THE DEVELOPMENT OF THE FROG 265 



a vertical fold of hypoblast, which extends from the pharynx 



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 ; a 2 , the hyoid arch; a s , the first branchial 

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

 gills ; ent, enteron \fbr, fore-brain ; inch, branching mesenchyme cells ; 

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

 vesicle; ph, pharynx ; //*, pronephros ; som, mesoblastic somites 

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

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



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



