214 THE SIXTH DAY. [CHAP. 



appears, the internal carotids remain as branches springing 

 from the distal ends of the second pair of arches ; they are 

 supplied with blood from that pair, the stream in which flows 

 chiefly towards the head instead of backwards towards the 

 dorsal aorta, as is the case with the succeeding arches. When 

 the second pair of arches is obliterated, the connecting branch 

 with the next arch is again left, and thus the internal carotids 

 appear as branches' from the distal ends of the third pair of 

 arches. 



On the third day, the dorsal aorta does not long remain 

 single in its backward course along the body, but soon 

 divides into two trunks which run one on either side of the 

 middle line of the body. On the fourth day however, the 

 point at which the aorta divides is carried very much further 

 back, .quite to the posterior end of the Wolffian bodies. The 

 two branches into which it there divides, form the origin 

 of the iliac (Fig. 65, 1 A) arteries supplying the hind limbs. 

 Each of these sends a short branch to the allantois (UA}. 

 As the allantois grows rapidly and becomes an import- 

 ant respiratory organ, these allantoic or umbilical arteries 

 increase so much in size that they speedily appear to be the 

 direct continuations of the aorta, and the iliac arteries to be 

 mere branches of them. As a general, though apparently 

 not invariable rule, the right umbilical artery gets gradually 

 smaller and soon disappears. 



From the main trunk of the aorta are given off small 

 transverse branches between the vertebrae (represented dia- 

 grammaticallyin Fig. 65, Ver. A.}, and also important branches 

 to the Wolffian bodies (Fig. 65, wf. A.}. 



The omphalo-mesaraic artery ( Of. A.) now leaves the aorta 

 as a single but quickly bifurcating trunk, which at the end 

 of the fifth day is still very large. 



By the fifth day, the ventricular portion of the heart 

 (compare Chap. vn. 7) is almost completely divided into 

 two chambers. The bulbus arteriosus is also divided by a 

 septum into two channels, which do not however run in s a 

 straight course, but have, according to Von Baer, a spiral 

 arrangement. One of the channels communicates with the 

 right ventricle of the heart and the other with the left. The 

 spiral of the former turns from the right and above (dorsal), 

 to the left and below (ventral) ; the latter from the left and 



