THE DEVELOPMENT OF THE FROG 253 



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



K. 



Fig. 59 



A, C, E, G, nnd If. Sagittal 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 // the egg 

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

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

 lateral lips of the bla^topore. bcl, blastocoele ; />/>, blastopore ; dt>, dorsal 

 lip of blastopo/e ; ent, mesenteron ; <?/, 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, 



