PREGNANCY 



6i 



of cells, known as a tnoniJu. By the time that the product of 

 division has reached the uterus the morula has become converted 

 into a hollow blastocyst through some of the cells in the inside 

 of the morula becoming vacuolated, the cavities running together 

 and then forming a space containing a fluid. Within the cavity 



Segmentation cavity Inner cell mass 



Endoderm 



Trophoblast 



Fig. 35. — Section through blastodermic vesicle of bat. 

 (After van Beneden, from Orcn/s Anatomy.) 



Inner cell mass 



Trojthoblast 



Endoderm 



Embryonic ectoderm 



Fig. 3G. — Section through embryonic disc of bat. 

 (After van Beneden, from Orai/s Anatomi/.) 



of the blastocyst a little knot of cells projects, which is called 

 the formative or inner cell mass. This eventually gives rise to 

 the three germinal layers of the embryo — ectoderm, mesoderm, 

 and endoderm — as well as to other structures concerned in the 

 attachment and nutrition of the embryo. 



The outer layer surrounding the blastocyst takes no part 

 in the formation of the embryo, but gives rise to a structure 



