CHAPTER V 



DRIESCH'S THEORIES OF DEVELOPMENT 



GENEEAL REFLECTIONS AND CONCLUSIONS 



To the inquirer into the causes of development the central 

 difficulty must always be the problem of differentiation. Growth 

 and division of the nucleus and the cell, processes which always 

 accompany differentiation, are, as we have seen, side issues ; but 

 the increase of structure, the production of form out of the 

 relatively formless germ, and the gradual passing of this into 

 a new individual which is like the parents that gave it birth, 

 this is the marvel which has always excited the wonder of the 

 observer, and demands all his wit to understand and to explain. 



Experimental investigation, as far as experiment has at present 

 gone, has shown, first, that a certain complexity of the physical 

 and chemical environment is a necessary condition of normal 

 development; that complexity may, it is true, vary within 

 certain limits, but those limits can only be transgressed under 

 pain of abnormality or death. In the second place it has been 

 demonstrated that the initial structure of the germ, and the 

 mutual interactions of its parts as they develop are both indis- 

 pensable internal factors. It now only remains for us to discuss 

 the value of those theories which are not only attempts to 

 explain, to give the most general account of, the phenomena in 

 causal terms, but also serve to provide a light to guide the 

 investigations of the future. One such hypothesis we have 

 already examined in some detail, Roux's hypothesis of self- 

 differentiation. According to this belief not only is the develop- 

 ment of each part determined by causes which reside in itself 

 alone, but the parts or rather their determinant representatives, 

 which are all ex hypothec necessarily present ab initio in the 

 undeveloped germ are located in the nucleus. The qualitative 

 division of the nucleus sunders these units from one another, 

 which then determine the characters assumed by the cytoplasm, 

 and so the whole process happens. 



As we have seen, the hypothesis, in this its original form, is 



