THE DEVELOPMENT OF THE FROG 253 



going on in the egg by means of a series of sagittal sections 



Fig. 59 



A, C, , G, and //. Sagittal sections through the stages indicated by the corre- 

 sponding letters in fig. 58. The vertical lines in A, C, A", G, are drawn through 

 the poles of the eggs ; the horizontal lines indicate the equators. In // the egg 

 has rotated through an angle of 120 degrees. K is a transverse section taken 

 through the horizontal line in stage , fig 58, to show the infolding of the 

 lateral lips of the blastopore. bcl, blastocoele ; bp, blastopore ; db, dorsal 

 lip of blastopore ; ent, mesenteron ; ep, epiblast ; //, lower layer ; , 

 epiblastic thickening marking the anterior end of the neural plate. 



that is, by sections of the principal stages depicted in fig. 58, 



