NERVOUS SYSTEM AND OTHER PARTS 1 21 



the level of the body from which the piece was taken. 

 In Planaria, for example, pieces taken from regions 

 near the original head and those from more posterior 

 levels (Fig. 19, a and b) behave in essentially the 

 same way in this respect. Although the anterior end 

 of piece b represents a body level which is normally 

 separated from the head by the whole pre-pharyngeal 

 and pharyngeal region, a new head develops directly 

 from the anterior end of b as well as from that of a and 

 the body regions corresponding to those between the 

 head and the level of b in the original animal arise later 

 by the reorganization of the tissues of b posterior to 

 the new head (Fig. 22). Moreover, this reorganization 

 never takes place unless a head does develop or begin to 

 develop. In the earthworm and other annelids which 

 are not too highly specialized to give rise to new heads 

 from postcephalic body levels the developmental process 

 is the same in these respects. In fact, in all cases where 

 isolated pieces give rise to new individuals the develop- 

 ment of the new apical end or head is the first new 

 differentiation at the anterior end, irrespective of the 

 body level represented by the piece. That is to say, 

 this process of so-called regulatory development is 

 essentially similar to embryonic development in that 

 differentiation of the new individual begins at the apical 

 or anterior end and proceeds basipetally or posteriorly. 

 In a piece like b (Figs. 19-22) the new head is "out of 

 place" when it arises, i.e., it adjoins parts which in the 

 normal animal are far removed from the head. The 

 normal relations are re-established by the redifferentia- 

 tion into the various intermediate regions of parts of 

 the piece posterior to the new head. The significance 



