1 5 2 AGE, GROWTH, AND DEATH 



plasm with age is an absolutely necessary corollary of 

 the discovery that differentiation is mainly a function 

 of the protoplasm. If there is to be a large degree of 

 differentiation it is necessary that the quantity of pro- 

 toplasm in the single cells should be increased, so 

 that there may be the raw material on hand out of 

 which the differentiated product can be manufactured. 

 If there is not such a preliminary increase of the pro- 

 toplasm, then the differentiation cannot occur. In 

 order that the perfection of the adult structure should 

 be attained, it is necessary that the mere undifferen- 

 tiated cells, each with a small body of protoplasm, 

 should acquire first an increased amount of proto- 

 plasm, and that then from the increased protoplasm 

 should be taken the material to result in differenti- 

 ation, in specialisation. 



An undifferentiated cell performs all the fundamen- 

 tal functions of life. An amoeba, or any unicellular 

 organism such as I have presented to you upon the 

 screen, does everything which is indispensable to life. 

 It takes food; it forms secretions and excretions; its 

 activity depends upon chemical alterations going on in 

 the food in the interior of its body ; it is capable of 

 sensation and of locomotion. It is probable that every 

 living cell has all of these fundamental properties of 

 protoplasm. When a cell becomes differentiated, 

 however, though it does not necessarily give up any 

 of its vital properties, it becomes different from other 

 cells because one of its properties is made conspic- 

 uous, and in order to acquire that conspicuousness, 



