REPRODUCTION. ~ 45 
The allantois is a structure of much physiological impor- 
tance. It arises at the same time as the amniotic folds are 
forming, by a budding or protrusion of the hind-gut into the 
Fic. 81.—Diagrammatic longitudinal section of a chick of the fourth day (after Allen Thom- 
son). ep, epiblast ; hy, hypoblast : sm, somatopleure ; vm, splanchnopleure ; af, pf, folds 
of the amnion ; pp, pleuroperitoneal cavity ; am, cavity of the amnion ; al, allantois; a, 
position of the future anus ; h, heart ; z, intestine ; vi, vitelline duct ; ys, yelk; s, foregut ; 
m, position of the mouth ; me, mesentery. 
pleuro-peritoneal cavity, and hence consists of an outgrowth 
of mesoblast lined by hypoblast. 
‘The outer membrane of the allantois fuses with the subzo- 
nal (serous) membrane, and, with the latter extending beyond 
the yelk-sac, incloses the albumen of the egg in a space termed 
som+alm 
ep } ‘al. at hy 
PA f 
Fic. 82.—A. Diagrammatic longitudinal section through the egg of a fowl. B. Detail of por- 
tion of same at a time when the allantois reached the spot marked x in A (after Duval). 
al, cavity of allantois ; alb, albumen ; ali, mesenteron ; al. hy, hypoblastic epithelium of 
allantois ; al.m, mesoblast of allantois; am, cavity of amnion; b, blood-vessel; emb, 
embryo; ep, epiblast of outer layer of amnion (serous membrane) ; ep. am, epiblastic 
A aeanien of inner layer of amnion (amnion proper) ; m. am, mesoblastic layer of latter ; 
sh, egg-shell ; som, somatic mesoblast of outer layer of amnion; v. m, vitelline membrane ; 
e point where the mesoblastic tissue of the allantois fuses with that of the serous mem- 
rane. 
