300 C. M. CHILD 



spond, and different individuals were cut into different numbers 

 of pieces. Since all these individual records show absolutely 

 nothing for our present purpose that is not more clearly shown by 

 the preceding tables and curves, their presentation in this paper 

 is quite unnecessary. Thsir existence is mentioned merely in 

 order to show that my conclusions concerning the factors of 

 size and piece and region of body are based on the study of a very 

 much larger number of individuals than is included in the records 

 presented above. These latter are based on 954 pieces from 314 

 worms, but my other data include an even larger number of 

 worms and pieces. 



DISCUSSION 

 1. The presence of a second zooid 



Some years ago I called attention (Child, '06b) to the apparent 

 presence of a second zooid in Planaria dorotocephala and P. 

 maculata, as indicated by the regional differences in the course 

 and results of regulation. The data presented above bear directly 

 upon this point, and it is necessary to consider the question before 

 proceeding farther. 



The facts are briefly as follows: the capacity for head forma- 

 tion decreases, with increasing distance of the level of section from 

 the original head until at some level in the postpharyngeal region 

 it increases again and becomes even greater than in the anterior 

 region. The tables and curves show this point very clearly. 

 The examination of the smaller pieces, e. g., the curve af fig. 40, 

 which shows the frequency of heads in the sixteenth pieces, and 

 similarly the series 197 of table I, shows that the region where the 

 power of head formation begins to increase again coincides very 

 closely with the region where fission occurs. 



So far as I have examined planarians which do not regularly 

 undergo fission in this region, e. g., Planaria simplicissima 

 Phagocata gracilis and Planaria velata Stringer no such increase 

 in the ability to produce wholes, or what here amounts to the 

 same thing, heads, occurs in the posterior regions of those forms. 



