390 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 7.5mm. 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 
