THE DEVELOPMENT OF THE FROG 265 



a vertical fold"of hypoblast, which extends|from the pharynx 



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, i?. 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. 

 «l,the mandibular arch ; a"^, the hyoid arch ; a'^, the first branchial 

 arch ; irv, blood-vessel in first and second branchial arch ; e^y external 

 gills ; ent. enteron ;y2'>-, fore-brain ; ?7zck, branching mesenchyme cells ; 

 na, nasal pits ; nc/i, notochord ; n^s, peritoneal funnel ; op, optic 

 vesicle; ^/i, pharynx ; pnj>, pronephros ; som, mesoblastic somites 

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

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



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



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