202 THE DEVELOPMENT OF THE CHICK 
from (4) the bulbus arteriosus by a slight constriction. The 
bulbus thus lies on the right side; it sweeps around the atrium 
anteriorly to the middle line and then bends up to enter the floor 
of the pharynx. 
From the ventral side one sees the looped ventricular division 
behind, in which we distinguish 
right and left limbs, the former 
of which enters the bulbus in 
front, and the latter the auricles. 
These two limbs represent ap- 
proximately the future right 
and left ventricles (Fig. 198, 
Chap: X11). 
In an ordinary entire mount 
of this stage the heart is seen 
from the right side, and the dis- 
position of the parts may be 
readily understood by reference 
to Fig. 117, and the preceding 
description. 
Fria. 116 .— Heart of a chick embryo i ; Hae Suen 
- other change that s 
of 72 hours, dissected out and drawn Another ¢ 1aee sie saan 
fromm therdareal cuviace: be noted here is the disappear- 
Aur. 1, Left auricle. Aur.r., Right ance of the mesocardium during 
sume a US grea the folding of the cardiae tube, 
Cuvier. D. V., Ductus venosus. Du Vep TE XCept in the region of the 
Sins venowic Tr runes arte sinus venosus where it remains 
permanently and becomes much 
broadened (seventy-two hours). 
(b) Changes in the Internal Structure of the Heart. We have 
already seen that the heart consists of two primary layers, viz., 
the endocardium, which is endothelial in nature, and the myo- 
eardium, which is derived from the splanchnic mesoblast. The 
distinction between the sinu-auricular and the bulbo-ventricular 
divisions of the heart is indicated internally at about the time 
the first external evidence is seen, by the fact that the endocar- 
dium is more closely applied to the myocardium in the former 
than in the latter division. In the sinus and atrium but little 
change takes place in the period under consideration. In the 
ventricle, on the other hand, and especially in the right limb, 
the wide space originally existing between endocardium and 
