190 



STUDY OF YOUNG CHICK EMBRYOS. 



This wall soon breaks through, and there results a single median tube of endo- 

 thelium which presently appears to be connected with the mesothelium, m.ht, 

 by long cell-processes across the wide intervening space. The heart is now a 

 double tube connected by the mesothelium with the tissues above. 



Section through the Wall of the Fovea cardiaca (Fig. 140). Underneath the whole 

 of the embryo and the germinative area is the extensive archenteric cavity, bounded 

 above by the cellular entoderm of the embryo and of the splanchnopleure. The 

 archenteron includes both the intestinal cavity of the embryo and the cavity of the 

 yolk-sac, and accordingly its lower floor is the mass of yolk. Into the head of 

 the embryo runs the closed prolongation of the archenteron, which we have studied 

 as the fore-gut. The posterior opening of the fore-gut is known as the fovea 

 cardiaca. The manner in which the entoderm at the fovea bends ventralward to 



Sont. 



Ao.d. Md. 



EC. 



Spl. COB. msth. Ent. Ph. V.om. 



FIG. 140. SECTION OF A CHICK EMBRYO WITH EIGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION 149. 



Ao.D, Dorsal aorta. Cos, Coelom. EC, Epidermis. Ent, Entoderm. G, Ganglionic crest. Md, Hind-brain. 



msth, Mesoderm of the septum transversum. Ph, Fore-gut. Som, Somatopleure. Spl, Splanchnopleure. 



V.om, Omphalo-mesaraic veins. X 100 diams. 



pass from the fore-gut on to the splanchnopleure may be readily understood from 

 figure 139. The present section passes through the nearly vertical wall of entoderm, 

 Ent, at the fovea. From this wall the anlage of the liver will arise. THe omphalo-. 

 mesaraic veins, V.om, pass by it to join the caudal or venous end of the heart. 

 The fore-gut, Ph, is still closed on its ventral side. The veins are in the splanch- 

 nopleure, and, being cut obliquely, appear somewhat elongated. They" each cause a 

 1 protuberance of the splanchnopleure toward the ccelom, Coe. The protuberance 

 is covered by a thick dense layer of mesoderm, Msth, which forms an arch over 

 the vein, so as to leave a clear space between it and the endothelium of the vein. 

 The two protuberances constitute the anlage of the septum transversum, which is 

 itself the anlage of the diaphragm. The region cephalad of the septum is the 

 cervico-thorax; the region caudad, the future abdomen. If the series of sections be 

 followed headward, the veins can be traced to their union with the heart in the 

 median line. If the series be followed tailward, the veins can be traced out into 

 the area pellucida, where they branch. The hind-brain, Md, is of smaller diameter 

 than in the previous sections. The epidermis, EC, is closely fitted against the dorsal 



