174 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Other authors have devoted chapters to " Developmental Processes," 

 but none of these cau be considered as at all complete. 



Thus 0. 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 ontogenetic 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 : 1. 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 ontogenetic processes comprise those of a general physio- 

 logical character. Of these, at least three are commonly recognized : — 



1. Growth (including both the results of assimilation and of imbibition 

 of water). 



2. 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 the form of 



