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



43 



Fig. 24. 

 Three stages in the formation and closure of the blastopore in Platy- 

 dactylus (after Will). The oval embryonic shield lies on the blastoderm ; 

 only part of the latter is represented. 



.H 







.2> 



■ffiMSngs 



Fig. 25. 

 Platy dactylus (after Will). 

 Sagittal sections of five stages in the development of the blastopore and 

 archenteron. 



A. The upper layer and the paraderm {p.d.) are fused in the primitive 

 plate (/./.) at the posterior margin of the embryonic shield ; the latter is 

 distinguished by its columnar cells from the fiat cells of the surrounding 

 blastoderm. 



B, C. Formation of the archenteron : dj. dorsal lip, y.p. yolk-plug, 

 tnes.v. mesoderm at ventral lip. 



E. Fusion of the archenteric and subgerminal (j.^.c.) cavities. 



