OTHER PROTOZOA 99 



taining as many as twelve thousand cells, a few of which are 

 reproductive cells and contain the germ-plasm. In the autumn 

 the germ-plasm is segregated in the eggs and spermatozoa, which 

 fuse two by two, an egg with a sperm, producing zygotes. The 

 somatic or body cells fall to the bottom and die, but the zygotes 

 live through the winter and germinate in the spring, thus assuring 

 the continued existence of the race. 



FIG. 47. Diagram to illustrate the theory of the continuity of the germ-plasm 

 by means of Volvox. The fertilized egg or zygote (zi) produces by divi- 

 sion in the spring a colony (c) containing a great many cells, some of 

 which are set aside for reproductive purposes. These produce new colo- 

 nies (n.c.) during the summer by the asexual method. Male cells (sper- 

 matozoa, sp) and female cells (eggs, e) are also formed. In the autumn 

 the body dies, but the fertilized germ cells (zz) produced by the union of 

 eggs and spermatozoa (/) survive the winter, dividing to form new colo- 

 nies the next spring. 



Certain processes in the development of the fertilized eggs of 

 some of the more complex animals strengthen the belief in the 

 passage of the germ-plasm from one generation to another, and 

 seem to indicate that the body is not the producer of the germ 

 cells, but is simply a vehicle which carries them about until they 

 reach maturity. The body then dies and disintegrates, the 

 important function of reproduction, and consequently the sur- 

 vival of the race, being left in charge of the mature germ cells. 



