ANALYSIS OF FORM REGULATION. 1 6/ 



VI. The Effect on Localization and Number of Parts. 



The localization of the pharynx in anaesthetics is different from 

 its localization in water. In prepharyngeal pieces the pharynx 

 appears nearer the posterior end and in postpharyngeal pieces, 

 nearer the anterior end than in water. This difference in the 

 localization of the pharynx means that the process of redifferen - 

 tiation does not extend so far from the posterior end in prephar- 

 geal pieces, or from the anterior end in postpharyngeal pieces as 

 in water. Moreover, the shifting of the pharynx toward the 

 middle, which occurs in each piece in water, is greatly retarded 

 and in many cases never completed in the anaesthetics. 



This change in the localization of a specific organ resulting 

 from a change in the external medium is of considerable interest 

 and has a bearing upon the problem of localization in general. 

 Driesch has maintained that the localization of morphogenetic 

 processes in "harmonious equipotential systems," of which he 

 regards Planaria as an example, cannot be accounted for on a 

 physico-chemical basis. In this case the facts certainly demon- 

 strate that a change in the constitution of the medium, of such a 

 character as to affect the metabolic processes, brings about a 

 change in the localization of a specific organ, the pharynx. 



Another case which involves not merely localization but num- 

 ber of parts as well, concerns the eye-spots. ' Normally two eye- 

 spots, symmetrically placed, appear in Planaria in regulation as 

 well as in ontogeny. As noted above, however, we find that short 

 pieces in water, especially those from the middle region of the 

 body, often give rise to only one median eye-spot. In fact the 

 formation of a single eye-spot instead of two is the first indication 

 of decreased or incomplete ability to form a head. If we com- 

 pare pieces of different lengths with anterior ends at the same 

 level of the body we find that with decreasing length of piece 

 single eye-spots appear more frequently, until in pieces of a cer- 

 tain length perhaps only single eye-spots are formed. With still 

 further decrease in the length of the pieces the heads with single 

 eye-spots give way to a headless condition. It should be noted, 

 however, that the relation between the length of the piece and 

 the character of the result is by no means constant, but is de- 

 pendent on the length of the animal, the physiological condition 



