CONTROL OF MORPHOGENESIS IN PLANARIA. 



317 



characteristic of the species in nature. In pieces below a certain 

 size which varies with the region of the body and with other 

 internal and external conditions, normal wholes develop from iso- 

 lated pieces but rarely or not at all. A brief description of some 

 of the characteristic results of regulation under these conditions 



9 



10 



one 



15 



CJc 



II 



CM; 



16 



12 



20 



21 



22 



23 



U 



FIGS. 8-27. 



is necessary. For convenience these are arranged under a number 

 of heads, but it must be understood that the "types" thus dis- 

 tinguished are by no means sharply denned. They grade into 

 each other in various ways, so that the whole series of types is 

 actually a continuous series. 



I. Tailless. (Fig. 8.) Very short pieces from the more 

 anterior regions often give rise, especially under certain experi- 



