Manchester Memoirs, Vol. l. (1906), No. 3. 



45 



floor corresponds to floor, and roof to roof; lastly the 

 sheets of cells projecting beneath the upper layer at the 

 sides of and behind the blastopore are the equivalents of 

 the mesoderm formed at the lateral and ventral lips in the 

 Amphibia. 



PSESSSSSBlSSSg5gg55|gjSSros; 



2> 



Fig. 27. 



Four stages in the formation of notochord, mesoderm, and endoderm 

 in Platydactyhis (after Will). 



A. The floor of the archenteron is still intact. 



B — D. The archenteron has come into communication with the 

 subgerminal cavity. Its roc)f is thickened in the middle line to form the 

 notochord (n.ck.) : the lateral parts of the roof are the mesoderm (mes.). 

 The paraderm is growing in from the sides to form the lining of the 

 alimentary tract. 



From this comparison it follows of course that cells 

 which are the morphological equivalents of the yolk-cells 

 of the Amphibia are to be found in the upper layer of 

 the Reptilian blastoderm ; and that that layer cannot 

 be termed the ectoderm until the process of invagination 

 is complete. 



