186 



EMBRYOLOGY. 



numerous villi, which are probably derived from the epiblast. 

 is known now as the chorion. 



What changes occur in the uterus? 



The decidua menstrualis has been mentioned as specially suitable 

 for the reception of the ovuni. But if impregnation occur, it is 

 not called by this name, as menstruation does not occur. It is 

 then known as the decidua vera. It consists of a thick, succulent 



Section of a Blastoderm at right angles to the long axis of the embryo, near its middle, 

 after eight hours' incubation (from Foster and Balfour) : A, epiblast formed of two 

 layers of cells; B, mesoblast thickened below the primitive groove ; C, hypoblast, 

 formed of one layer of flattened cells; pr, primitive groove; me. mesoblast cell; 

 bd, formative cells in the so-called segmentation or subgermiual cavity. (The line 

 of separation between the epiblast and mesoblast below the primitive groove is too 

 strongly marked in the figure.) 



mucous membrane caused by the proliferation of the subepithelial 

 cells. Into this decidua the ovum falls, and its shaggy chorion 

 implants its villi in the crypts of the mucous membrane. The 

 decidua soon envelops it, and the portion which is reflected over 

 the ovum is known as the decidua reflexa. 



What important changes now begin to occur in the blastoderm ? 

 The blastoderm, which is increasing in area from the rapid pro- 

 liferation of cells, folds at each end transversely and at the sides 

 longitudinally. These folds are of great importance, for it is in 

 this way that the contour of the body is outlined, and if it were 

 not for them the blastoderm .would continue to develop as a flat 

 surface. The transverse folds are at each end, and are known as 



