MUSEUM OF COMPAKATIVE ZOOLOGY. 211 



who also claims to have discovered the facts on which they were based. 

 Surprising is the statement tliat the external carotids are the continu- 

 ations of the ends of the anterior bifurcation of the ventral aorta. 



It is not possible at the present time to prove beyond doubt that the 

 ancestor of vertebrates possessed only a single dorsal vessel ; but the 

 best evidence at the present time (the anatomy and development of 

 the higher worms, and of those vertebrates retaining most of 'the ances- 

 tral features) points to an ancestral form having a single median dorsal 

 vessel. 



The embryological evidence cited by Macalister in support of his 

 views, is interpreted by Balfour, Gegenbaur, KoUiker, Hertwig, and 

 others, to be the effect of the shortening of the period of development, 

 the suppression of some of the stages and the adaptation to peculiar 

 embryonic environment. 



In passing over the intermediate stages between the fish and mammal, 

 our author has lost sight of the homologies of the vessels he deals with, 

 and, so fiir as his account runs, has not seen the precardiac aorta in any 

 vertebrate, but considers the two commou carotid trunks to represent 

 the pair of aortte which his theoiy calls for. 



The evidence which I have presented in the preceding pages shows 

 beyond question that the carotid arteries, instead of being derived from 

 the aorta or any of its branches, are derived from the commissures 

 which serve to connect the efferent branchial arteries with one another. 

 The bifurcated end of the aorta in the bird and mammal is only a rem- 

 nant of a previous complicated vascular apparatus. It is likewise 

 obvious that the carotid vessels cannot strictly be said to arise from, or 

 constitute the remains of, any particular pair of aortic arches, but repre- 

 sent all that is left of the commissural trunk from the most anterior 

 arch of the ancestral form to the most anterior arch of any given exist- 

 ing form. 



About the time Macalister's paper on the homology of the blood-ves- 

 sels of man appeared in England, T. J. Parker, working in Xew Zealand, 

 published a paper in the Pliilosophical Transactions on the vascular sys- 

 tem of Mustelus antai-ciiciis, in which he advances decidedlv interestinsr 

 views as to the homology of the carotids of this southern shark. In the 

 first place, Parker proposes to establish the terminology of these vessels 

 on a scientific basis, and as the result of his studies objects to the use 

 of tlie terms " internal" and '■ external " to designate the anatomical 

 relations of the carotids as really misleading. He would substitute and 

 use exclusively throughout vertebrates the terms "anterior " and " pos- 



