854 



PHYSIOLOGY. 



of cells (vertebral plate), which becomes laminated laterally (lateral 

 plate). From the vertebral plate develop the somites; the lateral 

 plate splits into two lamellae, of which the outer is the somatic meso- 

 derm; the inner, the splanchnic mesoderm. 



Fig. 388. 



Fig. 387. Formation of Decidua (the decidua is colored black, the 

 ovum is represented as engaged between two projecting folds of mem- 

 brane). (After DALTON.) 



Fig. 388. Projecting Folds of Membrane Growing Around the Ovum. 

 (After DALTON.) 



Fig. 389. Showing Ovum Completely Surrounded by the Decidua 

 Reflexa. (After DALTON.) 



The former unites with the ectoderm to form the somatopleure, 

 while the latter unites with the entoderm to form the splanchnopleure. 

 Between the somatcpleure and the splanchnopleure there is an open- 

 ing, the body-cavity, from which arise the serous cavities of the adult. 



