DYNAMICS OF MORPHOGENESIS 107 



essentially similar : as the concentration of the anesthetic or other 

 depressing agent increases, as the temperature becomes lower, 

 as starvation advances, etc., my observations show that in all 

 cases the processes at the posterior end of the piece are inhibited 

 first and after these others in sequence toward the anterior end.^ 

 It is possible in fact, with proper condition^, to inhibit only tail- 

 formation, or both tail-formation and pharynx-formation, or 

 everything except head-formation, or finally head-formation itself. 

 Moreover, when we compare similar processes in pieces from 

 different levels we find that they are not the same in all respects. 

 In the case of head-formation itself, for example, quantitatively 

 different conditions are necessary to produce a given effect on 

 morphogenesis in pieces from different levels. In general abnor- 

 mal heads are produced or head-formation is inhibited by less 

 extreme conditions in pieces from the posterior region of the first 

 zooid than in similar pieces from the anterior region. Besides 

 this the results differ characteristically at different levels of the 

 body according to the way in which the depressing agent or con- 

 dition is used. We can then obtain evidence concerning the 

 relation between the regulation of pieces and the axial gradient 

 in a variety of ways. Some lines of this evidence are presented in 

 the following pages. 



1 The fact that under certain conditions pieces fail to form heads and yet give 

 rise to long posterior outgrowths 'headless' pieces (Child, '11 c, '11 d) may seem 

 to be a direct contradiction of this statement, but it is not. It was pointed out 

 in the preceding paper (Child, '11 f, pp. 227-231) that the outgrowth at the ana- 

 tomical posterior end of headless pieces is not physiologically simply a posterior 

 end but is part of a new zooid or series of zooids and so possesses a higher rate of 

 reaction than the regions of the old tissue anterior to it. In such cases it is the 

 absence of a head that makes possible the establishment of a new head region in 

 the posterior part of the piece. This new head region may be and usually is the 

 region of highest rate of reaction in the piece and may dominate it (Child, '11 f, 

 pp. 239-241). In the headless pieces, then, instead of a new tail, a new zooid may 

 arise at the posterior end of the piece and we find that the development and growth 

 of the new zooid up to a certain stage is less readily inhibited by external factors 

 than is the formation of a tail. 



On the other hand, the statement that under the increasing action of depi-essing 

 factors the morphogenic processes are inhibited in sequence from the posterior 

 end anteriorly is true only for pieces which consist of a single zo5id or part of a 

 zooid and in which the axial gradient is simple and continuous. For such pieces, 

 however, it holds in all cases, so far as my observations go. 



