104: 



COMPARATIVE PHYSIOLOGY. 



The excretory system is also foreshadowed at an early period 

 by the Wolffian duct (Fig. 110), a mass of mesoblast cells near 

 which the cleavage of the mesoblast 

 takes place. 



During the latter part of the sec- 

 ond day the vascular system, includ- 

 ing the heart, makes great progress. 

 The latter, in consequence of excessive 

 growth and the alteration of the rela- 

 tive position of other parts, becomes 

 bent up on itself, so that it presents 

 a curve to the right which represents 

 the venous part, and one to the left, 

 answering to the arterial. The rudi- 

 ments of the auricles also are to be 



The arterial system is represented 

 at this stage by the expanded portion 

 of the heart known as the hulbus ar- 

 teriosus, and two extensions from it, 

 the aortas, which, uniting above the 

 alimentary canal, form a single poste- 

 rior or dorsal aorta. From these great 

 arterial vessels the lesser ones arise, 

 and by subdivision constitute that 

 great mesh- work represented diagram- 

 matically in Figs. 108, 109, from which 

 the course of the circulation may be 

 gathered. The beating of the heart 

 commences before the corpuscles have 

 become numerous, and while the tub- 

 ular system, through which the blood 

 is to be driven, is still very incomplete. 



The events of the third day are of 

 the nature of the extension of parts 



FiQ. 108.— Diagram of the em- 

 bryonic vascular system 

 (Wiedersheim). A, atrium; 

 A', A', dorsal aorta ; Ab, 

 branchial vessels; Acd^ cau- 

 dal artery; All, allantoic (hy- 

 pogastric) arteries; Am, vi- 

 telline arteries; B, bulbus 

 Rrterio&us; c, c'. external and 

 internal carotids; D. ductus 

 Cuvieri (precaval veins); Ej 

 external iliac arteries; H. V, 

 posterior cardinal vein ; Ic, 



STSenifij'T'rf^ht'^an^a already marked out rather than the 



left roots of the aorta; S.S', ' ' 



ing 

 iclav 



11 clefts ; S. A, 



formation of entirely new ones. The 

 following are the principal changes : 

 The bending of the bead-end down- 

 ward (cranial flexure) ; the turning 

 of the embryo so that it lies on its 

 left side ; the completion of the vitelline circulation ;. the in- 



nr veins ; Sb, subclavian ar- 

 tery ; ^6', subclavian vein; 

 Si, sinus venosus; V, ventri- 

 cle ; VC, anterior cardinal 

 vein; Vm, vitelline veins. 



