THE DEVELOPMENT OF THE FROG 259 



of mesoblast lying between the hypoblast and epiblast. This 



jicAr- 



Fig. 61 



A. Transverse section through the anterior part of a young 

 embryo of the frog, showing the formation of the noto- 

 chord, mesoblast, and hypoblast. B. A horizontal section 

 through the blastopore of an embryo of the same stage 

 as A, to show the mesoblast extending round to the lips 

 of the blastopore. &p, blastopore ; ec, epiblast ; ent, 

 enteron ; ^j/, hypoblast ; yjzes, mesoblast ; nek, notochord ; 

 »/, neural plate ; y/i, yolk. 



splitting does not at first extend to the tract of lower layer 



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