THE FCETAL CIRCULATION. 



273 



the posterior part of the primitive intestine in all Vertebrates but 

 Fishes (comp. Fig. 9, p. 11). As the allantois grows further and 

 further outwards, it comes, in the Sauropsida, to lie against the 

 internal surface of the egg-shell, and thanks to the porous nature 

 of the latter, which permits the air to pass through it, serves as an 

 important respiratory organ. In Mammals it serves in the embryo 

 both for respiration and nutrition (comp. pp. 10 and 274). The 

 further development of the embryonic vessels may take place in one 

 of three ways. 



Su 



FIG. 217. DIAGRAM OF THE VENOUS CIRCULATION IN THE CHICK AT THK 

 COMMENCEMENT OF THE FIFTH DAT. (After Balfour.) 



H, heart ; DC, ductus Cuvieri : into the ductus Cuvieri of either side falls J, the 

 jugular vein ; Su. V, the anterior vertebral ; W, the vein from the wing, and C, 

 the posterior cardinal vein ; S. V, sinus venosus ; Of, vitelline vein ; U, allantoic 

 vein, which at this stage gives off branches to the body -walls ; V.C.I, 

 postcaval. 



The embryo may either leave the egg, and take on an aquatic 

 existence (Anamnia), making use of its branchial vessels as a 

 gill-breather; the entire allantois, in the case of the Amphi- 

 bia, giving rise to the bladder. In the case of terrestrial animals 

 (Am n iota), which from the first breathe by means of lungs, 

 a modification and reduction of the branchial vessels and allantois 

 takes place, and the latter may even disappear entirely (certain 

 Reptiles and all Birds) (comp. p. 308) .* In the third case the 



1 Concerning the amnion, which encloses the embyros of Sauropdda and Mam- 

 malia, comp. p. 10 and Figs. 9 and 220. 



T 



