62 



THE STUDY OF CHICK EMBRYOS 



The somatopleuric folds of the amnion envelop the right side of the embryo and the 

 ectoderm of these folds now forms the outer layer of the chorion and the inner layer of the 

 armiion. The mesodermal components of these folds have not united. 



Section through the Anlage of the Liver (Fig. 63). — In this section the cavity of 

 the fore-gut is narrow, the gut being flattened from side to side. Ventrad there are evaginated 

 from the entoderm two elongate diverticula which form the anlages of the liver. On either side 

 of the anlages of the liver are sections of the vitelline veins on their way to the sinus venosus 

 at a higher level in the series. Note the intimate relation between the entodermal epithelium 

 of the liver and the endothelium of the vitelline veins. In later stages, as the liver anlages 

 branch, there is, as Minot aptly expresses it, "an intercrescence of the entodermal cells consti- 

 tuting the liver and of the vascular endothelium" of the vitelline veins. Thus are formed 

 the hepatic sinusoids of the portal system, which surround the cords of hepatic cells. 



Spinal cord 



Mesodermal segment- 



Entoderm- 



Cwlom 



Fore-gu 

 L. vitelline vein' 



Splanchnic mesoderm- 



Chorion 

 -Amnion 



Notochord 

 Dorsal aorta 



Posterior cardinal vein 



Splanchnic mesoderm 

 Liver anlage 



Vitelline vein 



Fig. 63. — Transverse section through the anlage of the liver of a fifty-hour chick embryo. X 50. 



The septum transversum is still present at this level and lateral to the fore-gut are small 

 body cavities. Lateral to the body cavities appear branches of the posterior cardinal veins. 



Section through the Cranial Portion of the Open Intestine (Fig. 64).— The intestine 

 is now open ventrad, its splanchnopleure passing directly over to that of the vascular area. 

 The folds of the amnion do not join, leaving the amniotic cavity open. The dorsal aorta 

 is divided by a septum into its primitive components, the right and left descending aorta: 

 Lateral to the aortje are the small posterior cardinal veins. The coelom is in communication 

 with the extra-embryonic body cavity. 



Section through the Seventeenth Pair of Mesodermal Segments (Fig. 65). The 



body of the embryo is now no longer flexed to the right. On the left side of the figure the 

 mesodermal segment shows a dorso-lateral muscle plate. The median and ventral portion of 

 the segment is being converted .into mesenchyme. On the right side appears a section of the 

 primary excretory or mesonephric duct. The embryonic somatopleure is arched and will form 



