82 COLONIAL PROTOZOA 



gametes, but macrogametes and microgametes may occur instead of 

 isogametes. Eudorina is a spherical colony, with more cells than in 

 Pandorina) and there are two kinds of colonies, one producing micro- 

 gametes and the other macrogametes. Pleodorina is a colony com- 

 posed of thirty-two cells. Twenty-eight of these cells are physio- 

 logically balanced and larger than the other four; any one of these 

 twenty-eight can reproduce by cell division to form a new colony. 

 Reproduction by the syngamy of microgametes and macrogametes, 

 formed by any of the twenty-eight cells, also occurs. The four small 

 cells take no part in these reproductive processes and disintegrate 

 when the reproduction, which occurs only among the twenty-eight 

 cells, has been completed. It appears, therefore, that these four cells 

 are not physiologically balanced, since they have lost the capacity 

 of reproduction ; hence they may be called somatic cells, or body cells, 

 in contrast to the twenty-eight cells which may be called germ cells. 

 In Volvox the number of somatic cells greatly outnumbers the germ 

 cells which are the physiologically balanced cells, as shown by their 

 capacity for reproduction by cell division to form new colonies or by 

 the formation of microgametes and macrogametes which reproduce 

 by syngamy. While differences occur in the several species of Volvox, 

 the following characteristics are important for the present purpose. 

 Three types of cells are found: numerous somatic cells; germ cells, 

 called parthenogonidia, which divide repeatedly within the parent to 

 form daughter colonies; and the zygote-forming germ cells, which dif- 

 ferentiate into either ova or spermatozoa. When an ovum has been 

 fertilized by a sperm, the resulting zygote secretes a cyst and after 

 an inactive period emerges to divide and form a new colony. Draw- 

 ings should be made as directed by the instructor. What eventually 

 becomes of the cells in a colony of Pandorinal In Pleodorina'? In 

 Volvox? How does Volvox differ from a many-celled animal? How 

 from a non-colonial protozoan? What is natural death? What is 

 the individual in these colonial species? 



