THE FUNCTIONAL PERIOD OF DEVELOPMENT 431 



The Stimulation of the muhiphcation of the nerve-cells in the 

 spinal cord (Chap, xi, p. 383) in Amhlystoma is another example of 

 the effects of nerve-endings. There are also the cases in which the 

 presence of a nervous connexion is necessary for the maintenance 

 of structure in an organ. As is well known, muscles atrophy when 

 the motor nerves to them are cut. But the best-analysed examples 

 concern the lateral-line organs, and the taste-buds on the barbels of 

 the catfish Amiuriis. When the nerves to these organs are cut, the 

 organs themselves undergo marked dedifferentiation, and rediffer- 

 entiation when the regenerating nerve restores their nerve-supply.^ 

 The trophic stimulus has been found to pass down the nerve from 

 the cell-body at a rate of 2 cm. per day, and the indications are that 

 it is due to percolation of a hormone-hke substance.^ It clearly 

 cannot be due to normal impulse-conduction (see p. 387). 



Though the precise mechanism of their action is still obscure, 

 the interest of these examples for the present purpose is clear. They 

 demonstrate that once the nervous system becomes functional, new 

 methods of influencing development are available in the organism. 

 These methods concern such diverse processes as local cell- 

 multiplication, large-scale regeneration, and the maintenance of 

 differentiation in organs. 



§6 



Finally, there are the effects of function per se. This is perhaps the 

 most pervading of all the new effects which take their origin at the 

 onset of the functional period. 



Function can influence the multiplication of cells and the size 

 of organs, the histological appearance of cells, and the arrangement 

 of cells and tissues within an organ. Often more than one of these 

 processes is involved at one time. The most obvious example of 

 purely quantitative change concerns compensatory hypertrophy. 

 When a portion of a functioning organ complex is removed, the 

 remainder increases in bulk in response to the increased demands 

 made upon it. The simplest instance concerns the kidneys. When 

 one kidney is removed, the other^ enlarges ; the enlargement is 

 considerable, though not to double its original bulk.^ 



1 Olmsted, 1920. ^ g. H. Parker, 1932 a, b. 



3 Ribbert, 1894. 



