THE EARLY DEVELOPMENT OF THE CHICK 297 



The allantois is clearly an outgrowth or appendage of the 

 hind-gut, and consequently its cavity is directly continuous 

 with the enteron; it is lined with endoderm, and its outer wall 

 is mesodermal. Before the close of the fourth day (Fig. 121) 

 the allantois has extended for some distance out into the exo- 

 coelom, lying to the right of the tail. Its terminal portion has 

 enlarged into a dilated vesicle connected with the gut by a 

 narrow tubular allantoic stalk. It has an abundant blood 

 supply, derived from a pair of branches of the dorsal aorta, 



FIG. 121. Median sagittal section through posterior end of four-day chick. 

 After Gasser (Maurer). al, Allantois; am, amnion (tail-fold) ; c, cloaca; m, cloacal 

 membrane; n, notochord; r, rectum; s, spinal cord; y, wall of yolk-sac (endoderm 

 and splanchnic mesoderm). 



the allantoic arteries; its blood collects into a single allantoic 

 'vein and is returned to the heart through the large left um- 

 bilical vein (Fig. 138). 



Once established, the allantois grows very rapidly through 

 the exoccelom, between the chorion and the yolk-sac and 

 amnion (Figs. 115, 116). Its vesicle enlarges rapidly and by 

 the end of the sixth day has covered the entire embryo; two 

 days later it has passed nearly around the yolk-sac. As the 

 allantois enlarges it effects certain important fusions with 

 other structures. Superficially it fuses with the chorion, the 

 mesodermal layers of the two membranes uniting so intimately 



