166 GENERAL BIOLOGY 



three thousandth generation. Whatever the causes 

 may be that induce conjugation in an ordinary cul- 

 ture, it seems to he evident that they are external to 

 the organism. 



In many of the Protozoa, as already described, con- 

 jugation is a complete mingling of the two organisms 

 in a zygote. In others, conjugation is temporary, 

 and there is an exchange of nuclear substance between 

 the two cells. In the ciliate infusoria, of which 

 Paramecium is the most familiar example, the 

 macronucleus degenerates and the micronucleus of 

 each conjugant divides twice. Three of the four 

 resulting nuclei degenerate. The fourth divides 

 again, one half passing into the other conjugating 

 cell and there fusing with the alternate half that does 

 not pass over. The two individuals then separate, 

 and after a number of divisions of the new, compound 

 nucleus, a rapid series of cell divisions results in the 

 production of a great many new individuals. 



Many of the Protozoa form spores under certain 

 circumstances. This is especially characteristic of 

 the parasitic forms. It is very interesting to dis- 

 cover that Amoeba proteus, which is usually con- 

 sidered one of the simplest of all organisms, and the 

 vital processes of which are frequently used to illus- 

 trate the functions of protoplasm at their lowest 

 level of specialization, has a very complicated process 

 of spore-formation. The nucleus of the Amoeba 

 divides repeatedly until about seventy small nuclei 

 result. These then fuse, two by two (still w r ithin 

 the same cell), and the fused nuclei redivide several 



