SEGMENTATION 



137 



apparently consists of a number of more or less com- 

 pletely fused segments, and in the vertebrates of a still 

 larger number. Even in the oligochaete annelids 

 Hyman (1916) has found indications in the behavior in 



FIG. 39. Stages of disintegration of frog embryo during late 

 cleavage, in side view; the secondary posterior region of growth is 

 already appearing as an area of high susceptibility in the equatorial 

 region (from Bellamy, 1919). 



regulatory development of a number of anterior seg- 

 ments, characteristic for each species, that they con- 

 stitute a more or less definite head region. 



The problem of the segmentation of the vertebrate 

 head has been widely discussed and an extensive litera- 

 ture exists which cannot be considered here. The 



