DEVELOPMENT OF THE OVUM. 



91 



them a cavity — the coelom (Fig. 69). The epiblast, with its 

 accompanying layer of mesoblast, makes up the somatopleure 

 which bounds the coelom without; the hypoblast and overly- 

 ing layer of mesoblast form the splanchnopleure which bounds 

 the cavity within. 



embryo 



prechorion 



amnion 

 somatopleure 

 — coelom 



- splanchnopleure 

 gut. 



Fig. 70. — Diagram of the Blastodermic Vesicle separating into Embryo and 



Membranes. 



Embryo and Membranes. — During the second week, while 

 the blastodermic vesicle is still within the Fallopian tube and 

 some time before it has reached the uterus, changes take place 

 whereby part of the blastodermic vesicle forms the embryo and 

 part is transformed into the enclosing membranes which con- 

 tain, protect, and nourish it (Fig. 70). Part also forms the 

 yolk sac. 



The Decidua. — When the uterine cavity is reached, the uterine 

 mucous membrane has become hypertrophied, the decidua being 

 thus formed. In the formation of the decidua, the uterine glands 

 become elongated and enlarged ; the inter-glandular tissue hyper- 

 trophied and the mouths of the glands closed, so that the surface 

 layer of the decidua is comparatively solid. The hypertrophy 

 of the mucous membrane is unequal, depressions or pits being- 

 formed on its surface. The deeper layer of the decidua is 

 cavernous, the spaces being formed by the distension of capillary 

 veins into venous spaces, and perhaps also by the enlargement of 

 the gland lumina. 



When the blastodermic vesicle reaches the uterus, it commonly 

 adheres to the posterior wall near the fundus (Fig. 71). The 



