304 Vertebrate Embryology 



over the ovum is the decidua reflexa ; and all 

 the remaining portions of the decidua are called 

 the decidua vera. As the embryo increases in 

 size and the chorionic vesicle becomes large, 

 the decidua reflexa is stretched out as a thin 

 layer, and the boundaries of the three regions 

 of the decidua become more distinct (Fig. 99). 

 The surfaces of the vera and reflexa remain 

 fairly smooth, while that of the serotina be- 

 comes more and more irregular as pregnancy 

 proceeds, until the projections may reach a 

 height of 10 to 15 mm., as seen in the mater- 

 nal placenta, to be described later. Further 

 details as to the structure of the human uter- 

 us may be obtained from any text-book of 

 histology. 



Let us now return to the growth of the 

 embryo and of the blastodermic vesicle. As 

 has been said, the folding off of the embryo 

 from the rest of the vesicle and the formation 

 of the amnion take place in much the same 

 way in the mammal as in the chick. As the 

 embryo develops and the amnion is completed, 

 the entire structure (embryo and amnion) comes 

 to lie inside of the chorion ; how this condi- 

 tion comes about will be easily understood by 

 examining Figures 93-96. 



