154 THE BIOLOGY OF AN ANIMAL. 



differentiated condition. The life of the ovum is confined to a 

 single cell. The blastula is composed of a number of nearly simi- 

 lar cells, which in the gastrula become differentiated into two dis- 

 tinct tissues. In later stages the cells become differentiated into 

 many different tissues, which in turn build up different organs 

 performing unlike functions. 



5. Lastly, it is a cycle, beginning with the ovum, and after all 

 the complicated changes of development resulting in the produc- 

 tion of new ova, which repeat the process and give rise to a new 

 generation. All other cells in the body (some of the sperma- 

 tozoa of course excepted) must sooner or later die. The ovum 

 alone persists as the starting-point to which the cycle of life con- 

 tinually returns (cf. p. 110). 



