Development of the Second Day 151 



The other embryonic vessels, at least their 

 endothelial lining, are formed by buds given 

 off from these first-formed vessels, just as hap- 

 pened in the case of the vessels of the vascular 

 area. 



The outer coats (media and adventitia) of 

 the veins and arteries are formed by differentia- 

 tion of the surrounding mesoblast. 



The Wolffian Duct 



The first indication of the uro-genital system 

 in the chick, as in the frog, is the Wolffian 

 duct. 



It arises at this time, when the embryo has 

 about ten mesoblastic somites, as a small ridge 

 from the uncleft mesoblast that lies at the 

 outer side of the last three somites. A cross 

 section through this region of the embryo (Figs. 

 52 and 54) shows the Wolffian duct project- 

 ing, on each side, into the triangular space 

 between the ectoblast above, the somite on the 

 inside, and the somatic mesoblast on the out- 

 side. At this time it is merely a solid rod of 

 cells, extending for two or three somites, and 

 without a central lumen. 



Summary — The most important changes of 

 the first half of the second day are : 



