THE DEVELOPMENT OF THE FROG 265 



a vertical fold^of hypoblast, which extends from the pharynx 



Of: 



fbr 



nek 



B. 



Fig. 64 



A. Horizontal section through an embryo frog some time before hatching, 

 showing the optic vesicles springing from the sides of the fore-brain, 

 the three anterior pairs of gill-slits, and five pairs of mesoblastic 

 somites. B. A similar section through a tadpole shortly after 

 hatching. The head is cut in a lower plane than in A, so only a 

 small part of the anterior end of the brain appears in the section. 

 a 1 , the mandibular arch; <z 2 , the hyoid arch; a%, the first branchial 

 arch ; bv, blood-vessel in first and second branchial arch ; eg, external 

 gills ; ent, enteron ',fbr, fore-brain ; mch, branching mesenchyme cells ; 

 na, nasal pits ; nch, notochord ; nps, peritoneal funnel ; op, optic 

 vesicle; ph, pharynx ; pnp^ pronephros ; som, mesoblastic somites 

 which in B are converted into muscle. /, mandibulo-hyoid slit ; 

 //, hyo-branchial slit ; III-V, branchial slits. 



to the surface epiblast, and meets and fuses with the deeper 



