430 INHERITANCE AND DEVELOPMENT 



differentiated into cells having the histological character of the nor- 

 mal entoblast ! One of the most fundamental of embryonic differen- 

 tiations is thus shown to be intimately conditioned by the chemical 

 environment. 



The observations of botanists on the production of roots and other 

 structures as the result of local stimuli are familiar to all. Loeb's 

 interesting experiments on hydroids give a similar result ('91). It 

 has long been known that Tubiilaria, like many other hydroids, has 

 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 

 Tubnlaria 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 eacJi end (Fig. 194); 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 organization. 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 germ 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 



