792 



DEVELOPMENT OF THE VASCULAR SYSTEM. 



from the hinder part of the aorta. The first vessels belonging to these 

 trunks are not, however, those of the lower limbs, for these are not yet 



Fig. 591. 



Fig. 591. — Transverse Section through the Dorsal Region op an Embryo-Chick 

 OP THE Second Day (from Foster and Balfour, after His). 

 M, medullary canal ; Pv, proto -vertebral column ; w, rudiment of Wolffian duct in the 

 intermediate mass; Ch, notocliord ; Ao, one of the two aortas ; A, epiblast ; C, hypo- 

 blast ; BC, splanchnopleure ; Pp, pleuroperitoneal space. 



formed ; but the umbilical or hypogastric arteries, developed at a very 

 early period in connection with the allantois, and subsequently attain- 



Fi£r. 592. 



Fig. 592.- — ^Transversb section through the Dorsal region op an Embryo Chick, 

 END op Third Day (from Foster and Balfour). 



Am, amnion ; m p, muscle plate ; CV, cardinal vein; Ao, dorsal aorta at the point 

 where its two roots begin to join ; Ch, notochord (the line does not quite reach it) ; Wd, 

 Wolffian duct; Wb, commencement of formation of Wolffian body ; ep, epiblast ; so, 

 somatopleure ; Sp, splanchnopleure ; hy, hypoblast. The section passes through the 

 place where the alimentary canal {hy) communicates with the yolk-sac. 



