104 TEXTBOOK OP ZOOLOGY 



tion advances; so it is Avith development of complexity in meta- 

 zoans. Another characteristic of Metazoa is the presence of a defi- 

 nite center of control localized in a particular group of cells which 

 becomes the nervous system in higher forms. 



Cellular Differentiation 



In Protozoa there is seen fair development of intracellular dif- 

 ferentiation, making it possible for one part of a cell to perform a 

 particular function, and for other parts to perform other functions. 

 The complexity of Metazoa is not the result of great complexity of 

 the individual cells, but it is due to the special differences between 

 them. The presence of a variety of cells within one body is spoken 

 of as intercellular differentiation. The modification of metabolic 



B 



Fig. 44. — Typical germ cells. A, ovum of the female; B, spermatozoa of the male. 



activity is the basic factor in the development of all differentiation. 

 Certain groups of cells become specialists in a particular phase of the 

 metabolic activity. Some become protective surface cells, others se- 

 crete special enzymes, still others specialize in excretion, and so on. 

 The entire metazoan body is usually divided into germ cells, which 

 are specialized for reproduction, and somatic cells or body cells, 

 which compose the remainder of the body and are grouped in layers. 

 The germ cells are set aside early in the life of the individual for 

 reproductive purposes. They develop in the reproductive glands or 

 gonads of the two sexes. The protoplasm of these cells is known as 

 germ plasm. The female germ cells are eggs or ova, and those of the 

 male are spermatozoa. When the germ cells reach maturity, they be- 

 come separated from the body and may give rise to a new generation. 



