VITALITY 



499 



The majority of isogametes show morphological characteristics 

 which easily distinguish them from agametes or vegetative indi- 

 viduals. In man\' cases the physiological differences at maturity 

 are expressed by a change in the type of diA'ision whereby binary 

 fission is replaced by multiple division. INIany daughter cells are 

 thus formed from one gametocyte and the term merogametes has 

 been applied to such a brood. The copulating gametes, howe\'er, 

 show no distinguishing morphological characteristics and the 

 differences between them if there are any, must be of a physiological 

 nature. In Foraminifera such isogametes are the rule and their 



Fiu. 208. — ^Scijtoinonas subtiUs, copulation. (After Dobell.) 



formation indicates a well-defined cyclical differentiation of the 

 parental protoplasm. Thus in PoJystomeUa crispa according to 

 Schaudinn (1903) and Lister (1905) ; in Penerojjlis pertusiis according 

 to Winter (1907) ; in Trichnsphcerivm siebddi according to Schaudinn 

 (1899) and in Foraminifera gene^all^^ the young protoplasm after 

 fertilization forms one ty])e of organism termed the microsph?eric 

 generation which reproduces by agamete formation (Fig. 119, 

 p. 239). Such agametes develop without fertilization into organisms 

 of a different type, the difference being shown by the character of 

 the initial shell chamber, hence a macrospha?ric generation. After 



