326 THEORIES OF INHERITANCE AND DEVELOPMENT 



the power to regenerate its " head " i.e. hypostome, mouth, and ten- 

 tacles after decapitation. Loeb proved that in this case the power 

 to form a new head is conditioned by the environment. For if a 

 Tubitlaria stem be cut off at both ends and inserted in the sand 

 upside down, i.e. with the oral end buried, a new head is regen- 

 erated at the free (formerly aboral) end. Moreover, if such a piece 

 be suspended in the water by its middle point, a new head is produced 

 at each end (Fig. 142); while if both ends be buried in the sand, 

 neither end regenerates. This proves in the clearest manner that 

 in this case the power to form a definite complicated structure is 

 called forth by the stimulus of the external environment. 



These cases must suffice for our purpose. They prove incontesta- 

 bly that normal development is in a greater or less degree the response 

 of the developing organism to normal conditions ; and they show that 

 we cannot hope to solve the problems of development without reckon- 

 ing with these conditions. But neither can we regard specific forms 

 of development as directly caused by the external conditions ; for the 

 egg of a fish and that of a polyp develop, side by side, in the same 

 drop of water, under identical conditions, each into its predestined 

 form. Every step of development is a physiological reaction, involv- 

 ing a long and complex chain of cause and effect between the stimu- 

 lus and the response. The character of the response is determined 

 not by the stimulus, but by the inherited organisation. While, there- 

 fore, the study of the external conditions is essential to the analysis 

 of embryological phenomena, it serves only to reveal the mode of 

 action of the idioplasm and gives but a dim insight into its ultimate 

 nature. 



I. DEVELOPMENT, INHERITANCE, AND METABOLISM 



In bringing the foregoing discussion into more direct relation with 

 the general theory of cell-action we may recall that the cell-nucleus 

 appears to us in two apparently different roles. On the one hand, it 

 is a primary factor in morphological synthesis and hence in inheri- 

 tance, on the other hand an organ of metabolism especially concerned 

 with the constructive process. These two functions we may with 

 Claude Bernard regard as but different phases of one process. The 

 building of a definite cell-product, such as a muscle-fibre, a nerve- 

 process, a cilium, a pigment-granule, a zymogen-granule, is in the last 

 analysis the result of a specific form of metabolic activity, as we may 

 conclude from the fact that such products have not only a definite 

 physical and morphological character, but also a definite chemical 

 character. In its physiological aspect, therefore, inheritance is the 

 recurrence, in successive generations, of like forms of metabolism ; 



