76 



COMPARATIVE PHYSIOLOGY. 



yelk-sac (umbilical vesicle of the mammalian embryo). The 

 manner in which this takes place will appear upon an inspec- 

 tion of the accompanying figures. 



Very early in the history of the embryo two eminences, the 

 head and the tail folds, arise, and, curving over toward each 



F.So. 



Fio. 76. — Diagrammatic longitadlnal section throngh the axis of an embiyo chick 

 (after Foster and Balfour). N. C, Neural canal; Oh, notocbord; Fg, foregut; 

 F. So, somatopleure; F. Sp, splanchnopleure; Sp, splanchnopleure, forming lower 

 wall of foregut; Ht, heart; ^, pleuroperitoneal cavity; Am, amniotic fold; F, 

 epiblast; M, mesoblast; H, hypoblast. 



other, meet after being joined by corresponding lateral folds. 

 Fusion and absorption result at this meeting-point, in the 

 inelosure of one cavity and the blending of two others. These 

 folds constitute the amniotic membranes, the inner of which 



Fig. 77. — Diagrammatic longitudinal section of a chick of the fourth day (after Allen 

 Thomson), ep. epiblast; hy, hypoblast; em, somatopleure; vm, splanchnopleure; 

 af, pf, folds of the amnion; 2>P, pleuroperitoneal cavity; am, cavity of the am- 

 nion; al. allantois; a, position of the future anus; h, heart; i, intestme; vi, vitel- 

 line duct; 1/8, yelk; s, foregut; m, position of the mouth; me, mesentery. 



forms the true amnion, the outer the false amnion (serous mem- 

 brane, subzonal membrane). Within the amnion proper is the 

 amniotic cavity filled with fluid (liquor amnii), while the space 

 between the true and false amniotic folds, which gradually in- 



