194 Studies in Animal Behavior 



part matters of the behavior of cells. If we can 

 interpret development anywhere in terms of the 

 tropisms and other responses of individual cells this 

 peculiar group of organisms would seem to present 

 unusually favorable opportunities for attacking the 

 problem. 



In a very interesting series of experiments Prof. 

 H. V. Wilson has cut various sponges and hydroids 

 into pieces, pressed the tissues through fine bolt- 

 ing cloth so as to reduce them practically to masses 

 of isolated cells. It was found that these cells be- 

 gan to come together and form aggregations which 

 subsequently differentiated into the form of the spe- 

 cies from which the fragments were taken. Out 

 of a hodge-podge of all sorts of cells one sees the 

 gradual emergence of an organic body. The pro- 

 ponents of the conception of a cell state could 

 scarcely hope for a better illustration of their point 

 of view. It is as if a group of independent cells 

 had said among themselves: "Behold, let us creep 

 together and form an organism. If some of us 

 do this, some that, and others something else we 

 will all get along in peace and harmony and more- 

 over much to our mutual advantage." 



How a cell differentiates as well as what it does 

 in the way of behavior depends largely on the re- 

 lations in which it stands to other cells. The di- 

 rection of differentiation which a cell follows may be 

 looked upon as a response to environmental stimuli, 

 just as the movements of a cell may be so regarded. 



