THE PROTOZOAN LIFE CYCLE. 237 



formation. These erstwhile questionable bodies must now go 

 into the protozoan literature under the generic name of Neuro- 

 ryctes given by Dr. Williams. When the same method comes 

 to be applied to the small-pox organism, I am confident that the 

 last doubter will be convinced that it, too, is a protozoon, and of 

 the rhizopod type. 



Still another feature of this second period is the change in form 

 of the conjugating individuals. We have seen that, with increas- 

 ing maturity, the organisms of a cycle lose their definite shape, 

 become plastic or amreboid in some cases and conjugate while in 

 this condition. Now it is probably due to the same underlying 

 causes that gametes of relatively minute size are formed. In 

 Polytoina, for example, size differences are entirely facultative, 

 two normals, two reduced individuals, or one normal and one 

 reduced individual may unite. From such an indifferent condi- 

 tion we find all intermediate stages to fully established obligatory 

 differences in size between conjugating forms and vegetative forms. 

 This might be interpreted as a purely physiological matter de- 

 pending upon the general chemical and physical balance in the 

 cell, but with it is bound up very often a second phenomenon, 

 sexual differentiation, which has a deeper significance than the 

 mere change of form at this period of adolescence. The study 

 of protozoa has thrown no light as yet on this problem, which 

 according to recent experimental and cytological findings, espe- 

 cially Wilson's discoveries on the extra sex-determining chromo- 

 some in certain insects, would seem to be a matter of inheritance 

 rather than of controllable physiological balance. In many cases 

 what may be called secondary sexual characters in protozoa are 

 evident from the very outset after conjugation, even the first 

 progeny being sexually differentiated as in Trypanosoma or 

 Adclca, and this certainly can not be traced to advancing age or 

 to changes in chemical relations of nucleus and cytoplasm. 



In some cases these differentiations are not established until 

 some change in external conditions brings them out. Klebs, 

 Dangeard, and others have made different types of flagellates 

 conjugate by changing the temperature or increasing the den- 

 sity in the surrounding medium. The same experiment is per- 

 formed by mosquitoes and other insects on various parasitic 



