248 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



Ect. 



Ms. spl. End. 



living chick embryos (Miller; Reagan, 1915) leaves little doubt of the 

 correctness of the Ruckert-Mollier view. 



The delicate injection methods of Mall and his students show that 

 capillary plexuses precede the formation of definite arterial and venous 

 trunks (Fig. 254). Only by the selection, enlargement, and differentia- 

 tion of appropriate paths do the 

 definitive vessels arise. Capillaries, 

 from which the flow has been di- 

 verted, atrophy. The primitive, 

 paired aortas are formed from the 

 medial margins of such plexuses. 

 -Ent. Exceptions to the general rule are 

 the intersegmental arteries which 

 arise .as single trunks from the 

 aorta (Evans). 



. Inheritance, as well as the hy- 

 drodynamic factors incident to the 

 blood flow, participates in the 

 selection of channels from the 

 capillary bed. 



Origin of the Tubular Heart. 

 The heart of the lower fishes 

 and of amphibians arises in the 

 ventral mesentery of the fore-gut. 

 A tubular cavity first appears, 

 about which the cells differentiate 

 directly into endo-, myo-, and 

 epicardium. 



In bony fishes, reptiles, birds, 

 and mammals, the heart is formed, 

 while the embryo is still flattened 

 FIG. 255. Diagrams to illustrate the origin on the surface of the yolk, from 

 of the mammalian heart. Ect., Ectoderm; pa i red an l a ges which later grow 

 End., endothelial tubes; Ent., entoderm; Fg., , 



fore-gut; Msc.d., dorsal mesocardium; Ms.spl., mesad and fuse - Aggregates of 

 splanchnic mesoderm fepi- and myocardium), mesodermal Cells, which SOOn form 



thin-walled tubes, first appear be- 

 tween the entoderm and splanchnic mesoderm; these are flanked by folds 

 of splanchnic mesoderm that bulge laterally into the ccelomic cavity (Figs. 

 255 A and 35). Such paired cellular masses (endothelial anlages) are 

 present in the Spee 1.54 mm. human embryo (Fig. 77). As the embryo 

 grows away from the yolk and the fore-gut is formed, the entoderm 

 withdraws from between the endothelial tubes, allowing these as well as 

 the mesodermal folds to fuse (Figs. 255 B, C; 36 and 37). 



Msc. d.i- 



