EARLY DEVELOPMENT IN THE FROG. 



327 



an area of disintegration that has just begun in the region where 

 the tail bud appears later. In neural fold stages disintegration 

 begins at the anterior end of the floor of the neural groove usually 

 at two points on either side of the median line, where the primor- 

 dia of the optic vesicles have appeared. Often, and especially 

 in earlier neural fold stages, the disintegration begins in the 

 median line at the anterior end of the neural groove. From this 



C 



FIG. 6. Disintegration stages during late cleavage. The eggs are shown in 

 side view with the animal pole uppermost and the gray crescent to the right. 



point or points as the case may be, disintegration spreads back 

 along the neural groove nearly to the posterior end, where it 

 joins an area that has just begun to undergo dissolution. In 

 the meantime, the medullary folds, ventral suckers, and anterior 

 part of the head region have begun to disintegrate. 



With the closure of the neural folds and the further elongation 

 of the embryo, local differences in susceptibility begin to appear 

 with the differentiation of certain organs. While in general, 

 the apical-medial-dorsal region begins to disintegrate first, the 

 dissolution of cells in this region may be followed shortly, or 

 even preceeded by, the disintegration of rapidly proliferating 



