242 THE PROTOZOA 



In forms of Protozoa other than the Infusoria, there have been no 

 careful observations and experiments to determine the limits of the 

 potential of vitality. Nevertheless, certain facts have been recorded 

 which appear to be of the same order of phenomena as those seen in 

 the Infusoria. Thus, in some Monadida {Cercomonas, Tetramitus, 

 etc.) the loss of the customary body-form and the assumption of a 

 plastic condition prior to conjugation may be interpreted as an indi- 

 cation of degeneration, and comparable to the loss of membranelles, 

 cilia, etc., in the Infusoria. A similar interpretation may account for 

 the fusion of two cells of Difflugia lobostoma (Fig. 1 18), which under 

 normal conditions will not unite, but which fuse readily under "certain 

 conditions of the plasm" (Rhumbler). Also in Polytoma and Cldorogo- 

 nium, where facultative conjugation occurs between two full-sized cells, 

 between a full-sized and a smaller cell, or between two smaller cells, 

 the difference in size may be interpreted as an expression of degener- 

 ation, comparable to that which is indicated by the senescent, small- 

 sized individuals of Onychodromus or Paramcccium, where fusion may 

 also occur between two small conjugants or between a normal-sized 

 cell and a small one (Figs. 120 and 132). In all cases the protoplasm 

 has reached that state which, for the want of a better term, may be 

 called mature, when conjugation is possible. The sex-difference, 

 which is facultative in Polytoma and Chlorogonimn, becomes obliga- 

 tory in the allied form, Phacotus, a condition which may be compared 

 with that of the peritrichous Ciliata, where, in the Vorticellidas, the 

 free-swimming microgametes are dwarfed forms of the normal indi- 

 viduals with which they fuse (Fig. 122). 



In the more complex colonies of Flagellidia, and in the Coccidiida, 

 the microgametes, although they arise by spore-formation, are to be 

 compared with spermatozoa of Metazoa, and the macrogametes are 

 equivalent to eggs. The obligatory sex-difference is here indicated 

 by the storage of a reserve food supply with concomitant quiescence 

 in the larger forms (females), and by loss of cytoplasm and concomi- 

 tant increase of motion in the smaller ones (males). The researches 

 upon Coccidiida have established the fact that, as in Infusoria, there 

 is a period of conjugation which alternates with a period of spore- 

 formation, and the changed form of the conjugants into spermatozoa 

 and eggs may be taken as evidence of exhaustion as in the less 

 specialized cases among the Protozoa ; although with our present 

 knowledge an interpretation of these conjugants as degenerate cells 

 can only be based upon analogy with other forms, where a similar 

 cycle brings about more or less similar conjugating individuals. 



It is apparent from the facts cited that the study of Protozoa can 

 throw but little light upon the question of sex-difference. It may be 

 due either to differences in nutriment or to some more deeply lying 



