SECT. 3] AND ORGANISATION 559 



how one part of the cell acts on another; thus there remains an 

 enormous qualitative task to accomplish before we can begin to 

 enter on a numerical phase. I have been thinking of all this in reading 

 the monograph of M. Rapkine on the energetics of development, 

 where the physico-chemical facts are presented in a plain sort of 

 manner lending itself well to meditation. But I confess that it all 

 remains sibylline, enigmatic, to me, and some acute friends have 

 admitted the same embarrassment. Where is the explicative value, 

 the light, the link between all these facts so interesting, yet so 

 isolated? I cannot hit on it." This excellent passage, which my 

 friend has kindly allowed me to quote, expresses admirably the 

 doubtful air with which many biologists regard the extension of 

 physico-chemical concepts into morphology. In answer I pointed out 

 that chemical embryology is a very young science and if more 

 attention had been devoted to it in the past, would not show the 

 blank spaces and the gaps of which my friend complained. In a 

 word, Roux's decision to concentrate solely on the revelation of the 

 ''secondary components" in embryogenesis has had momentous 

 effects, but there were always those who could not resist the tempta- 

 tion of going deeper down to the "primary components" before all 

 the mass of facts had been dealt with on the more superficial level. 

 Among such miners at the deeper levels are the physico-chemical 

 embryologists. It is true that an enormous qualitative task remains 

 to be accomplished, but there is room enough for both kinds of 

 work and it will surely be through the close co-operation of both 

 kinds of worker that the facts will lose their sibylline and enigmatic 

 character. 



3-4. The Types of Morphogenetic Action 



In order to have some concrete idea as to how the processes of 

 morphogenesis may be supposed to go on, and to form a background 

 for chemical facts, one may construct a table in which are placed 

 all the different kinds of process that may go on during differentia- 

 tion. Such classifications have been made by KeUicott and by 

 Jenkinson, and tables constructed from them are given here. (Charts 

 IV and V.) 



Jenkinson's table may be taken first. The movements of single cells 

 which he enumerates are of much importance. The lower-layer cells 

 of the blastoderm of elasmobranch fishes, for instance, are free and 



