104 C. M. CHILD 



phogenetic development as well as in function in the adult, and 

 in the third paper (Child, '11 f) the process of asexual reproduc- 

 tion in Planaria was shown to be a necessary consequence of the 

 existence of a distance limit in the dominance of the head region. 

 Parts which come to lie beyond this limit, either in consequence 

 of growth or of decrease in the effective distance of correlative 

 factors originating in the dominant part (Child, '11 a) return to 

 the fundamental reaction of planarian protoplasm, which is repre- 

 sented in morphogenesis by the formation of a head or the initial 

 steps in this process, according to the degree of physiological isola- 

 tion and the rate of the dynamic processes. 



It was shown that in Planaria the failure of the posterior zooids 

 to undergo visible morphogenetic development while still attached 

 to the parent body is due to the fact that they are not completely 

 isolated physiologically from the dominant head region of the 

 animal. 



The purpose of the present paper is to present certain data 

 which throw light upon the problem of the nature of the axial 

 gradient. These data concern certain features of the regulatory 

 morphogenesis of pieces under experimental conditions. 



II. EXPERIMENTAL DATA 



A. THE AXIAL FACTOR IN REGULATION UNDER CONSTANT EXTERNAL 



CONDITIONS 



In the first paper of this series the axial factor in regulation 

 as it appears under the usual conditions of existence was consid- 

 ered at some length: the most important points of that consid- 

 eration must be briefly stated here. They are as follows: first, 

 the axial differences in rate consist of a decrease in the rate of 

 regulation with increasing distance from the original anterior 

 end; second, in longer pieces the size of the head and in shorter 

 pieces the frequency of normal head-formation also decrease with 

 change of level in the posterior direction; third, in all pieces the 

 pharynx is farthest posterior in the most anterior pieces, i. e., 

 the prepharyngeal region is longest in such pieces, and its length 



