards and forwards, the dorsal lip remain- 
ing as a fixed point; as the proliferating epiblast extends 
the opening is re-formed from the dorsal to the ventral side, the 
ventral portion of the blastoporic rim being the last part com- 
pleted. Ifthe increase of the ventral hypoblast takes place 
more anteriorly a “yolk hernia” results; that is, the forma- 
tion of the epiblast is interfered with a short distance in front 
of the ventral lip of the blastopore, and it is not difficult to 
understand how, by the subsequent increase of the more pos- 
teriorly situated ventral hypoblast, such a yolk hernia might 
be prolonged backwards until it became continuous with the 
blastopore. In a case of this kind the process of completion 
of the epiblastic layer would follow in the reverse way, and the 
posterior lip of the blastopore would be completed before the 
“‘ volk hernia” was coveredin. Apparently the Elasmobranchii 
present us with examples of a process of this kind in its 
simplest form. The supposititious opening in the epiblast, 
through which the yolk hernia projects, is the yolk blasto- 
pore, and at its margins the epiblast and yolk are in close 
contact, if not absolutely fused. 
In lizards and birds there is also a yolk blastopore which at 
its origin is apparently situated further forward than in the 
Elasmobranchs, but as in them, by the great increase of the 
