158 



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 



OF 



BF 



BR.1 



BR.2 



KA 



KA 



Fiu. 74. Horizontal section of a Tadpole at the time of hatching, x 40. 



AF, afferent branchial vessel of first branchial arch. BF, fore-brain. BR.l, first 

 branchial arch. BR.2, second branchial arch. BR.3, third' branchial arch. C, body 

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

 mandibular gill-pouch. HY, hyoid arch. IN, infundibulum. KA, archinephric duct 

 of right side. KA', archinephric 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. 



before the closure of the neural canal is completed ; they develop 

 rapidly, reaching the external epiblast, and fusing with it, at an 

 early stage. 



