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 (19 16) 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 



