DEVELOPMENT OF BIRDS AND MAMMALS 249 



Head 

 fold 



Primitive streak 



which in certain mammals, including many rodents, insecti- 

 vores, bats, monkeys, and man. is retained as the amnion. In 

 others, that part of the serosa which roofs in the embryonic 

 area disappears, and an amnion is developed as in birds and 

 reptiles. 



The outer layer, serosa, chorion, or trophoblast, is applied 

 to the wall of the uterus and develops processes which invade 

 the wall. It often becomes double over the embryonic hemi- 

 sphere or completely, the outer layer of the two thickening 

 to form a plasmodioblast which 

 replaces the epithelium of the 

 uterine wall, and it is invaded by 

 processes of the inner of the two 

 layers, which is formed of cylindrical 

 cells. By this means the maternal 

 vessels are brought into close re- 

 lationship with the outer part of 

 the serosa, and later the mesoderm 

 brought from the embryo by the 

 allantois invades the processes on 

 the embryonic side. The serosa 

 thus forms the placenta, and it 

 provides for the exchange of food 

 material, and oxygen for waste 

 material, between the two blood 

 systems. In certain rodents the 



upper part of the serosa is converted into a pouch. These are 

 some of the modifications of the serosa connected with the 

 retention of the embryo in the uterus. 



When these preliminary processes are accomplished the 

 area pellucida, consisting of ectoderm and endoderm, enters 

 upon developmental phases which are similar to those of 

 birds and reptiles. It enlarges, becoming oval in shape and 

 the posterior end pointed. The embryonic area becomes 

 more strictly marked out by a central area of cylindrical cells, 

 and along the mid-line there gradually appears the primitive 

 streak, and, as in the bird, it at once commences forming meso- 

 derm. As in the bird also, it leaves the surface about the 

 middle of the pellucid region and is continued, uniting with 



\Neur.fold 

 Margin 

 of mesoderm 



FIG. 128. Dorsal views of 

 the embryonic area in 

 early mammalian stages 

 (about seven and eight 

 days in the rabbit) to 

 indicate their similarity 

 to those of the bird. 

 Compare with fig. 121. 



