28 THE ANATOMY OF INVERTEBRATED ANIMALS. 



starting-point of their development. But the first process in 

 that development is the division of the germ into a number 

 of morphological units or blastomeres, which, eventually, give 

 rise to cells ; and, as each of these possesses the same physio- 

 logical functions as the germ itself, it follows that each mor- 

 phological unit is also a physiological unit, and the multicellu- 

 lar mass is strictly a compound organism, made up of a mul- 

 titude of physiologically independent cells. The physiologi- 

 cal activities manifested by the complex whole represent the 

 sum, or rather the resultant, of the separate and independent 

 physiological activities resident in each of the simpler con- 

 stituents of that whole. 



The morphological changes which the cells undergo in 

 the course of the further development of the organism do 

 not affect their individuality ; and, notwithstanding the modi- 

 fication and confluence of its constituent cells, the adult or- 

 ganism, however complex, is still an aggregate of morphologi- 

 cal units. Nor is it less an aggregate of physiological units, 

 each of which retains its fundamental independence, though 

 that independence becomes restricted in various ways. 



Each cell, or that element of a tissue which proceeds from 

 the modification of a cell, must needs retain its sustentative 

 functions so long as it grows or maintains a condition of 

 equilibrium ; but the most completely metamorphosed cells 

 show no trace of the generative function, and many exhibit 

 no correlative functions. Contrariwise, those cells of the adult 

 organism which are the unmetamorphosed derivatives of the 

 germ exhibit all the primary functions, not only nourishing 

 themselves and growing, but multiplying, and frequently 

 showing more or less marked movements. 



Organs are parts of the body which perform particular 

 functions. In strictness, perhaps, it is not quite right to 

 speak of organs of sustentation or generation, each of these 

 functions being necessarily performed by the morphological 

 unit which is nourished or reproduced. What are called the 

 organs of these functions are the apparatuses by which cer- 

 tain operations, subsidiary to sustentation and generation, are 

 carried on. 



Thus, in the case of the sustentative functions, all those 

 organs may be said to contribute to these functions which are 

 concerned in bringing nutriment witliin the reach of the ulti- 

 mate cells, or in removing waste matter from them ; while in 

 the case of the generative function, all those organs contribute 

 to the function whicli produce the cells from which germs are 



