34 EARLY AMPHIBIAN DEVELOPMENT 



point (which does not occur at the same time for all the organs 

 of an organism) development can only proceed with function. 

 Function then perfects the results of the differentiation which 

 has been achieved without it, and is necessary for full and final 

 differentiation.^ 



The onset of function of the organs therefore marks an important 

 epoch in development, and, following Roux, it is possible to dis- 

 tinguish a prefunctional period during which morphological and 

 histological differentiation proceed to make the organs ready to 

 enter upon their functions, from a functional period during which 

 functional differentiation effects the final elaboration, interde- 

 pendence, and control of the rudiments, and converts them into 

 the perfected organs of the free-living organism.^ It will be neces- 

 sary to say more on this point in the final chapter. 



This book concerns itself almost entirely with the prefunctional 

 period. As has been shown, this period is characterised by certain 

 remarkable sequences of morphological and histological processes 

 of differentiation. Complications of structure and texture appear 

 which had previously been absent. The next problem to be tackled, 

 therefore, is the origin of differentiation. This concerns the question 

 as to how developmental processes are causally related to one 

 another in the sequence of events, i.e. whether the development of 

 any given rudiment would take place as it normally does if it had 

 not been for the previous development of some other rudiment, and 

 also the question as to what are the factors, causes, or conditions 

 which are responsible for initiating these sequences of processes 

 of development and differentiation. 



1 The term function is here used to denote function in the ordinary physio- 

 logical sense, as some specialised activity performed by the organ, normally for 

 the physiological benefit of the organism as a whole. The tissues are always 

 "functional" in the sense of being alive and working, and in addition they may 

 be performing special developmental functions even in that period which is here 

 denoted as the prefunctional period. Nevertheless, the distinction is an important 

 and useful one. 



2 Roux, i88i. 



