REPRODUCTION AND EARLY DEVELOPMENT. 103 



109. First Steps in Development. The earliest stages 

 in the development of the new individual after fertiliza- 

 tion is the multiplication of its cells. One cell divides 

 in two, these into four, and so on, until a large number of 

 small cells is produced, which in the aggregate may be 

 no larger than the original single cell. In this process 

 some of the stored food of the ovum is used up in bring- 

 ing about the divisions and in developing the new parts 

 of all these cells, as nuclei, cell walls, etc. The daughter 

 cells may be all of like size or there may be the greatest 

 difference among them, but eggs of the same species 

 always develop in much the same way. (See Fig. 20.) 



no. The Conditions of Development. The external 

 conditions that favor development are, in general, those 

 that are favorable to life itself, and it is unnecessary to 

 enumerate them here. The internal conditions, those 

 which the egg inherits from its parents and influence the 

 way in which the fertilized egg develops, are enumerated 

 below : 



1. The size of the egg. 



2. The amount of food substance stored in the egg. 



3. The manner in which this, food substance is dis- 

 tributed in the egg. 



4. Whether the egg is encased in a hard shell. 



5. Whether the egg is to be developed in the water 

 or in a dry condition. 



If the ovum is small and has little food, it will prob- 

 ably need to hatch and become active enough to secure 

 its own food quickly. This is true of many water ani- 

 mals, as sponges, echinoderms, worms, etc. In the 

 higher mammals there is little food in the egg, but the 



