416 ARTHUR ROBINSON. 
yolk, it has been extended back into the true blastopore. 
We may imagine that the true blastopore in lizards and birds 
has been split, as compared with the opening of the yolk 
blastopore, at a comparatively late period; therefore its 
margins are not so far apart as the margins of the yolk blasto- 
pore, and as the epiblast extends they are swung back into 
position long before the yolk blastopore is closed. 
In all the Vertebrata below the Sauropsida and mammals the 
fusion of the hypoblast and epiblast in the margins of the 
true blastopore remains during the closure of that orifice and 
for some time after. 
In lizards and birds there is an early separation of the two 
primitive layers in the region of the true blastopore; the 
separation is, however, only temporary, and before the yolk 
blastopore is closed the two layers have reunited in the primi- 
tive streak, which becomes perforated by the neurenteric 
canal. It is difficult to suggest any satisfactory explanation 
of this phenomenon ; possibly the proliferation of the posterior 
part of the ventral hypoblast in the lower Vertebrates has been 
in some way associated with a tendency to increased rapidity 
of epiblast production in order that the early enclosure of the 
hypoblast might be ensured. In the passage from the lower 
Vertebrates to the Protamniota the tendency to rapid epiblast 
formation was retained, but the situation of the ventral hypo- 
blastic proliferation was transferred to a more anterior posi- 
tion ; and consequently, as the area to be covered in the region 
of the true blastopore was reduced, the rapidly growing epi- 
blast encroached upon the orifice and brought about its early 
occlusion, after which the two layers, previously united in its 
margins, became separated ; and when this separation occurred, 
the cells forming the boundary of the orifice, which may be 
termed from this position peristomal cells, remained attached 
to the epiblast, and continued quiescent until the period of 
mesoblast formation commenced, when they proliferated and 
produced a ridge-like thickening on the under surface of the 
epiblast, which was the earliest rudiment of the primitive 
streak. These phenomena are repeated in lizards and birds. 
