248 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



chymal tissue spaces. Moreover, direct experimental proof on living chick em- 

 bryos (Miller; Reagan, Anat. Rec, vol. 9, 1915) leaves little doubt of the correct- 

 ness of the Riickert-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 differentiation of definite paths do the 

 definitive vessels arise. Capillaries, from which the flow has been diverted, 

 atrophy. The primitive paired aortas are formed from the medial margins of 



such plexuses. Exceptions to the 

 general rule are the intersegmental 

 arteries which arise as single trunks 

 from the aorta (Evans). 



Inheritance, as well as the hydro- 

 dynamic factors incident with the 

 blood flow, doubtless 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 mes- 

 entery 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 

 on the surface of the yoUc, from 

 paired anlages which later grow 

 mesad and fuse. Aggregates of meso- 

 dermal cells, which soon form thin-walled tubes, first appear between the ento- 

 derm and splanchnic mesoderm; these are flanked by folds of splanchnic meso- 

 derm which bulge laterally into the coelomic 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). 



Fig. 254. — The caudal end of a chick embryo 

 of 32 somites, showing the primary capillary plexus 

 in the posterior limb buds from which the sciatic 

 artery will differentiate. Aortas have formed from 

 the mesial margins of the plexuses (Evans). 



