CARDIAC-LOOP FORMATION IN CHICK 385 
In the formation of the cardiac loop, the U-shaped ventricular 
bend becomes compressed (pl. 1, F, G, H). Coincidently, the 
apex of the bend is dilated so that the ventricle loses its U shape 
and becomes more saccular. The same process of dilation short- 
ens, almost to obliteration the ventricular portion of the limbs 
of the U, so that the atrioventricular canal and bulbus cordis 
appear to lead off side by side, from a common ventricular sac 
(pl. 1, 1). Meanwhile, as has already been described in connec- 
tion with the formation of the loop, the entire ventricle has 
shifted more toward the sagittal plane (pl. 1, F to J). It has 
at the same time been bent caudad and then dorsad (fig. 1, H 
to L). In this manner the apex of the ventricle, which was 
originally the most right-hand portion of the U-shaped heart 
tube, becomes the most caudal part. 
The first external manifestation of the impending division of 
the ventricle into right and left chambers shows in the oldest 
stages here studied. During the fourth day, a slight groove 
appears on the ventral surface of the ventricle, which extends 
ecaudad from the angle between the bulboventricular constric- 
tion and the atrioventricular constriction (pl. 1, K, L). This 
groove in later stages extends still farther caudad and marks 
externally the position at which the septum interventriculorum 
develops. 
In the bulbotruncus region the early changes are shown so 
definitely by the figures that little can be added by description. 
The most interesting phases of the development of the bulbus 
occur in stages of development more advanced than those with 
which this study is concerned. They have been described in 
detail by Lindes (65), Langer (94), Masius (89), Hochstetter 
(06), and Lillie (’08). 
The early differentiation of the sinus venosus has been less 
fully described and calls for more detailed attention. A definite 
dorsal line of demarcation between the sinus venosus and the 
atrium, can first be made out at the close of the second day of 
incubation (chicks of 25 somites). At this time the middorsal 
portion of the sino-atrial region of the heart becomes dilated. 
This dilation is situated just where the persistent caudal portion 
