THE RISE OF THE MAMMALIA 113 



nutrition of the foetus is now brought about, to 

 understand the principal features in the development 

 of the foetus itself (Fig. 24 A and B). The repeated 

 division of the fertilized ovum results in the forma- 

 tion of an embryo, only part of which is destined 



Fig. 24. A. Transverse section through the uterus of a Mammal 

 in the wall of which is embedded an embryo at an early 

 stage before the formation of the placenta. The region where 

 the placenta will form is shaded in. am. amniotic cavity; 

 I. lumen of uterus; tr. trophoblast; ys. yolk-sac; ex. c. extra- 

 embryonic coelom; pi. maternal portion of placenta; pl f . foetal 

 portion of placenta. 



to form the future animal. The external investing 

 layer of cells, known as the trophoblast (tr.), is simply 

 used to put the foetus inside it in close connection 

 with the maternal tissues and so to convey the 



S. A. K. 8 



