304 



COMPARATIVE ANATOMY 



the branchial vessels and allantois takes place, and the latter may 

 even disappear entirely (see under Urinary organs). In the third 

 case, the embryo undergoes a longer intra-uterine existence, the 

 allantois coming into close connection with the walls of the uterus 

 by means of the chorionic villi : the allantoic vessels extend into the 

 wall of the uterus and come into more or less close relations with 



AA 



Pig. 244. — Diageam or the Circulation of the Yolk -Sac at the end op 

 THE Third Day of Incubation in the Chick. (After Balfour.) 



H, heart ; AA, the second, third, and fourth aortic arches : tlie first has become 

 obliterated in its median portion, but is continued at its proximal end as the 

 external carotid, and at its distal end as the internal carotid ; Ao, dorsal 

 aorta; i.O/.^, left vitelline artery; R.Of.A, right vitelline artery; S.T, 

 sinus terniinalis ; .L. 0/, left vitelline vein ; R.Of, right vitelline vein; 8.V, 

 sinus venosus ; D.C, ductus Cuvieri ; S.Ca.V, anterior cardinal or jugular 

 vein; F.Oa, posterior cardinal vein. The veins are marked in outline, and 

 the arteries are made black. The whole blastoderm has been removed from 

 the egg, and is supposed to be viewed from below. Hence the left is seen on 

 the right, and vice versa. 



the maternal vessels, thus serving for the respiration and nutrition 

 of the foetus. In this way a placenta and a placental circulation 

 arise (comp. Fig. 9, and pp. 9 and 337). 



On the appearance of pulmonary respiration, important changes 

 take place in the branchial vessels and heart. The formation 

 of a septum in both the atrium and ventricle leads to the presence 



