98 AN INTRODUCTION TO ZOOLOGY 



of the larger cells without flagella. Some of these grow larger and 

 may be recognized as female cells or macrogametes (o); others 

 produce by simple division a flat plat, containing about one hun- 

 dred and twenty-eight spindle-shaped male cells or microgametes 

 (s). These escape into the central cavity and fuse with the 

 macrogametes. The zygote thus formed secretes a surrounding 

 wall consisting of two layers, the outer of which is reddish in color 

 and covered with short spines. In this condition the winter is 

 passed. The following spring the zygote breaks out of the wall 

 and by division produces a new colony. The smaller somatic 

 cells contained in the mother colony fall to the bottom and disin- 

 tegrate as soon as the new colonies produced by the fertilized 

 germ cells have escaped. 



In Volvox, true somatic cells are encountered for the first time, 

 that is, cells which function only vegetatively and are unable to 

 reproduce the colony. In the other forms described every cell 

 has the capacity of reproducing the whole. Volvox also contains 

 true germ cells, that is, cells that have given up nutritive functions 

 to carry on reproduction. Furthermore, a clear case of natural 

 death occurs in the somatic cells when they fall to the bottom of 

 the pond and disintegrate. The bodies of higher animals consist 

 of many cells which may be separated into somatic and germ cells. 

 The latter are either male or female. In most cases a fusion of a 

 male cell with a female cell is necessary before a new animal can 

 be reproduced. At any rate, some of these germ cells maintain 

 the continuation of the species by producing new individuals 

 while the somatic cells perish when the animal dies. 



The Germ Plasm Theory. Figure 47 illustrates the theory 

 of the continuity of the germ-plasm which is held by most zoolo- 

 gists at the present time. By germ-plasm is meant that part of 

 the protoplasm which is set aside for reproductive purposes and 

 determines that the offspring shall resemble their parents; 

 this special material is stored in the germ cells. In the spring 

 of the year the Volvox race is represented by fertilized eggs 

 (zygotes) only; each of these divides, producing an animal con- 



