310 C. M. CHILD. 



increase represents the anterior region of the second zooid. Where 

 the second zooid is of considerable length a similar decrease in 

 the capacity for head formation occurs from its anterior end to 

 about the posterior one half to one third of its length, where 

 again a sudden increase in the ability of the pieces to form heads 

 occurs. Posterior to this level there is a more or less diffuse 

 region of very great capacity for head formation probably in 

 reality a series of head regions, extending to the end of the body. 

 Other features of regulation, the rate, the relation between re- 

 generation and redifferentiation, the size of the head and the 

 position of the pharynx show corresponding regional changes. 

 Most of these facts were briefly stated in an earlier paper (Child, 

 'o6b) and will be considered more fully elsewhere. 



Secondly, it is possible by various means to induce fission in a 

 very short time in animals which are far below the size at which 

 fission would occur in nature, or which for other reasons would 

 be incapable of fission under natural conditions. In all cases 

 where fission can be induced it occurs at a level where sudden 

 increase .in the capacity for head formation appears. Some of 

 the methods of inducing fission have been described in another 

 paper (Child, '106). 



And finally, the length of the second zooid, together with other 

 zooids which may arise from its posterior region varies with the 

 length of the whole, but not proportionally. In very small 

 animals the second zooid cannot be found by any methods thus 

 far employed, but as the animal grows longer, it appears at the 

 posterior end as a short region of high capacity for head formation 

 and as growth continues it increases in length more rapidly 

 than other parts; consequently in very large animals the post- 

 pharyngeal region is often double or more than double the length 

 of the prepharyngeal, while in very small animals the prepharyn- 

 geal region is the longer. In Planaria the second zooid does not 

 develop a head as visibly differentiated region before its separa- 

 tion, as do the posterior zooids of the Microstomidae and annelids. 

 This is because the processes which make this region morpho- 

 logically posterior are more firmly fixed in Planaria than in 

 those forms. So long as it remains attached to more anterior 

 regions, the development of the second zooid can proceed only 



