EARLY DEVELOPMENT 



Fig. 171. Formation of the am- 

 nion and the endoderm from the 

 inner cell mass in humans (dia- 

 gramed from a l^^-day embryo). 

 A slitlike cavity appears in the inner 

 cell mass and enlarges to form the 

 amniotic cavity. A layer of cells 

 interpreted as endoderm separates 

 from the lower surface of the inner 

 cell mass. 



TROPHOBLAST 



One of the striking differences in development between birds and mam- 

 mals is the very early formation of the amnion in the latter. The chick em- 

 bryo is in a very advanced stage of development before the amnion is com- 

 pleted. In the mammal, on the other hand, the amnion forms before the 

 primitive streak appears. In the human, the cells of the inner cell mass re- 

 arrange themselves to form a cavity which enlarges, becoming the amniotic 

 cavity. Thus there is no folding of layers of cells as in the chick embryo. 

 With the formation of the amnion a group of cells becomes segregated from 

 the opposite side of the inner cell mass, and these cells become the endoderm 

 lining of the yolk sac (Fig. 171). Here again there seems to be a direct re- 

 arrangement and migration of cells to form a sac. While the amnion and the 

 yolk sac are forming, other cells from the inner cell mass migrate around 

 these two structures, forming mesoderm. This mesoderm extends in all direc- 

 tions to cover the walls of the blastocyst completely. These processes result 

 in a blastocyst in which the future embryo is suspended by mesoderm from 

 the wall of the cyst. The top of the future embryo is covered by the amnion; 

 the bottom is bounded by the yolk sac (Fig. 172). 



While these changes are going on inside the blastocyst the walls become 

 converted into the trophoblast. Part of the trophoblast invades the uterine 

 mucosa, forming a spongy layer which becomes part of the placenta. The 

 maternal blood flows through this spongy layer of tissue, the syntrophoblast 

 (Fig. 172). The rest of the trophoblast remains as a discrete layer and is 

 coated with mesoderm. These combined two layers form the chorion, which 

 sends out fingerlike projections into the syntrophoblast to form the chorionic 



