46 



THE STUDY OF CHICK EMBRYOS 



of three lateral diverticula, the pharyngeal pouches. Cephalad the pharynx is 

 closed ventrally by the pharyngeal membrane. 



Heart and Blood Vessels. — After receiving the vitelline veins cephalad to 

 ih-t fovea cardiaca the double-walled tube of the heart dilates and bends ventrad 

 and to the embryo's right (Fig. 41). It then is flexed dorsad and to the median 

 line, and narrows to form the ventral aorta. The aorta Kes ventrad to the pharynx 

 and divides at the boundary line between the mid- and hind-brain into two 

 ventral aortcB. These diverge and course dorsad around the pharynx. Before 

 reaching the optic vesicles they bend sharply dorsad and caudad, and, as the 

 paired descending aorta, may be traced to a point opposite the last primitive seg- 

 ments. In the region of the fovea cardiaca they'he close together and have fused 

 to form a single vessel, the dorsal aorta. They soon separate and opposite the 

 last primitive segments they are connected by numerous capillaries with the 



Hind-brain 



Fore-! 



Neural tube 



Mid-brain 



Fore-brain 

 Amnion fold. 



Pharyngeal membrane 



cM^s^iaiSsr Notochord 

 Fovea cardiaca 



Vitelline vein 



Ventral 

 aorta Dorsal Heart Pericardial cavity 

 mesocardium 



Fig. 42.— a median longitudinal section of the head of a thirty-eight-hour chick embryo. X about 50. 



vascular network. In this region at a later stage the trunks of the paired vitelline 

 arteries will be differentiated. The heart beats at this stage ; the blood flows from 

 the vascular area by way of the viteUine veins to the heart, thence by the aortae 

 and vitelline arteries back again. This constitutes the vitelline circulation and 

 through it the embryo receives nutriment from the yolk for its future develop- 

 ment. 



In studying transverse sections of the embryo it is not sufficient mereh- to 

 identify the structures seen. The student should determine also the exact level 

 of each section with respect to Figs. 40, 41 and 42, and trace the organs from sec- 

 tion to section in the series. It is important to remember that the transverse 

 sections figured and described in this manual (except those of the fifty-hour chick) 

 are all drawn viewed from the cephalic surface; hence the right side of the embryo 

 is at the reader's left. 



