. The development of the chick; an introduction to embryology. Birds -- Embryology. 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 (F
. The development of the chick; an introduction to embryology. Birds -- Embryology. 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.
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