im 



THE FROG. 



the side walls of the pharynx (Fig. 74, HM, HC), which grow 

 outwards towards the exterior. They appear first at a very 

 early stage, while the blastopore is still open (Fig. 60), and even 



BR.2 



Fig. 74. — Horizontg,! section of a Tadpole at the time of hatching, x 40. 



AF, afferent branchial vessel of first branchial arch. BP, fore-brain. BIl.1, first 

 brancliial arch. BB..2, second branchial arch. BR.3, third branchial arch. C, body 

 cavity or coelom. EF, efferent branchial vessel of first branchial arch. HM, hyo- 

 mandlbular gUl-pouoh. HIT, hyoid ai'oh. IN, infundibulum. KA, arohinephric duct 

 of right side. KA', arohinephric duct of left side, seen in section. KP, head kidney 

 or pronephros. KS, third nephrostome of right pronephros. KS', third nephrostome of 

 left pronephros, seen in section. OF, olfactory pit. OS, optic stalk. TP, pharyngeal 

 region of mesenteron. TI, intestinal region of mesentcrou. Y, yolk-cells. 



jefore the closure of the neural canal is completed ; they develop, 

 ■apidly, reaching the external epiblast, and fusing with it, at an 

 sarly stage. 



