274 C. M. CHILD 



tinguished from animals nearly ready for fission by the differ- 

 ence in position of the pharynx. In animals approaching 

 fission the postpharyngeal region is longer than the prepharyn-. 

 geal, while for a considerable period after fission the reverse is 

 the case in the anterior product. In the large individuals the 

 growth is chiefly in the postpharyngeal region, more specifically 

 in the second zooid, consequently after a certain length is attained 

 further increase in length is almost wholly postpharyngeal. 

 After fission the postpharyngeal region of the anterior product 

 gradually elongates again. By selecting individuals of a certain 

 length in which the position of the pharynx is approximately the 

 same it is possible to attain a certain degree of uniformity as 

 regards the physiological condition in respect to fission. 



Furthermore, uniformity w4th respect to nutritive conditions 

 must also be considered. Starved animals give different results 

 from those which are well fed. In a stock collected at one time 

 from a single locality or kept in captivity and fed at regular 

 intervals, the individuals of a certain length may be regarded as 

 uniform in nutritive condition within certain limits. As I 

 have shown elsewhere (Child, 'lib), the nutritive reserves in well 

 fed individuals of Planaria probably last for some three or four 

 weeks under average conditions, i. e., the animals can go for 

 three weeks or more without food before they begin to exhibit 

 the symptoms of extreme starvation and rapid decrease in size. 

 Animals left for a week or two without food show no very great 

 differences in their regulatory capacity from those fed daily up 

 to the beginning of the experiment. 



And finally, since temperature and oxygen supply influence 

 the course and results of regulation, uniformity or control of 

 these external conditions during experiment is also necessary 

 if results which are really comparable are to be obtained. 



For all the series described in the following section and for 

 others, which, together with these, form the basis of my work on 

 the factors of size and position, well nourished individuals were 

 selected averaging about 15 mm. in lenoth in some cases 12-15 

 mm., in others 14-18 mm., and with the postpharyngeal region 

 longer than the prepharyngeal. 



