LATER DEVELOPMENT OF VASCULAR SYSTEM 



355 



completely along its entire free border with the endothelial 

 cushions of the latter. It would thus establish a complete par- 

 tition between the two auricles were it not for the fact that 

 secondary perforations arise in it before its free edge meets the 



endothelial cushions (Fig. 203), 

 logical significance as the fora- 

 men ovale in the mammalian 

 heart, and persist through the 

 period of incubation, closing 

 soon after hatching. 



(d) The Cushion-septum (Sep- 

 tum of the Auricular Canal). 

 This septum completes the en- 

 tire system by uniting together 

 the three septa already consid- 

 ered. It forms as two cushion- 

 like thickenings of the endothe- 

 lium in the floor and roof re- 

 spectively of the auricular canal 

 (cf. Figs. 202, 203 and 204). 

 These cushions rapidly thicken 

 so as to restrict the center of 

 the atrioventricular aperture, 

 and finally, fusing together, di- 

 vide the latter into two verti- 

 cally-elongated apertures, right 

 and left respectively. The time 

 of formation of this large endo- 

 cardial cushion dividing the au- 

 ricular canal is coincident with 

 the formation of the other septa. 



(e) Completion of the Septa. 



These have the same physio- 



FiG. 203. — Reconstruction of the 



heart of a chick embryo of 5.7 mm. 



head-length, seen from right side. 



Part of the wall of the right auricle 



is cut away. (After Masius.) 



B. Co., Bulbus cordis. D. C, Duct 

 of Cuvier. E. C. d., v., Dorsal and 

 ventral endothehal cushions. O.S. v.. 

 Opening of the sinus venosus into the 

 right auricle. 0. 1, 0. 2, Primary and 

 secondary ostia or inter-auricular con- 

 nections. 



Thus bv the end of the fifth 



or the beginning of the sixth day of incubation, the heart is 

 prepared for the rapid completion of a double circulation. The 

 embryonic circulation is never completely double, however, for 

 the reason that the embryonic respiratory organ (allantois) 

 belongs to the aortic system, and full pulmonary circulation does 

 not begin until after hatching. However, between the sixth 

 and eighth days the right and left chambers of the heart become 

 completely separated, except that the interauricular- foramina 



