20 THE HEL1OZOA 



pseudopodia which they retained during the plastogamy. Schaudinn 

 thinks it probable that all recorded cases of division without mitosis 

 and without retraction of the pseudopodia are really cases in which 

 plastogamic individuals have been seen to separate. 



An observation recently made by Calkins on Paramecium suggests 

 a possible eft'ect of plastogamy. The work of Maupas has shown 

 that, after a certain number of asexual divisions, Paramecium and 

 other Ciliata, when grown in artificial culture-media with a constant 

 supply of food of one kind, exhibit phenomena of degeneration, 

 which quickly lead to the death of the whole culture, unless 

 individuals produced by another zygote are introduced. If such 

 individuals are introduced, plastogamy occurs, which is quickly 

 followed by a complicated sexual (karyogamic) process ; and after 

 this the "rejuvenated" culture can enter upon another period of 

 asexual multiplication (cf. Chap. I. Fasc. II. pp. 386, 387). Calkins 

 has, however, shown that a culture which exhibits signs of degenera- 

 tion may be completely "rejuvenated" by purely chemical stimuli, 

 such as an appropriate change of food, and that if plastogamy alone 

 be allowed to occur, the conjugating individuals being shaken apart 

 before the nuclear changes which precede karyogamy have taken 

 place, these individuals can still go through a further cycle of 

 asexual divisions. Nothing analogous to the phenomena of 

 "senile degeneration" described by Maupas has been observed 

 among the Heliozoa, but it is possible that it may occur, and that 

 the rejuvenescent effect of natural plastogamy is similar to that of 

 the artificial plastogamy observed by Calkins. 



Although plastogamy is often followed by a complete separation 

 of individuals, it may be the beginning of a sexual karyogamic 

 process, which has been carefully studied by Schaudinn. In this 

 case the mass of individuals, united by ectosarc, sinks to the bottom 

 of the water; the pseudopodia are withdrawn, and a common 

 gelatinous cyst is secreted, like the outer layer of a solitary cyst. 

 Each individual within the gelatinous common cyst secretes a 

 membrane, which is thrown into wrinkles, so that in optical 

 section it looks as if made of spicules joined together. These 

 cysts lie in pairs within the common jelly, the two members of a 

 pair in contact (Fig. 3). The nucleus of each cyst now goes 

 through a mitosis (infra, pp. 25, 27), which results in the extrusion 

 of a single polar body. When the pronuclei of a pair of adjacent 

 cysts have returned to the resting condition, the walls of the cysts 

 break down at the point of contact, the two cell-bodies fuse, their 

 pronuclei also fusing, and the completed zygote becomes spheroidal 

 within the membrane derived from the cyst -walls of the two 

 gametes. After a period of quiescence the nucleus of the zygote 

 divides into two, by a process identical with that observed in 

 asexual cysts, and the division of the nucleus is followed by that 



