360 



REPRODUCTION 



as far as the blastocyst stage (see Fig. 68). It is a mass 

 of cells containing a vesicle. In this form the ovum 

 buries itself in the decidua. As the embryo increases in 

 size it projects into the cavity of the uterus. In the 

 decidua three parts are now distinguished : (1) the decidua 



enveloping epiblast 



enclosed epiblast 



hypoblast 

 enveloping epiblast 



Fig. 08. — Bilaminar blastocyst (Keith, after Van Eeneden). 



serotina or basalis, where the embryo is attached to the 

 uterus ; (2) the decidua reflexa, which covers the embryo ; 

 and (3) the decidua vera, which hues the remainder of the 

 uterine cavity. With further growth the decidua reflexa 

 and the decidua vera come into direct contact (Fig. 69). 

 At an early stage the nutrition of the embryo is prob- 



