174 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY, 
Other authors have devoted chapters to “Developmental Processes,” 
but none of these can be considered as at all complete. 
Thus O. Hertwig, in his “Text-Book of the Embryology of Man and 
Mammals" (English Translation by E. L. Mark, 1892, p. 76), has a 
“General Discussion of the Principles of Development." He recog- 
nized two main ones: (1) the principle of unequal growth (producing 
folds which are either invaginations or evaginations, and which may unite 
along their edges) ; and (2) the principle of histological differentiation. 
Minot, in his Human Embryology, in a chapter on “ Differentiation," 
seems to think also that these two processes are sufficient to explain the 
differentiation of organs. 
More important in this connection than either of the preceding is the 
recent paper of Herbst in the “ Biologisches Centralblatt ” (Vol. XIV. 
'Nos. 18-22). This author, after reviewing the literature upom taxism 
and tropism, explains as phenomena of the same order certain ontogenetic 
processes. He has not, however, attempted to catalogue all the onto- 
genetic processes. 
Before beginning the present catalogue, I may state that I distinguish 
between ontogenetic principles and ontogenetio processes. Under the 
first head I include such laws of development as terminal growth, repeti- 
tion of parts and bilateral symmetry in development: These I have not 
attempted to catalogue. The present paper is concerned only with the 
latter group, which comprises the different elementary operations or ac- 
tions exhibited in ontogeny. 
These may be divided into two classes : ]. the grosser ontogenetic pro 
cesses; and 2, histogenic processes. This paper deals with the first 
class only. 
In discussing the grosser ontogenetic processes we may distinguish 
(A) those of a more general nature from (B) the more special ones. 
A. The general ontogenetie processes comprise those of a veneral physio- 
logical character. Of these, at least three are commonly recognized : — 
1. Growth (including both the results of assimilation and of imbibition 
of water). 
9. Nuclear division. 
3. Secretion. 
B. The special ontogenetic processes are differential in character, i. e. 
the differentiation of the body is effected by them. 
These processes may be classified, first of all, according to tho form of 
