Scharff — On Dohrn'' s Theories on the Origin of Vertebrates. 17 



"We thus find at a later stage that the so-called nasal organ of 

 Petromyzon is in reality almost wholly formed by the hypophysis, 

 only the external part belonging to the former. (See figs. n. and in.) 



The hypophysis ends blindly, underneath the part of the brain 

 known as the infundibulum (in.). 



In closing 1 , this chapter on the development of the hypophysis 

 in Petromyzon, Dohrn alludes again to his view, that this organ 

 was originally a pair of gill-clefts in front of the mouth. When 

 the coalition of the two gill-clefts in the ancestral vertebrate took 



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



in. = infundibulum. 

 c. = chorda dorsalis. 

 11. = lower lip. 



na. = nasal invagination. 



hy. = hypophysial invagination. 



ect. = ectoderm. 



mo. = mouth, or stomodacum. I ent. = entoderm. 



place in order to form the mouth, the anterior hypophysial gill- 

 clefts also lost their independent character, and in one group of 

 vertebrates they were drawn within the mouth and in the other, 

 owing to the enormous development of the upper lip, they united 

 within the nasal organ. 



In Petromyzon the so-called nasal organ probably still retains 

 to some extent, the function of a gill. 



IV. — The Visceral Arches of Elasmobranchs. 1 



Balfour was opposed to Dohrn's theories about the origin of 

 the mouth. He held the view, that the present mouth of verte- 



1 Ibid., vol. v. 1884, pp. 102-151. 



SCIEN. PROC. R.D.S. — VOL. VI., PT. I. 



