THE SECRET OF DEVELOPMENT 417 



do not know the secret of development, which is part of the 

 larger secret of life itself. 



No doubt the process of development is objectively an orderly 

 sequence of chemical and physical events, just as the growth of 

 a crystal is. The developing embryo is the arena of intricate 

 processes of chemical construction and disruption, of physical 

 attractions and repulsions ; but the characteristic feature of 

 the whole business is, that it is co-ordinated, regulated and 

 adaptive in a manner for which it seems at present, to say the 

 least^ very difficult to suggest any analogue in inanimate nature. 

 For this reason not a few embryologists, such as Driesch, believe 

 themselves warranted in frankly postulating a vitalistic factor — 

 an Aristotelian " Entelechy." 



An Outline of what is known. — We know that the germ-cells, 

 and their nuclei more particularly, form the physical basis of 

 inheritance ; that there is a genetic continuity between the 

 fertilised egg-cells which gave rise to the parents and those 

 which gave rise to their offspring and those of their offspring; 

 that fertilisation implies an intimate and orderly union of two 

 individualities, condensed and integrated for the time being 

 in the ovum and spermatozoon ; that the sperm acts as a libera- 

 ting stimulus on the ovum, as well as being the bearer of the 

 paternal half of the inheritance and of a peculiar little body, 

 (the centrosome), that plays an important part in the subse- 

 quent division of the fertilised egg-cell ; that the mode of all 

 development is by division of nuclei and the integration of 

 the living matter into unit areas or cells, each presided over by 

 a nucleus ; that differentiation comes about very gradually — 

 the obviously complex slowly arising out of the apparently 

 simple ; that paternal and maternal characteristics are dis- 

 tributed in exact equality by the nuclear or cellular divisions, 

 and that the paternal and maternal contributions usually form 

 the warp and woof of the web which we call the organism, and 

 persist in the germ-cells thereof, though the expression or realisa- 



