Origin of Sexuality 275 



at least Infusoria, as amongst Algae, that after a certain period 

 of dividing activity this is followed by differentiation of the 

 cell-contents into complemental gamete cells, with or without 

 previous encystment. For, when such a stage is not reached, 

 senile decay sets in, unless indeed some new and different 

 stimulant form of energy incites to continued division. 



We can now examine phenomena of sexuality in the Pro- 

 tozoa. Amongst the Rhizopoda-Lobosa enough is kno'WTi to 

 indicate that, after a period of division-activity, a stage is 

 reached when the nuclear chromatin undergoes reduction- 

 di\T[sion by the extrusion of a certain amount of its substance, 

 that then undergoes disintegration. During this process, 

 cells that are nearly related to each other in origin (Endamoeba), 

 or that may belong to distinct broods, become gamete cells. 

 These show such energizing reactions that pairs come together, 

 fuse, and produce a zygospore from which a new indi\ddual 

 develops. 



In Centropyxis and Trichosphcerium transitions from the 

 above simple conditions to the complex details seen in For- 

 aminifera are presented. For, during reduction-di\asion in 

 the former, two sizes of shell arise, the individuals in the smaller 

 of which probably decay, while those of the larger continue 

 to increase, and eventually produce large conjugating cells 

 or megagametes, and also — by subdivision — four smaller cells 

 or microgametes. One of the latter conjugates with the former 

 to produce a new and fully rejuvenated individual. 



In Trichosphcerium one generation arises that seems to be 

 mainly nutritive in function, and wliich forms spicules in the 

 gelatinous outer coat. It shows numerous nuclei in its pro- 

 toplasm, and multiplies by division. This stage in time gives 

 place to another that is devoid of spicules, which after repeated 

 divisions encysts, produces flagellate gametes, and in order to 

 fertihzation a gamete from one cyst must unite with a gamete 

 from another cyst, so as to form a zygospore that starts again 

 the first or tropliic generation. 



Amongst Foraminifera two types of shell are — at least in 

 some genera — produced, the megalospheric and the micro- 



