Manchester Memoirs, Vol. I. (1906), No. 3. 39 



cavity becomes divided, as it does in Ceraiodtis, into three 

 portions ; a narrow median strip, the notochord, and two 

 lateral sheets of mesoderm {Fig. 23, B) which are con- 

 tinuous with one another behind the yolk-plug by the cell 

 plate invaginated at the lateral and ventral lips. The 

 mesoderm has in fact precisely the same relations as in 

 the frog (or other Anamnian) at a similar stage. 



Fig. 23. 

 Hypogeophis (after Brauer). 

 Differentiation of the roof of the archenteron into notochord and meso- 

 derm ; formation of the roof of the definitive gut {al.c.) by ingrowth and 

 undergrowth of yolk-cells. 



Brauer states expressly that no additions are made to 

 either the notochord or the mesoderm from any other 

 source. 



The roof of the gut is completed by upgrowth and 

 ingrowth of vegetative cells below the middle layer 

 {Fig. 23, B). 



Let us now, before proceeding to the Amniota, 

 endeavour to express, in as brief a form as possible, the 

 principal facts of germ-layer development as they occur in 

 the Anamnia. 



