102 AN INTRODUCTION TO ZOOLOGY 



pose of performing the functions described above as characteristic 

 of nervous tissue. 



2. THE GERM CELLS 



We have seen that the substance of which all animals are com- 

 posed may be separated into reproductive and non-reproductive. 

 The latter is called somatic-plasm, the former germ-plasm. No 

 organism arises fully developed, but usually originates from a 

 union of two germ cells which have been produced by a preexist- 

 ing organism. There is a gradual development of an individual 

 from the fertilized germ cell, the organism passing through a 

 number of well-defined stages before the adult condition is at- 

 tained. To bring the various steps of this process clearly before 

 us we shall now discuss the origin and ripening of the germ cells, 

 their union in fertilization, and the subsequent developmental 

 history of the zygote thus formed. These subjects will be con- 

 sidered in some detail for each type studied in the following chap- 

 ters, so we shall here give only a general outline which will serve 

 as a foundation for future discussions. 



Early in the history of the individual certain cells are set aside 

 for the sole purpose of reproduction; these are the germ cells. 

 All the other cells of the developing organism become more or 

 less specialized for the various vegetative functions, and may 

 eventually be recognized as nerve cells, muscle cells, etc. Only 

 the germ cells remain in their primitive condition, and, since 

 they take no part in the daily life of the animal, we may think of 

 them as being passively carried about and protected by the soma- 

 tic cells. Just before the individual becomes sexually mature, the 

 germ cells awake to activity and a number of complex processes 

 are inaugurated which result in the casting out of certain of them 

 by the body to produce new individuals similar to the parent 

 organism. 



Spermatogenesis. The development of the male germ cell or 

 spermatozoon is termed spermatogenesis. As shown in Figure 49, 

 this process maybe divided into three periods: (i) the multiplica- 



