57° 



THE POPULAR SCIENCE MONTHLY 



viduals. The individual cell appears to be entirely self-sufficient; it 

 captures food, digests it, grows to maximum size and then divides 

 (Fig. 1). The original individual cell no longer exists, although it has 

 not died; its protoplasm is now distributed between two daughter cells. 

 In the same way these cells grow old in turn, divide into two each, and 

 so on apparently in endless succession of cell generations. Obviously 

 if this could keep on indefinitely there would be a basis for the view 

 that Protozoa are immortal. They do not keep this up, however, but 

 there comes a time when the nature of the protoplasmic make-up 

 changes, and processes similar to fertilization in Metazoa supervene. 



Tn the great majority of parasitic 

 protozoa and in most free-living 

 forms that have been studied in 

 culture, there comes a period 

 when ceitain cells of the race, or 

 specialized parts of the proto- 

 plasm of all of the cells of the 

 gig race, undergo marked changes. 



** different from any vegetative 



phase, and reorganization of the 

 old individual or formation of 

 new ones is the outcome. This 

 result is brought about by conju- 

 gation or the union of two cells 

 in more or less complete coales- 

 cence, during which an inter- 

 change and mixture of germ plasms is accomplished (Fig. 2). 



In some of the best-known forms of Protozoa, notably in Par- 

 amecium caudatum, the conditions are quite different from those of 

 the majority of protozoa and too many generalizations have been made 

 upon the comparatively rare phenomena which are manifested in this 

 " slipper animal " and its immediate allies. In the conjugation of 

 Paramecium two individual cells unite very much as do Blepharisma 

 cells. In each individual there are two types of nuclei, one, a large 

 macronucleus, plays no part in the fertilization process, but, sooner or 

 later, disintegrates and dissolves in the cell. The other is a minute 

 micronucleus, which divides three successive times, giving rise to a 

 number of micronuclei, which, with the exception of two germ nuclei, 

 also disintegrate and dissolve in the cell. These two germ nuclei are 

 sexually differentiated, one is smaller than the other, and migrates into 

 the other cell of the pair, there uniting with the stationary larger form 

 of nucleus (Fig. 3). Thus there is a mutual fertilization of the two 



Fig. 



Original. 



