202 



THE DEVELOPMENT OF THE CHICK 



from (4) the bulbus arteriosus by a slight constriction. The 

 bulbus thus Ues 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. XII). 



In an ordinar}^ 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. 



Another change that should 

 be noted here is the disappear- 

 ance of the mesocardium during 

 the folding of the cardiac tube, 

 except in the region of the 

 sinus venosus w^here it remains 

 permanently and becomes much 

 broadened (seventy-two hours). 



(6) Changes in the Internal Structure o] 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- 

 cardium, 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 Umb, 

 the wide space originally existing between endocardium and 



Fig. 116 . — Heart of a chick embryo 



of 72 hours, dissected out and drawn 



from the dorsal surface. 



Aur. 1., Left auricle. Aur. r., Right 

 auricle. B. a., Bulbus arteriosus. 

 D. C. r. 1., Right and left ducts of 

 Cuvier. D. V., Ductus venosus. S. V., 

 Sinus venosus. Tr. a., Truncus arte- 

 riosus. V. r., Right limb of ventricle. 



