114 



protopIasmic age of protozoa 



make-up of the protoplasm, for in the early periods of activity the body 

 wall, while plastic, always retains its firm contour and cortical density. 

 Pearl ('07), furthermore, has shown by biometric analysis that con- 

 jugating paramecia are markedly smaller and less variable than non- 

 conjugating forms. 



Similar changes in density mark this period in other kinds of pro- 

 tozoa. Thus, among the flagellated forms like tetramitus or cerco- 

 monas the ordinarily firm contour of the cell becomes plastic and 

 highly changeable in form, and two of them upon meeting fuse in 

 conjugation. Here again a physical change is well illustrated. 



Fig. 41 



Polystomella crispa. Liberation of pseudopodiospores from the niicrospheric 

 individual. (Photo by J. J. Lister.) 



Still more remarkable is the change in form which some types of 

 sarcodina undergo at this time. The rhizopods are especially note- 

 worthy in this connection, Schlumberger ('83) noting for the first time 

 a peculiar dimorphism in the shells of foraminifera (Fig. 42), a differ- 

 ence which Schaudinn ('03) and Lister ('05) were the first to explain. 

 These observers found that the voung forms, immediatelv after fer- 

 tilization, give rise to what Schlumberger termed the " microspheric"^ 

 type of shell. Upon reproduction, such a cell ultimately gives rise to 

 pseudopodiospores which leave the old shell and secrete new ones of a 

 different type, termed the "megalospheric" type (Fig. 41). The 

 latter generation, when fully grown, gives rise to flagellispores which 

 conjugate and thus complete the cycle (see Fig. 52, p. 123). 



