Studies on Regulation. 279 



appear. In other words, during the first period the machine is 

 constructed and during the second it functions. Here again the 

 morphological conception of development appears as the basis of 

 this analysis. In my opinion, all stages of development are to be 

 regarded as functional though the kind of function and its visible 

 results differ. 



It is important, moreover, to distinguish between the visible 

 substances or those which future investigation may prove to exist in 

 the nucleus or cytoplasm of the germ-cell and the structures into 

 w^hich they develop. Suppose a certain substance or region of the 

 egg can be followed to the entoderm of the larva. It is not con- 

 ceivable that this substance or region if isolated can form a typical 

 intestine, although its cells may differentiate into typical intestinal 

 cells. In other words this region may continue to function in the 

 characteristic manner after isolation, and each of its cell units may 

 undergo the differentiation corresponding to this function, but the 

 typical form of the whole does not appear because the typical 

 relations of the elements to each other, and to the environment 

 are not established. The differentiation of the cell units is doubt- 

 less in large part the result of their chemical constitution while the 

 formation of a characteristic organ, tlie intestine, is largely the 

 result of physical factors, the natural pressures and movements 

 of cells, surface tensions, tensions due to conditions in other parts, 

 pressure of fluids, etc. Development of the typical form is due 

 to these conditions as well as to the character of the substance 

 itself. And these conditions have been ignored to a large extent 

 in the study of development. 



Morphogenesis is very commonly regarded as the result of the 

 composition of the substances in the germ-cell. From this point 

 of view have arisen the hypotheses which regard morphogenesis 

 as analogous to crystallization, and the theories of formative stuffs 

 which, though perhaps correct for certain elements of structure, 

 are, nevertheless, without general significance because thev are 

 based on inadequate conceptions. 



The relation between the composition of the germ-cell and the 

 structure of the developed organism cannot be a direct one, but is 

 rather exceedingly remote. To look for the equivalents of mor- 

 phological characters in the germ-cell must involve us in many 

 difficulties, because most so-called morphological characters are 

 primarily typical space-relations of masses and these necessarily 



