350 



THE DEVELOPMENT OF THE CHICK 



sively ventral and posterior. The definitive ventral surface of the heart 

 is a cranial rather than a ventral surface during the critical period of 

 development described below, up to eight days (cf. Figs. 148 and 150). 

 In other words, the apex of the heart is directed ventrally rather than 

 posteriorly, though it has a posterior inclination. For simplicity of de- 

 scription, however, it seems better to use the definitive orientation in the 

 following account; that is, to regard the apex of the heart as posterior 

 instead of ventral, and the bulbus face of the heart as ventral instead 

 of cranial, in position. 



Fig. 200. — Ventral view of the heart of a 

 chick embryo of 7.5 mm. head-length. (After 



Masius.) 



Atr. d., s., Right and left auricles. B. Co., 

 Bulbus cordis. V., Ventricle. 



Division of the Cavities of the Heart. The embryonic 

 heart is primarily a single continuous tube; during development 

 a complex series of changes brings about its complete division 

 into right and left sides, corresponding to the pulmonary and 

 systemic circulations. Partitions or septa arise independently 

 in each primary division of the cardiac tube, excepting the sinus 

 venosus, and subsequently these unite in such a way as to make 

 two independent circulatory systems. During this time the 



