94 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



there is in the first place growth, which must be regarded 

 as a very essential one. 



Analytically we must carefully discriminate between the 

 increase in the size of the cavities of an organism by a 

 passive extension of their surfaces and the proper growth of 

 the individual cells, which again may be due either to mere 

 extension or to real assimilation. Osmotic pressure, of 

 course, plays an important part both in the growth of the 

 body-cavities and in simple cellular extension. We repeat 

 the caution against believing too much to be explained by 

 this phenomenon : it is the organism which by the secretion 

 of osmotic substances in the cavities or the protoplasm of 

 the cells prepares the ground for growth even of this 

 osmotic sort. The real cellular growth which proceeds on 

 the basis of assimilation cannot, of course, be accounted for 

 by osmotic events, not even in its most general type. 



Ontogenetical growth generally sets in, both in animals 

 and in plants, after the chief lines of organisation are laid 

 out ; it is only the formation of the definite histological 

 structures which usually runs parallel to it. 



On Cell-division. We have already said a good deal 

 about the importance of cell -division in ontogeny: it 

 accompanies very many of the processes of organisation in 

 all living beings. But even then, there are the Protozoa, 

 in the morphogenesis of which it does not occur at all, and 

 there have also become known many cases of morphogenesis 

 in higher animals, mostly of the type of regulation, in which 

 cellular division is almost or wholly wanting. Therefore, 

 cellular division cannot be the true reason of differentiation, 

 but is only a process, which though necessary in some cases, 

 cannot be essential to it. It must be conceded, I believe, 



