SPECIALIZATION AND ORGANIZATION. \J 



different courses, each of which goes on into almost 

 endless sub-division, is not the whole of reproduction. 

 Reproduction of the species of course includes all other 

 kinds of reproduction ; nevertheless it is as distinct 

 from them as the individual is from its component cells. 

 The individual may be the product of a single cell, but, 

 once formed, the heterogeneous components must sev- 

 erally have their own methods of reproduction, other- 

 wise the organism could not keep up its reserves, nor 

 supply the places of exhausted, disabled, or worn out 

 laborers. These specialized modes of reproduction, as 

 varied and as distinct as the histolos^ical elements of 

 the organism, although derived from the process which 

 continues the species, yet differ from it in this impor- 

 tant respect, that their products are isogeneous rather 

 than heterogeneous. 



This distinction is already well marked in Hydra, 

 where we find the entoderm cells so specialized that 

 they can reproduce only cells of their own kind. How 

 different it is with the ova, which reproduce all kinds of 

 cells represented in the Hydra community. 



Certain kinds of work exclude the power of reproduc- 

 tion, and such cases call for special provisions of still 

 another class. The loss of such power by any class of 

 cells is generally made good by a closely allied class, 

 or by the younger cells of the same class. Such 

 reserves may play a relatively passive part, until the 

 time arrives for them to take the place of their prede- 

 cessors ; and they may be capable of assuming any one 

 of several different roles. Again Hydra furnishes us 

 with a simple illustration. The superficial ectoderm 

 cells of Hydra, consisting of nettle-cells, nerve-muscle 



