46 ANIMAL AGGREGATIONS 



come together to form viable aggregations. Galtsoff 's statement con- 

 cerning sponge cells is: "The examination of the behavior of dis- 

 sociated cells shows that the formation of aggregates is chiefly due 

 to random movement of the archaeocytes which collect all the cells 

 lying in their route." Child is more certain of the absence of definite 

 cytotropism around dissociated Corymorpha cells and has observed 

 cells when near together to move apart without aggregating ap- 

 parently as often as he has found movement in the opposite direc- 

 tion. 



The cytotropism observed by Roux can be interpreted as analo- 

 gous with a very simple social appetite, or at least showing that mu- 

 tual attraction between living units extends to dissociated embryo 

 cells. From forces similar to those causing such simple mutual at- 

 tractions of cells, we might expect social appetites to develop. Such 

 a reaction may be regarded as a forerunner of the social instincts of 

 many observers. On the other hand, in the formation of aggrega- 

 tions of dissociated sponge and hydroid cells there is no evidence of 

 such mutual attraction. The method is essentially the same as that 

 just outlined for the formation of aggregations of land isopods and 

 of starfishes. Yet under favorable conditions these cell aggregates 

 formed without evidence of mutual attraction may develop into well 

 integrated animals. 



PROTOTAXIS AND INSTINCT 



Wallin (1927) has postulated a factor or principle which he re- 

 gards as of fundamental importance for many interrelations between 

 cells or between whole organisms and which he has called the prin- 

 ciple of "prototaxis." This is defined as "the innate tendency of one 

 organism or cell to react in a definite manner to another organism or 

 cell." This reaction may be either positive or negative. The latter 

 results in a mutual repulsion of organisms or cells, for, since organ- 

 isms may be found separated for a number of reasons, Wallin recog- 

 nized that negative prototaxis can be demonstrated only if the actual 

 process is observed. On the other hand, positive prototaxis, which is 

 "the affinity of one organism or cell for another organism or cell," 

 may result in such well-known phenomena as those of conjugation. 



