THE YELK SACK. 



661 



foetus occupies the opening which has been gradually made by 

 the fluid wedge. The outer part of the amniotic membrane, 

 derived from the mesoblast, is of a tougher character than the 

 inner epithelial layer, and it is said to possess muscular fibre and 

 to be capable of rhythmical contractions. 



(2) The Yelk Sack is that part of the blastoderm which grows 

 and envelops the yelk, which previously was only surrounded by 

 the vitelline membrane. After the mesoblast has split into two 

 layers, the splanchnopleure becomes bent inward at a point 

 some distance from its origin, carrying with it the hypoblast. 

 By this curve an upper constricted canal is differentiated from 



Diagrammatic longitudinal section of a chick on the fourth day. (Allen Thomson.) 

 ep, Epiblast. hy, Hypoblast. sm, Somatopleure. v.m, Splanchnopleure. af.pf, Folds 

 of the am n ion. pp, Pleuroperitoneal cavity, am, Cavity of amnion. al, Allantois. a, 

 Position of the future anus, h, Heart, i, Intestine, vi, Vitelline duct, ys, Yelk. *, 

 Fore gut. m, Position of the mouth, me, The mesentery. 



the large lower cavity. This upper canal becomes eventually 

 the alimentary tract, the lower cavity the yelk sack, while the 

 constricted portion leading from the one to the other is the canal 

 leading from the intestine to the yelk, called the ductus vitello- 

 intestinalis. 



At first the splanchnopleure incloses only the upper part of 

 the yelk, but as development proceeds it grows around, and at 

 last completely encircles it. The yelk sack is thus derived from 

 the splanchnopleural layer of the mesoblast, and its lining hypo- 

 blast. 



