114 



PRIMITIVE ANIMALS 



nourishment from the mother to the young. Within 

 the investing trophoblast the cells continue to divide 



Fig. 24. B. Transverse section through the uterus of a Mam- 

 mal and a developing foetus after the allantoic placenta has 

 formed. The foetus em. is pushed up into the amniotic cavity 

 am. and is attached to the yolk-sac ys. by the umbilicus, and to 

 the placenta pi. by the allantois al. The coelom of the foetus c. 

 is continuous with the extra-embryonic coelom ex.c. The placenta 

 is formed partly of uterine pi. and partly of trophoblastic tissue 

 pi'. 



and to arrange themselves into the three germ-layers 

 of the embryo the ectoderm, endoderm, and meso- 

 derm. Just as in the Reptile or Bird, the endoderm 



