72 



THE STUDY OF CHICK EMBRYOS 



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. 



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. 61). — 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 



Splanchnic mesoderm 



Ectoderm 



Fig. 6i. — Transverse section through the cranial portion of the open intestine of a fifty-hour chick 



embryo. X 50. 



is divided by a septum into its primitive components, the right and left aorta. The ccelom is in 

 communication with the extra-embryonic body cavity. 



Section through the Seventeenth Pair of Mesodermal Segments (Fig. 62). — 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 left side appears a section of the 

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

 the future ventro-lateral body wall of the embryo. The fold lateral to the arch of the somato- 

 pleure gives indication of the later approximation of the ventral body walls, by which the embryo 

 is separated from the underlying layers of the blastoderm. 



Section through the Origin of the Vitelline Arteries (Fig. 63).— At this level the 

 embryo is more flattened and simpler in structure, the section resembling one through the 

 mid-gut region of a thirty-six-hour chick (Fig. 47). The amniotic folds have not appeared. 

 On the left side of the figure the vitelline artery leaves the aorta. On the right side the con- 



