LATER DEVELOPMENT OF VASCULAR SYSTEM 300 
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). These have the same physio- 
logical significance as the fora- 
men ovale in the mammalian 
heart, and persist through the 
period of incubation, closing 
soon after hatching. 
(d) TheCushion-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 
(ef. Figs. 202, 203 and 204). 
These cushions rapidly thicken 
so as to restrict the center of Fra. 203. — Reconstruction of the 
the atrioventricular aperture, heart of a chick embryo of 5.7 mm. 
and finally, fusing together, di- head-length, seen from right side. 
widelthe latter into two verti Part of the wall of the right auricle 
is cut away. (After Masius.) 
vally-elonge apertures, right ‘ 
cally elong ited apertures, right B. Co., Bulbus cordis. D.C., Duct 
and left respectively. The time of Cuvier. E. C. d., v., Dorsal and 
, : 2 : ventral endothelial cushions. O.S.v., 
“mé his large endo- . 
of 101 on on of this large endo Opening of the sinus venosus into the 
cardial cushion dividing the au- right auricle. O.1,0.2, Primary and 
ricular canal is coincident with secondary ostia or inter-auricular con- 
: nections. 
the formation of the other septa. 
(e) Completion of the Septa. Thus by 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 
