CONTROL OF MORPHOGENESIS IN PLANARIA. 311 



to a certain stage because existing conditions inhibit it. But, 

 as I have pointed out, this development does proceed far enough 

 to permit some degree of independent motor reaction in the 

 second zooid, and it is this which brings about fission. 



From the fact of the existence of the second zooid it follows that 

 the region which represents physiologically the posterior end of 

 the first zooid is somewhere near the middle of the postpharyn- 

 geal region, or in very large animals, in its anterior third. If 

 we wish to compare anterior and posterior regions of the same 

 zooid, this fact must be kept in mind. Turning now to the ques- 

 tion of the dominance of the head region, the facts which indi- 

 cate this dominance are briefly as follows : A piece above a certain 

 size from any level of the body is capable of reproducing all 

 parts which lie posterior to its level whether it produces a head 

 or not, but it is incapable of giving rise to any of the parts which 

 lie anterior to its level, unless some approach to head formation 

 occurs first. 



For example, a piece like cd, Fig. I, is capable of producing 

 a new pharynx and the characteristic postpharyngeal intestinal 

 branches, even though it may fail to form a head, as is the case 

 under certain conditions. On the other hand the piece ef, from 

 the postpharyngeal region, i. e., the posterior region of the first 

 zooid, never produces even a pharynx unless some approach to 

 head formation occurs first. If it remains headless as it usually 

 does, it also remains without a pharynx or mouth and no pre- 

 pharyngeal intestinal region with an axial intestine ever appears. 

 Anterior regulation in such cases is limited practically to closure 

 of the wound. 



The dominance of the head region is also ctearly shown by the 

 relation between the rapidity of development and the size of the 

 head on the one hand and the position of the pharynx on the 

 other. In pieces from near the anterior end (e. g., ac or 12, Fig. i) 

 the head is disproportionately large and develops very rapidly 

 and the pharynx is situated near the posterior end of the piece 

 (Fig. 3), while in pieces such as eg, Fig. I, from the posterior part 

 of the first zooid, the head is relatively small and forms slowly 

 and the pharynx is anterior to the middle of the piece, Fig. 3. 

 Moreover, it is possible to demonstrate experimentally in pieces 



