172 



THE FOURTH DAY. 



[CHAP. 



their appearance unite close to the allantois into a single 

 trunk, the umbilical vein, which, running along the splanch- 

 nopleure, falls into the omphalo-mesaraic vein (Fig. 53, U). 



23. Meanwhile the heart is undergoing considerable 

 changes. Though the whole organ still exhibits a marked 

 curvature to the right, the ventricular portion becomes 

 directed more distinctly downwards, forming a blunted cone 

 whose apex will eventually become the apex of the adult 

 heart. 



The concave (or dorsal) walls of the ventricles become 

 much thicker, as did the convex or ventral walls on the third 

 day. 



"Well-marked constrictions now separate the ventricles 

 from the bulbus arteriosus on the one hand, and from 

 the auricles on the other. The latter constriction is very 

 distinct, and receives the name of canalis auricularis 

 (Fig. 54, C.A.) ; the former, sometimes called the /return 

 Halleri, is far less conspicuous. 



The most important event is perhaps the formation of 

 the ventricular septum. This, which commenced on the 

 third day as a crescentic ridge or fold springing from the 

 convex or ventral side of the rounded ventricular portion of 

 the heart, now grows rapidly across the ventricular cavity 

 towards the concave or dorsal side. It thus forms an in- 

 complete longitudinal partition extending from the canalis 

 auricularis to the commencement of the bulbus arteriosus, 

 and dividing the twisted ventricular tube into two somewhat 



Fig. 54. 



Heart of a Chick on the Fourth Day of Incubation viewed from 



the Ventral Surface. 



1. a. left auricular appendage. C. A. canalis auricularis. V. ventricle, b. bulbus 



arteriosus. 



