THE DEVELOPMENT OF THE FROG 259 



of mesoblast lying between the hypoblast and epiblast This' 



mes 



1 ig. 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. l>p, blastopore ; ec, epiblast ; ent, 

 enteron ; hy, hypoblast ; tnes, mesoblast ; nc/i, notochord ; 

 np, neural plate ; yk, yolk. 



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



