CHICK EMBRYO OF EIGHTEEN PRIMITIVE SEGMENTS 



Transverse Section through the Caudal End of the Heart (Fig. 44.) The 

 section passes through the hind-brain. The descending aortas are separated only by a thin 

 septum which is ruptured in this section. The mesothelial wall of the heart is continuous with 

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

 myocardium of the heart and the somatic mesoderm. Lateral to the aortae are the anterior cardinal 



Ectoder. 

 Mes. seqmenf 



'eural tube 



ForequT 



Ant. cardinal Vein 



c I L i Coelo ( 



opianchmc. mesoaer/n 



Entoderm 



Vitelllne Vein / I '^v x Splanchnic fnesoderm 



Heart , 



FIG. 44. Transverse section through the caudal end of the heart of a thirty-six-hour chick embryo. 



X75- 





FIG. 45. Transverse section through the fovea cardiaca of a thirty-six-hour chick embryo. Ant. 

 card, -vein, anterior cardinal vein; L. vit. vein, left vitelline vein; Mes. segment, mesodermal segment; 

 Spl. mesoderm, splanchnic mesoderm. X go. 



veins. A pair of primitive mesodermal 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. 



Transverse Section through the Fovea Cardiaca (Fig. 45). 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 



