Manchester Memoirs, Vol. I. (1906), No. JJ. 35 



is practically identical with that which is observed in the 

 Anura, except in one in:iportant respect. In the bilateral 

 closure of the blastopore, the presence of a ventral as well 

 as a dorsal lip {^Fig. 20, A) and the formation of the 

 mesoderm from a double source, the two groups closely 

 resemble one another ; but while in the frog the under 

 layer of the roof of the archenteron persists as the dorsal 

 lining of the digestive tract, in the Urodeles the sheet of 

 cells in question becomes wholly converted with the upper 

 layers of the roof into the notochord, just as is the case in 

 Petrojiiyzon, and the roof of the gut is formed by an in- 

 growth of vegetative cells from the sides {Fig. 20, B, C). 



GymnopJiiona. 



The last group of the Anamnia whose germinal layers 

 remain to be considered is the Gymnophiona. We here 

 fbijlow Brauer's excellent description of HypogeopJiis. 



In the Gymnophiona the o.^^ is so laden with yolk 

 that it nearly approaches the meroblastic type, and the 

 result of segmentation is what may fairly be described as 

 a blastoderm resting on a partially divided yolk. The 

 blastoderm comprises a superficial epithelium of columnar 

 cells, thinner and lower at the margin, and covers several 

 irregular layers of scattered cells, which are more abund- 

 antly supplied with yolk. The cavities between these 

 cells are equivalent to the ordinary segmentation cavity 

 {Fig. 22, A). Below the segmentation cavity is the yolk, 

 divided at its surface into cells, and containing scattered 

 nuclei throughout its substance. Immediately round the 

 blastoderm the surface of the yolk is also partially 

 segmented. 



At one point — the posterior middle point — of the 

 edge of this blastoderm, the dorsal lip appears ; it 

 exhibits the usual radial arrangement of cells {Fig. 22, A). 



