266 



THE PROTOZOA 



generations namely, the monad form, product of syngamy, and 

 the adult, mastigamo3ba-form, which ultimately produces the 

 monad-like gametes. Hence the life-cycle in such a type is an 

 alternation of generations (metagenesis), which, as in so many other 



FIG. 112. Life-cycle of Mastigdla vitrea, diagrammatic. 1, 2, and 3, Different 

 forms assumed by the adult " vegetative " type of individual ; 3o. 36, repro- 

 duction by binary fission; 4 10, gamete - formation ; o (in each case), 

 microgamete-formation, 6, macrogamete-formation ; in the former the gamont 

 becomes encysted, and the principal nucleus degenerates early in the process ; 

 in the latter the gamont remains motile and the principal nucleus persists 

 to the last : 4 6, extrusion oi chromidia from the nucleus and formation 

 of secondary nuclei ; 7, 8, formation of the gametes round the secondary 

 nuclei ; 9, extrusion of the gametes ; 10a, the small, non-flagellated micro- 

 gametes ; 106, the larger, flagellated macrogametes ; 11, copulation of the 

 gametes ; 12, 12o, 126, multiplication by binary fission of the monad-like 

 zygote ; 13, 14, growth of the monad-form, after a period of multiplication, 

 into the adult mastigamoeba-form. After Goldschmidt (41). 



oases in the animal kingdom, appears to have come about by mul- 

 tiplicative processes taking place in a larval type, phylogenetically 

 older namely, the monad form, the only form of individual that 



