AVES. 193 



splanchnic and somatic stalks of the embryo, on its way to the 

 space between the external and internal folds of the amnion, 

 which it will be remembered is directly continuous with the 

 pleuroperitoneal cavity (fig. 121 K). In this space it eventually 

 spreads out over the whole body of the chick. On the first half 

 of the fourth day the vesicle is still very small, and its growth is 

 not very rapid. Its mesoblast wall still remains very thick. In 



FIG. 124. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR 

 END OF AN EMBRYO BlRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS. 



ep. epiblast; Sp.c. spinal canal; ch. notochord; n.e. neurenteric canal ; hy. hypo- 

 blast ; p.a.g. post-anal gut ; pr. remains of primitive streak folded in on the ventral 

 side; al. allantois; me. mesoblast; an. point where anus will be formed ', p.c. peri- 

 visceral cavity ; am. amnion; so. somatopleure; sp. splanchnopleure. 



the latter half of the day its growth becomes very rapid, and it 

 forms a very conspicuous object in a chick of that date (fig. 118, 

 A I}. At the same time its blood-vessels become important. It 

 receives its supply of blood from two branches of the iliac arteries 

 known as the allantoic arteries 1 , and the blood is brought back 

 from it by two allantoic veins which run along in the body walls 

 (fig. 119) and after uniting into a single trunk fall into the 

 vitelline vein close behind the liver. 



Before dealing with the later history of the fcetal membranes, 

 it will be convenient to complete the history of the yolk-sack. 



Yolk-Sack. The origin of the area opaca has already 

 been described. It rapidly extends over the yolk underneath 

 the vitelline membrane ; and is composed of epiblast and of the 



1 I propose to call these arteries and the corresponding veins the allantoic arteries 

 and veins, instead of using the confusing term ' umbilical.' 



B. III. 13 



