18 



Scientific Proceedings, Royal Dublin Society. 



brates was also the ancestral mouth which, in the vertebral ancestors, 

 had a suctorial function. Balfour's chief argument lay in his 

 opinion of the homology of the so-called external gill- arches in 

 elasmobranchs with the branchial skeleton in the lamprey. 



In order to disprove this view, Dohrn commences his fourth 

 study with a consideration of the development of the visceral 

 arches. He choses the arches lying between the second and fourth 

 clefts, being more typical in their form than the others. 



In speaking of the branchial veins, Dohrn alters the current 



I! 



Figure II., longitudinal section through the head of an embryonic Petromyzon (after Dohrn). 



ep. = epiphysis. 



na. = nasal invagination. 



hy. = hypophysial invagination. 



ul. = upper lip. 



mo. = mouth, or stomodacum. 

 11. = lower lip. 

 ect. = ectoderm, 

 ent. = entoderm. 



terminology by calling them the anterior and posterior vein of a 

 certain arch, while formerly they were spoken of as the anterior 

 and posterior vein of a gill-pouch, which, of course, is exactly the 

 reverse of the former. (See fig. iv.) 



The original root of the aorta is identical with the original 

 branchial artery whose dorsal part is called aortic root. As 

 development proceeds, branchial veins originate from subsidiary 

 streams, which at first open into the dorsal part of the artery. The 

 anterior vein now develops more strongly, and the posterior vein, 



