156 



A SURVEY OF THE LATER EMBRYO OR LARVA 

 .Olfactory pit. 



Stomodeal cleft 



External gills, 



M Jm:... aafe:-. 



Development of the external gills of Raiia pipiens. 



Adaptations for respiration become a more and more important 

 biological function as the embryo develops organ systems. Thus the 

 third and fourth visceral arches develop finger-like external gills and 

 the fifth visceral arch gives rise to a rudimentary pair of such gills. 

 The mouth opens through to the gills and a constant current of water 

 is taken into the mouth and pharynx. This water passes out through 

 the second and third branchial clefts (otherwise designated as the 

 fourth and fifth visceral clefts) and over the associated external gills. 

 In this way these thin-walled, ectodermally covered, and highly vas- 

 cular gills are able to exchange CO.j and Oo. Subsequently, when the 

 tadpole develops a set of internal gills, the hyoid arch gives rise to a 

 posteriorly directed flap-like membrane which covers the degenerat- 



Tadpole with external gills. 



