158 A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



wise can not be traced from dorsal to ventral aorta. The apparently absent 

 portions of both of these arches are near the ventral aorta. 



As shown in plate 5, figure 2, there is on the left side a small arterial twig 

 which branches off from the dorsal aorta just behind the third arch. The signifi- 

 cance of this is uncertain, but from its position it seems quite probable that it may 

 be the beginning of a fourth aortic arch. 



Whether the second and third arches are not yet completely developed or 

 whether their lumens have become secondarily occluded I am unable to determine. 

 It would seem, however, that the former is the more probable, even though in the 

 somewhat younger embryos of Watt and Van den Broeck, 47 and in Thompson's 

 embryo as described by Felix, the second arch is complete. The beginning third 

 arch (and the probable beginning fourth) indicates that in this respect my speci- 

 men is older than any of the above-mentioned embryos. The incomplete second 

 arch may be regarded as having been slightly retarded in its development. 



AORT DORSALES. 



The dorsal aortse are two large vessels which extend from about the level of 

 the anterior end of the chorda dorsalis to within a very short distance from the 

 tip of the tail. Between the eighth and nineteenth segments the dorsal aortse are 

 fused together in the mid-line, forming a single median vessel; elsewhere two 

 distinct vessels are apparent. Where two vessels are present they lie one on 

 either side of and slightly ventral to the notochord; where single it lies directly 

 ventral to the notochord. Throughout their entire courses, whether paired or 

 single, the dorsal aortse lie just dorsal to the digestive tube. 



The dorsal aortae, when traced from their anterior to their posterior extremi- 

 ties, continually change in shape and size. The median dorsal aorta is smallest 

 in cross-sectional area at about the level of the eleventh segment and largest at 

 the level of the fourteenth. The vascular bed (cross-sectional area) at this level 

 is even larger than that of the paired dorsal aorta? combined. The endothelium 

 of the dorsal aorta? is distinct throughout. In the region of the fourteenth segment 

 it forms an incomplete septum, undoubtedly the remains of the originally fused 

 medial walls of the paired vessels, which have not as yet disappeared. 



BRANCHES OF THE DORSAL AORT/E. 



Anterior Branches. At the point where the dorsal aorta and the first aortic 

 arch join, two small arteries (plate 5, fig. 2) are given off from the dorsal wall of 

 the dorsal aorta. The one situated more caudally is the smaller of the two. It 

 extends dorsally and medially and terminates in the region of the posterior end 

 of the prosencephalon, a short distance behind the anterior end of the notochord. 

 I am unable from the specimen or from other descriptions to identify this vessel 

 with any degree of certainty, but presume that it gives rise to one of the cerebral 

 arteries. The anterior branch is larger. It extends medially and anteriorly and 

 comes to lie close to the side- wall of the prosencephalon. Here it divides into 

 two branches, one of which extends forward on the ventral wall of the prosen- 

 cephalon, while the other passes medially along its ventral wall. The latter 



