210 BULLETIN OF THE 



so that after the fifth day there is but a single dorsal vessel for the middle 

 and the hinder part of the body, — the dorsal aorta. In the region of 

 the head and neck of mammals, the foremost ends of the two vessels re- 

 main permanently separate as the internal carotid arteries. [Italics mine.] 

 . . . There were also originally two ventral longitudinal vessels, but 

 their union probably occurred even earlier than that of the dorsal. . . . 

 The setting apart of one portion of the single ventral vessel to form the 

 heart differentiates the pre- from the post-cardiac portions of the ventral 

 vessel. As a consequence of the cardiac differentiation, the only places 

 where complete metameric arcades remain are the precardiac segments. 

 . . . Behind the heart in higher vertebrates a series of vessels extend from 

 the dorsal aorta through the mesogastric fold, and end in the splanch- 

 nopleure ; these correspond to the dorsal extremities of the postcardia 

 lateral metameric arcades. With the condensation of the anterior seg- 

 ments which takes place in the formation of the skull, all distinct vas- 

 cular metamerism is lost, and the anterior segmental arches become 

 displaced backward or lost. The common and external carotids are con- 

 tinuations of the ventral aorta, while the root of the internal carotid is 

 the altered relic of the third arch, and the ascending continuation of 

 that vessel is the upper part of the dorsal aorta. . . . The only carotid 

 branches which in any way represent rudimental arcades are the occipi- 

 tal and posterior auricular arteries. . . . The cervical dorsal aorta (in- 

 ternal carotid) has only rudimental branches in the neck, represented 

 by the intercarotid ramuli. Its intracranial continuation gives off three 

 lateral neural branches, the posterior, middle, and anterior cerebrals 

 (the first originally being a carotid branch, its root being the so called 

 posterior communicating, but its anastomotic internal branch, which 

 joins the median anastomosis, dilates so as to form its functional 

 root). The ventro-lateral branches are reduced and modified as tym- 

 panic, vidian, receptacular, and ophthalmic branches." 



As the foregoing quotations show, Macalister tacitly assumes that the 

 double nature of the dorsal vessel in mammalian embryos is comparable 

 with that of some vermian type. He claims that the anterior ends of 

 the double dorsal vessel are transformed into the internal carotid arteries 

 of the adult, which is certainly true, but not in the sense our author 

 intends it, as appears from the context, for he says further on, that the 

 root of the internal carotid is the altered relic of the third arch. These 

 views were, I believe, first expressed by Allen Thompson 26, 27 in 1831, 



26 Thompson, Allen. The Development of the Vascular System. New Phil. 

 Journ., Edinburgh, 1831. 



27 The same. Quain's Anat., 9th ed., II., 1882. 



