THE DEVELOPMENT OF THE FROG 261 



it were not for the comparison afforded by other Am- 

 phibia. 



When the notochord has been established, the mesoblast 

 forms two lateral sheets lying between epiblast and hypoblast, 

 and extending right round or nearly round the yolk, as the 



nch 



enl, 



jnes. 



Fig. 62 



Transverse section through the middle of the body of an embryo of about 

 the age depicted in fig. 60, C. ec, epiblast ; ent, enteron ; hy, hypoblast ; 

 mes, unsplit mesoblast on the ventral side ; mes.som, mesoblastic somite ; 

 nek, notochord ; nt, neural tube ; snip, somatic layer of mesoblast (somatopleur) ; 

 s&'Pi splanchnic layer of mesoblast (splanchnopleur). The space between these 

 two layers is the coelom ; yk, yolk. 



case may be, in different regions of the embryo. The edges 

 of these sheets abutting on the notochord now become 

 thickened (fig. 62) to form the so-called segmental plates, 

 and shortly afterwards the lateral parts of the sheets are 

 divided into an outer or somatic layer adjacent to the epiblast, 



