THE LATER STAGES OF DEVELOPMENT 157 



organs have become established, the former begins to influence 

 the development of the latter. It is true that practically normal 

 differentiation can take place in amphibian limbs without 

 nerves, but such limbs remain smaller than normal ones (Ham- 

 burger, 1929). Evidently, the nervous system has a trophic 

 influence which intensifies growth. In other cases, however, the 

 connection with the nervous system is indispensable for the 

 maintenance of the structure of the organ. Marked atrophy 

 occurs, for instance, in muscles as soon as the motor nerves 

 are severed. This is even more true in the case of the lateral 

 line sense organs and the skin taste buds of fishes, which 

 degenerate entirely after transection of the afferent nerve 

 (Olmsted, 1920). This influence of the nervous system is 

 probably due to the secretion of a substance by the nerves. 



The development of the blood circulation also has important 

 consequences for further development, as it supplies the various 

 organs with the food substances needed for their growth. The 

 distribution of these substances between the organs will depend 

 upon the total quantity that is available. The body proportions 

 of maximally fed animals may differ sharply from those of 

 starved specimens. In extreme cases, certain organs or parts of 

 the body may be completely reduced in consequence of want of 

 food. 



The possibility of hormone transport is another important 

 consequence of the development of the vascular system. Hor- 

 mones are substances produced by certain glands, which become 

 distributed throughout the body by the blood so that they can 

 exert their influences on the development of other organs in 

 remote places. In another context, we have already mentioned 

 the "gene hormones" of insects (p. 87). Hormones play a very 

 important role, in particular in the later stages of development, 

 and in the adult animal. A more extensive discussion of these 

 phenomena lies outside the scope of this book. 



Finally, in the later stages, the function itself of each organ 

 has an important influence on its development. Once differentia- 

 tion has proceeded so far that the organs begin to take over 

 their own specific functions, new processes are started under 

 the influence thereof. These processes influence the structure 



