PROTOZOAN GERM PLASM 



579 



In Paramecium there are no gametes of this kind, but portions of the 

 adult individuals fuse as in OpTiryocystis mesnili. Unlike this Gregar- 

 ino, the fusion of the adult cells is only temporary and the two parties 

 to the conjugation do not die. During the temporary fusion there is a 

 mutual interchange of micronuclei and a mutual fertilization, while the 

 only portion to disintegrate and die is the macronucleus of each con- 

 jugant, and this is replaced by a specially differentiated fragment of the 

 new micronucleus. 



In ciliated protozoa such as Paramecium and its near relations the 

 germ plasm is concentrated in the micronucleus, while the somatic 

 plasm is represented by the macronucleus. As we have seen, the micro- 

 nucleus enlarges and divides during conjugation, first into two, then 

 these into four, all but one of the four degenerating. The fourth 

 divides for the third time, but this time unequally (Fig. 3) into a 

 smaller migratory and a larger stationary form. The smaller micro- 

 nucleus migrates into the other cell of the pair and there unites with 

 the stationary larger nucleus. The macronucleus then degenerates and 

 is absorbed in each of the conjugating cells. 



The aberrant conditions in these Infusoria can be interpreted if we 

 regard the three divisions of the micronucleus as the reminiscence of a 



Fig. 13. 



Original. 



process of gamete formation which obtained in remote ancestral forms. 

 A parallel, but less extreme, case is seen in the Gregarine Opliryocystis, 

 where one gamete only is formed by the conjugating cells. This is an 

 isolated case among these Sporozoa, for in the typical forms a great 

 number of gametes are developed, as shown in the accompanying pho- 

 tograph of Monocystis (Fig. 13). The one pair of gametes in Ophryo- 

 cystis, plus the nuclear derivatives of the pro-conjugants, represent the 



