THE PANCREAS, -POST- ANAL GUT, AND OTLLs. 



157 



the tadpole (Fig. 69), the only trace of the post-anal gut is a 

 solid cord of cells, running in a slightly irregular course beneath 

 the notochord, from the hinder end of the spinal cord to the 

 mesenteron. 



DEVELOPMENT OF THE GILL-CLEFTS 

 AND THE GILLS. 



The gills and gill- clefts, which form the main respiratory 

 apparatus of the tadpole, are developed in connection with the 

 side walls of the pharynx. The gill-clefts are a series of slit-like 



oc- 



BR.l 



BR.2 



MT- 



BR.2 



FIG. 72. 



FIG. 73. 



FIG. 72. Side view of a Tadpole at the time of hatching, x 16. 

 FIG. 73. Ventral view of the same Tadpole. 



BR.1, external gill of first branchial arch. BR.2, external gill of second branchial 

 arch. DS, stomatodaaal pit. MT, inesoblastic somites seen through the skin. OC, 

 olfactory pit. Q,, sucker. TJ, proctodaeal or cloacal aperture. 



perforations in these walls, leading from the pharynx to the 

 exterior ; while the gills themselves are vascular tufts developed 

 on the gill-arches, i.e. on the parts of the pharyngeal wall between 

 the successive gill-clefts. 



1. The Gill-clefts. 



The gill-clefts are formed as vertical, pouch-like foldings of 



