THE STUDY OF CHICK EMBRYOS 



Section through the Caudal End of the Heart (Fig. 46). The section passes through 

 theihind-brain. The descending aortas are separated only by a thin septum which is 

 ruptured in this section. The anterior cardinal veins are cut at the level where they bend 



Ectoderm 

 Mesodermal segment 



Hind-brain 



fore-gut 



Anterior cardinal vein 

 Somatic mesoderm Caslom 



Entoderm 



Splanchnic mesoderm 

 Vitelline vein/ ^^ 



Heart Myocardium 



FIG. 46. Transverse section through the caudal end of the heart of a thirty-eight-hour chick 



embryo. X 75. 



ventrad to enter the heart. The mesothelial wall of the heart is continuous with the 

 splanchnic mesoderm. On the right side of the section there is apparent fusion between 

 the myocardium of the heart and the somatic mesoderm. A pair of primitive mesodermal 



Ectoderm 

 Mesodermal segment 



Somatopleure _ 



Extra-embryonic caiom 



Right mtelline vein 



mesoderm Entoderm Fore-gut 

 Fro. 47. -Transverse section through the intestinal portal of a thirty-eight-hour chick embryo. 



X 90. 



segments may be seen in this section lateral to the hind-brain. It may be noted here that 

 the primitive segments were not present in the sections of the head previously studied. 

 Section through the Intestinal Portal (Fig. 47) . The descending aorta} now form a single 

 vessel, the dorsal aorta, the medium septum having disappeared. The section passes through 

 the entoderm at the point where it is folded dorsad and cephalad into the head as the fore- 



