DIFFEKRNTIATION IN HYDROID COLONIES. 139 



that the chemical causes of death are identical with the chemical 

 processes that lie at the basis of development itself, differs with it . 

 radically. He asserts, as a result of experiments with the eggs 

 of sea urchins, that natural death and development are determined 

 by chemical processes that have absolutely nothing to do with 

 each other. 



My own investigations, impeded, among other causes, by 

 scarcity of material, have not yet reached a definite conclusion in 

 this direction, although the results so far obtained are not incon- 

 sistent with the results of Loeb's brilliant experiments. Mean- 

 while, the facts as they appear in seven species of Aglaoplienia, 

 may be of some service to experimenters interested in the general 

 problem. 



As is very well known, a typical colony of the plumularian 

 hydroid Aglaoplunia closely resembles a feather, of which the 

 shaft is represented by the stem and the vanes by two ranks of 

 alternating branchlets, or hydrocladia, corresponding to barbs. 

 Each hydrocladium is divided by more or less definite nodes 

 into internodes and bears on one aspect — the same in all hydro- 

 cladia — a compact series of hydranths, one to each internode, 

 with tooth-rimmed hydrothecae. Associated with each hydro- 

 theca are three tubular nematophores, a pair laterally placed, to 

 the rear, and one in front in the median line (mesial). 



Inspecting such a colony for signs of the differentiation con- 

 nected so clearly with age in C. bakeri^ one would naturally 

 look for differences associated, first, with the growth of the colony 

 as a whole, second, with the growth of individual hydrocladia. 

 The former might be determined by a comparison of correspond- 

 ing internodes of different hydrocladia, the latter by a comparison 

 of successive internodes of the same hydrocladium. 



In both cases, this procedure has established a correlation 

 between differentiation and growth.^ Since the picture of the 

 correlation is not equally clear and elaborate in all species 

 studied, I will describe it first for one of the most satisfactory in 

 these respects, namely, A. octocarpa. 



^ Loc. cit. 



^ See Pearl, Publ. Carnegie Inst. 58, 1907, and, more recently, Ritter, Univ. Cal. 

 Publ. Zool., 6, 1909, p. 64. 



