198 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



It will be noted that No. I. 7 (absorption by mesenchyme) is not 

 assigned to any one of the three categories ; and that certain other pro- 

 cesses (III. 7, Detachment from a layer; II. 2, Splitting; III. 4 and 5, 

 thickening and thinning of a layer ; and IV. 3, Vacuolization) are of so 

 doubtful a nature that, although assigned to the special categories, this 

 assignment can be regarded as provisional only. 



The process of absorption does not readily fall into one of the three 

 categories, and at the same time it does not seem worth while to erect a 

 special category for it. 



As for the doubtful cases, the doubt is not whether they are referable 

 to one of these categories, but rather in knowing in which one to place 

 them. 



Regarding the three general categories, it has long been recognized 

 that taxic and tropic processes are responses to stimuli. It has not 

 been so generally recognized that all growth processes are such. A 

 moment's consideration will, however, make this probable. 



Let us consider for a moment what it is that controls differential 

 growth, — What makes one part of a membrane grow faster than another, 

 causing a folding of that part 1 



Inequality of growth is clearly not due to inequality of food supplied, 

 since folds arise in uniformly nourished membranes, — bathed, that is to 

 say, uniformly by the nutritive fluids. It must therefore be due to ine- 

 quality of the activities which lead to growth ; namely, the taking in of 

 food and its assimilation, and the imbibition of water. Now it is our fun- 

 damental assumption that activities of all sorts, including ingestion and 

 imbibition, are responses to stimuli. In so far, then, as differential 

 growth is dependent upon the inequality of these activities in differ- 

 ent parts of the membrane, it is dependent upon stimuli acting upon 

 that membrane. 



Whenever the activities are diverse in the different parts of a mem- 

 brane, it must be either that the stimulus applied to the different parts 

 is diverse, or, if not, that the protoplasm is diverse in its different parts, 

 for what the result shall be depends upon two factors, — the quality of 

 the stimulus and that of the protoplasm. 



Let us now consider somewhat more in detail the taxic and tropic 

 processes. As is well known, the stimuli which control these movements 

 result either in migration towards the source of the stimulus or away 

 from it, so that positive or negative taxis or tropism occurs. In ontogeny 

 it is often impossible to say where the source of stimulation is, and 

 therefore whether the tactic or tropic movements are -f- or — . Cer- 





