AN INTRODUCTION TO THE STUDY OF GROWTH. 27 



by what means the tiny organs to which they first give 

 rise become the bulky organs of the adult. This 

 increase must be due either to the swelling of the cells 

 that are already formed, or to the formation of new ones 

 by divisions of the old ; and in every instance it is to be 

 determined whether one or both of these processes have 

 been at work, and what share in the result may be 

 attributed to each of them. 



The conception of a complicated animal body, as com- 

 posed of a number of cells, was at first accompanied by 

 the idea that the cells themselves were comparatively 

 simple, and the simplicity of the cell was contrasted 

 with the complexity of the whole organism. Unfortu- 

 nately this notion of simplicity is misleading, for there 

 are instances, the most striking being among the 

 protozoa, in which a number of functions are carried 

 on by different parts of the cytoplasm, although, accord- 

 ing to definition, the entire animal forms but a single 

 cell. But the facts in connection with which the idea 

 of simplicity is least fit, are those relating to develop- 

 ment. From the human ovum, for example, is gradually 

 elaborated the complex adult, and it is difficult to escape 

 the conviction that in some measure at least the com- 

 plexity of the latter is represented in the structure of 

 the former. 



In carrying out this elaboration the course of events 

 is mainly determined, not by the surrounding conditions, 

 although these have some influence, but by conditions 

 within the egg. The beginning o'f the growth process 

 immediately follows fertilisation, and hence that may be 

 looked upon as the initial impulse, but in what manner 

 this impulse sets in motion the marvellous series of 

 changes which are to result in the completed animal is 

 not thereby explained. A few generations ago the 

 development of animals was regarded as due to evolu- 



