670 



HANDBOOK OF PHYSIOLOGY. 



contracts (Fig. 455), shrivels up, and together with the part of its duct 

 external to the abdomen, is detached and disappears either before or at 

 the termination of intra-uterine life, the period of its disappearance 

 varying in different orders of Mammalia. 



When blood-vessels begin to be developed, they ramify largely over 

 the walls of the umbilical vesicle, and are actively concerned in absorb- 

 ing its contents and conveying them away for the nutrition of the em- 

 bryo. 



FIG. 453. Diagrams showing three successive stages of development. Transverse vertical sec- 

 tions. The yelk-sac, ys, is seen progressively diminishing in size. In the embryo itself the medul- 

 lary canal and notochord are seen in section, a', in middle figure, the alimentary canal, becoming 

 pinched off, as it were, from the yelk-sac; a', in right hand figure, alimentary canal completely 

 closed; a, in last two figures, amnion; oc, cavity of amnium filled with amniotic fluid; pp, space 

 between amn ion and chorion continuous with the pleuro-peritoneal cavity inside, the body; vt, 

 vitellme membrane; ys, yelk-sac, or umbilical vesicle (Foster and Balfour). 



At an early stage of development of the foetus, and some time before 

 the completion of the changes which have been just described, two im- 



FIG. 454. 



FIG. 455. 

 a, area pellucida; 6, area vasculosa; 



FIG. 454. Diagram showing vascular area in the chick 

 c, area vitellina. 



FIG. 455. Human embryo of fifth week with umbilical vesicle; about natural size (Dalton). 

 The human umbilical vesicle never exceeds the size of a small pea. 



portant structures, called respectively the amnion and the allantois, 

 begin to be formed. 



Amnion. The amnion is produced as follows: Beyond the head- 

 and tail-folds before described (p. 667), the somatopleure coated by epi- 

 blast, is raised into folds, which grow up, arching over the embryo, not 

 only anteriorly and posteriorly but also laterally, and all converging 



