THE GENERAL SCHEME 



many divisions it becomes old; the vigor of its members 

 diminishes. Individual animals become abnormal and en- 

 feebled, and the rate of fission slows down. The clone is in 

 danger of extinction. It needs a shake-up, which it cannot 

 get through the process of complete inbreeding (or rather, 

 lack of outbreeding) by which the clone develops. 



Conjugation or mating. This seems to be the reason that even 

 in such simple animals a process much like sexual union occurs. 

 Fig. 6 shows the mating of the one-celled animal Scytomonas. 

 Animals of this species are not attached, like Cothurnia, but 

 swim about in the water. As we watch them under the micro- 

 scope, two individuals that are going to mate swim near each 

 other, come into contact and actually cohere side to side. One 

 of them loses its whiplike flagellum. The protoplasmic sub- 

 stance of which they are made becomes continuous from one 

 animal to the other, and the nuclei move toward each other 

 and unite. In this particular species the conjugated animal 

 then becomes dormant for a time, but ultimately resumes 



Fio. (i. Conjugation of the one-celled animal Scytomonas. Proceeding 

 from A, which shows a single individual, through B, C, D, and E, two 

 cells are seen to join, fuse their nuclei (the dark round objects) and 

 unite into one cell. This single individual remains dormant for a time 

 but ultimately becomes active again. Greatly magnified. After Dobell, 

 simplified. 



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