EXPERIMENTAL MORPHOGENESIS 95 



that the same conclusion can be drawn from all our 

 experiments on very young stages of the germ. 



The investigations of the last few years have made it 

 quite clear that even in organisms with a high power of 

 morphogenetic regulation it is always the form of the whole, 

 but not the individual cell, which is subjected to the regula- 

 tion processes. Starting from certain results obtained by 

 T. H. Morgan, I was able to show that in all the small but 

 whole larvae, reared from isolated blastomeres, the size of 

 the cells remains normal, only their number being reduced ; 

 and Boveri has shown most clearly that it is always the 

 size of the nucleus more correctly, the mass of the 

 chromatin which determines how large a cell of a certain 

 histological kind is to be. In this view, the cell appears 

 even more as a sort of material used by the organism as 

 supplied, just as workmen can build the most different 

 buildings with stones of a given size. 



ft". The External Means of Morphogenesis 



We now know what internal means of morphogenesis are, 

 and so we may glance at some of the most important 

 " outer means " or " conditions " of organisation. 



Like the adult, the germ also requires a certain amount 

 of heat, oxygen, and, when it grows up in the sea, salinity 

 in the medium. For the germ, as for the adult, there 

 exists not only a minimum but also a maximum limit 

 of all the necessary factors of the medium ; the same factor 

 which at a certain intensity promotes development, disturbs 

 it from a certain other intensity upwards. 



Within the limits of this minimum and this maximum 



