196 



Frvceediiigs of the Royal Physical Society. 



lying between two visceral clefts, and in existing vertebrates 

 the distribution of external gills is as follows : — 



Visceral Arch. 



Ontogenetic Development. 



In ontogeny the development of the external gills follows 

 the same lines in all three groups — Crossopterygii, Dipnoi, 

 and Urodela. The external gill makes its first appearance 

 as a rounded elevation of the surface of the body, consisting 

 of mesoderm covered by ectoderm, and situated between the 

 positions where two visceral clefts will appear later. The 

 external gill rudiment appears long before the clefts are 

 perforated. The hypoblastic walls of the gill outgrowths 

 of the pharyngeal wall are sharply distinguishable from 

 the ectoderm by their cells being packed with large yolk 

 granules. There can be no possibility of hypoblast entering 

 in any way into the formation of the external gills, which 

 are purely dermal outgrowths covered with ectoderm. 



The external gill rudiment comes to project more and more 

 freely above the general surface, and presently it develops 

 its characteristic relation to the vascular system, as described 

 long ago by Boas. The main aortic arch passes right out 

 into the external gill, looping back at its tip. The aortic 



! Cf. Driiner, Zool. Jahrh. AnaL, B.l. 19, s. 434. 



