SEGMENTATION 



133 



We may conceive the process somewhat as follows: 

 the physiological isolation of this posterior region of 

 the developing organism results first in increased cell 

 division and growth, i.e., it becomes or remains less 

 specialized instead of contin- 

 uing to differentiate, like other 

 parts. This is the same pro- 

 cess that we find in other cases 

 of physiological isolation. 

 Second, in this mass of em- 

 bryonic cells the development 

 of a new individual begins, 

 but in this case the individual 

 becomes a segment instead of 

 a complete organism. Its 

 failure to develop a head may 

 be due either to incomplete 

 isolation from the head region 

 or to the presence of some 

 degree of specialization in the 

 cells concerned which makes 

 them incapable of giving rise 

 to a head. 



In this connection it may 

 be pointed out that among 

 the lower invertebrates, e.g., 

 the flatworms, the first limi- 

 tation in the power of regeneration of pieces of the body 

 with advancing specialization is the loss of capacity 

 to give rise to a new head. In the triclad turbellarian, 

 Planaria, for example, pieces above a certain length 

 from any level of the body are able to regenerate heads 



Fig. 37. — Early larval stage 

 of Chactoptcrus, a polychaete 

 annelid, showing the primary 

 apical and the secondary poste- 

 rior regions of high susceptibil- 

 ity as they appear when the 

 cells begin to swell and separate 

 in various agents; at this stage 

 the mouth is developing and 

 also appears as a region of high 

 susceptibility. 



