REPRODUCTION. 



77 



creases in size as the yelk-sac diminishes, forms the pleuro- 

 peritoneal cavity, body cavity, or coelom. The amniotic cavity 

 also extends, so that the embryo is surrounded by it or lies 

 centrally within it. The enlargement of the coelom and exten- 

 sion of the false amniotic folds lead finally to a similar meeting 

 and fusion like that which occurred in the formation of the true 

 amniotic cavity. The yelk-sac, gradually lessening, is at last 

 withdrawn into the body of the embryo. 



Fig. 76 shows how the amniotic head fold arises, from a 

 budding out of the epiblast and mesoblast at a point where the 

 origiiial cell layers of the embryo have separated into two folds, 

 the somatopleure or body fold and the splanchnopleure or vis- 

 ceral fold, owing to a division or cleavage of the mesoblast 

 toward the long axis of the body. Eemembering this, it is 

 always easy to determine by a diagram the composition of any 

 one of the membranes or 

 folds of the embryo, for 

 the components must be 

 epiblast, mesoblast, or 

 hypoblast ; thus, the 

 splanchnopleure is made 

 up of hypoblast internally 

 and mesoblast externally 

 — a principle of great sig- 

 nificance, since, as will be 

 learned later, all the tis- 

 sues of the body may be 

 classified simply, and at 

 the same time scientifi- 

 cally, according to their 

 embryological origin. 



The allantois is a 

 structure of much physi- 

 ological importance. It 

 arises at the same time as 

 the amniotic folds are 



forming, by a budding or protrusion of the hind-gut into the 

 pleuro-peritoneal cavity, and hence consists of an outgrowth of 

 mesoblast lined by hypoblast. 



Fio. 78. — Diagrammatic longitudinal section 

 through the egg of a fowl (after Duval). 

 al, cavity of allantois; cUb, albumen; ati, mes- 

 enteron; am, cavity of amnion; emb, embryo; 

 sh, egg-shell; v. m, vitelline membrane. 



