256 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



methods. He believes that Mollier and Tursting have overlooked the angioblast cords between 

 the capillary spaces and have thus described them as vascular anlages independent of the 

 extraembryonic plexus. 



It thus seems probable that the endothelium of the primitive heart and ves- 

 sels has a common origin from the endothelial cells of the area vasculosa. After 

 the development of the endocardium and primitive aortas it is certain that most 



other vascular trunks are formed 

 first as capillary plexuses. By en- 

 largement and differentiation of 

 definite paths in such a capillary 

 plexus the arterial and venous 

 trunks are developed. By the in- 

 jection methods of Mall and his 

 students such capillary plexuses 

 have been demonstrated in the 

 limb buds, in the head and in 

 many organs of chick and pig em- 

 bryos (Fig. 247). Exceptions to the 

 general rule are the intersegmental 

 arteries which arise as single trunks 

 from the aorta (Evans in Keibel and 

 Mall, vol. 2). 



Origin of the Tubular Heart. — 

 In chick and mammalian embryos 

 it is known that paired endothelial 

 anlages of the heart bulge into a 

 fold of the splanchnic mesoderm 

 on each side when the embryo is 

 still flattened on the surface of the 

 yolk (Fig. 248,.!). Paired endothelial anlages are present in the Spee human 

 embryo 1.54 mm. long. As the embryo grows away from the yolk, and the head- 

 fold elongates, the entoderm is withdrawn from between the endothelial anlages 

 and these at once fuse (Fig. 248, B, C). The heart is now an unpaired endothelial 

 tube lying in the folds of the splanchnic mesoderm. Soon the ventral attachment 

 of the mesoderm disappears, leaving the heart suspended by the dorsal meso- 

 cardium in the single pericardial chamber (Fig. 248, C). The endothelial tube 

 forms the endocardium, the splanchnic mesoderm later gives rise to the epicardium 



Fig. 247. — The caudal end of a chick embryo of 

 32 somites, showing the primary capillary plexus 

 in the posterior limb buds. 261I1 Dor. Seg. Vein, 

 twenty-sixth dorsal segmental vein, i. c, that in the 

 twenty-seventh interspace (Evans). 



