STUDIES ON THE DYNAMICS OF MORPHOGENESIS 301 



Moreover, the process of regulation in pieces from the region 

 thus marked off as a second zooid is similar to that in younger 

 animals, as compared with that in other regions of the body. In 

 young individuals of Planaria, external and nutritive conditions 

 being as nearly as possible similar, regulation is more rapid than 

 in older and the younger piece approaches the usual proportions 

 more rapidly than the older; moreover, a larger piace is necessary 

 for the formation of a whole in the older than in the younger ani- 

 mal. In all of these respects pieces from the posterior quarter of 

 the body appear "younger" than pieces from other regions. As 

 regards the rapidity of regulation no data are given except inci- 

 dentally in the present paper, but the fact that pieces from the 

 region of the second zooid regulate more rapidly than pieces from 

 other regions appears in every series of experiments involving these 

 regions. 



As regards the relation between regeneration in the stricter 

 sense and redifferentiation, a comparison of figs. 38 and 39 from 

 the region of the second zooid, with figs. 29, 30 and 31 from the pre- 

 pharyngeal region will suffice to show the differences in the rela- 

 tion between regenerated and redifferentiated parts in the form- 

 ation of the new head, as well as the differences in the process 

 of change of shape. Moreover, all the series which include this 

 region and in which the pieces are sufficiently small to permit 

 conclusions on this point, show that a smaller piece is capable of 

 forming a whole in the region of the second zooid than in any 

 other region of the body. 



My experiments with alcohol (Child 'lib) also show that this 

 region is physiologically younger than any other, at least at cer- 

 tain stages of the development of the new zooid, and in these 

 experiments a perfectly distinct demarcation between the intact 

 second zooid and the disintegrating first zooid often appears. 



And finally, my experiments on the experimental control of 

 fission (Child, '10) add demonstrative evidence along another 

 line to that cited above. There can then be no doubt that in 

 individuals of P. dorotocephala and P. maculata without sexual 

 organs and above a certain size a certain posterior portion of the 

 body is physiologically specified as a second zooid. 



