STUDIES ON THE DYNAMICS OF MORPHOGENESIS 273 



EXPERIMENTAL DATA 



1. Material and methods 



My experimental work on the genus Planaria has extended 

 over a period of more than ten years: during almost every year 

 of this time at least a month or more has been devoted to this 

 work and since 1907 work on Planaria has been going on during 

 at least eight months of the year. Planaria dorotocephala has 

 served as the subject of experiment to a greater extent than any 

 other species, though a considerable amount of work has been 

 done on P. maculata and some on P. simplicissima and on a 

 species to which C. E. Stringer ('09) has recently given the name 

 Planaria velata. I have also been able to extend my experiments 

 along certain lines to Phagocataand Dendrocoelum and the marine 

 Procerodes. P. dorotocephala and P. maculata are very similar 

 in their general constitution, both reproduce by fission and both 

 show much the same regulatory capacity. The formation of new 

 zooids at the posterior ends of these animals introduces certain 

 complications into the results of experiments, but when once the 

 presence of a second zooid as a physiological, if not a visible 

 morphological system is recognized, these complications are not 

 difficult to interpret. My conclusions are based largely upon the 

 study of P. dorotocephala and P. maculata. I shall refer to the 

 other genera and species only so far as they possess a special 

 interest in certain connections. 



In experiments concerned, as many of mine have been, with 

 the factors of size of the piece and the regions of the body, it is 

 necessary to select the individuals for experiment first with respect 

 to uniformity of size, for differences in size usually mean differ- 

 ences in age or nutritive condition: second with respect to the 

 length of time which has elapsed since the preceding fission. 

 In animals which have undergone fission very recently the ante- 

 rior individuals still show the lighter-colored new tissue at the 

 posterior end and are less slender than others, while the posterior 

 individuals are of course small and possess new heads. The ante- 

 rior products of fission, however, can for a long time be dis- 



