198 BULLETIN: MUSEUM OF COMPARATIVE ZOÓLOGY. 
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 ailayer; 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. 
Hegarding 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 thé nutritive fluids. It must thereforo 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 inpestion 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 iu tho 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 baxic and tropio 
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 + or —. Cer- 
