CONTROL OF MORPHOGENESIS IN PLANARIA. 313 



from the postpharyngeal region that any external conditions 

 which alter quantitatively the processes which give rise to a 

 head also alter the position of the pharynx. A single example 

 is given here to illustrate this relation. The piece eh, Fig. i, 

 including the whole postpharyngeal region and the second zooid, 

 always gives rise, under anything like natural conditions, to 

 animals like Fig. 4 with large heads, normal eyes and the pharynx 

 somewhat anterior to the middle. If, how r ever, we decrease the 

 rate of the metabolic processes by means of low temperature, 

 anesthetics, CO 2 , etc., the head forms much more slowly, remains 

 much smaller, often possessses only a single median eye and the 

 pharynx appears close behind the head and remains of small 

 size. Fig. 5 shows the position of the pharynx in such a piece 

 after regulation in 1.5 per cent, alcohol, 0.3 per cent, ether or in 

 water containing CO 2 . By varying the external conditions 

 quantitatively all gradations between the conditions of Figs. 4 

 and 5 can be obtained. Changes in the other direction can be 

 brought about by high temperature and also by other conditions 

 which increase the rate of- reactions in the body. In such cases 

 the head is larger and develops more rapidly and the pharynx lies 

 further posteriorly. 



In general I have found it possible by these and other methods 

 to alter the localization of the pharynx in pieces within very wide 

 limits. Since the pharynx is situated at the posterior end of the 

 median axial intestinal branch of the prepharyngeal region, the 

 localization of the pharynx in the piece is directly related to the 

 length of this axial intestine. In these experiments a definite 

 spatial relation between the rate of the dynamic processes and 

 the localization and size of organs and regions appears. 



Moreover, the dominance of the head region and the limit of 

 its effectiveness also appear in the conditions determining the 

 origin of a second zooid. In newly hatched P. maculata I have 

 not as yet had opportunity to examine newly hatched P. doro- 

 tocephala, but have no doubt that they are similar the second 

 zooid is not present and even the whole postpharyngeal region 

 is incapable of forming a head when isolated. In this respect 

 it is like the posterior region of the first zooid in larger animals. 

 As the worm grows longer the capacity to produce a head appears 



