THE DEVELOPMENT OF THE FROG 253 

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





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Fig. 59 

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

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

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

 has rotated through an angle of 120 degrees, /f is a transverse section taken 

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

 lateral Hps of the blastopore, bcl, blastoccele ; bp, blastopore ; dd, dorsal 

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

 epiblastic thickening marking the anterior end of the neural plate. 



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



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