Vll 



CHROMIDIA IN PROTISTA 



207 



Another example of multiplication of nuclei by chromidia formation 

 alternating with multiplication by mitotic division, though attended by 

 more compHcations, is afforded by Arcella (Fig. 88), the life history of 





Fig. 87. 



stages in the formation of macrogametes in Mastigella vitrea. (After Goldschmidt, A.P.K., 1907.^ A. 

 inacrogametocyte nucleus surrounded by extruded chromidia. B, a later stage. As the chromidia get 

 further away from the nucleus they are formed into the macrogamete nuclei. C, gametocyte full of macro- 

 gametes. Some are undergoing the (? reduction) division. D, four of the macrogametes from C on a larger 

 scale. Three are in mitosis, and the fourth has completed the mitosis and contains the mature gamete nucleus 

 and the polar body-like " reduction body." 



ch, chromidia ; m, macrogamete nuclei ; ;•, " reduction body." 



which has been studied by many workers. The essential features from 

 our present point of view are as follows. At the beginning of the asexual 

 cycle the young Arcella consists of a minute uninucleated cell. Soon 

 the nucleus divides into two, by mitosis. At about the same time a 

 great number of chromidia are emitted from the nuclei. These become 



