THE CELL 



39 



very small micronucleus, and a rather intricate "neuromotor" 

 apparatus, which is a nervous system in miniature (Fig, 285). 

 Euplotes is a highly organized individual though but a single 

 cell. It is an organism because it lives alone; it is a cell because 

 it is all included within a single membrane. 



Fig. 29. — The fusion (conjugation) of two Chlamydomonas. {After Smith.) 



All the tissue cells so far mentioned are vegetative, or somatic, 

 ones; that is to say, they are cells concerned in building up and 

 maintaining the plant or animal body in its purely vegetative 

 existence, in growth and in nutrition. Besides the business of 

 living, every organism is con- 

 cerned with perpetuating its 

 kind. In higher forms of life, 

 certain cells are set aside solely 

 for reproduction. These are 

 the highly specialized cells. 

 In Protista, there is naturally 

 no such division of labor, for 

 the organism is but a single 

 cell. One-celled plants and 

 animals carry on a vegetative 

 life today and tomorrow turn 

 their attention to multiplica- 

 tion, which is usually accom- 

 plished by the simple nonsexual 

 method of fission, or pinching 

 in two. Occasionally, a primi- 

 tive form of sexual reproduction takes place among the Protista ; 

 two entire and similar organisms come together, unite by fusion, 

 and later divide (Fig. 29). 



In the higher plants and animals, definite cells are formed for 

 reproduction. These cells are the eggs and sperm. Egg cells 



Fig. 30. — An egg of the seaweed 

 Fucus surrounded by sperm preliminary 

 to fertilization. {From G. A. Thuret.) 



