HISTORY OF THE HUMAN 



account of the law just mentioned, the mass of a growing 

 animal increases faster than its external surface, and the time 

 is soon reached at which it is in danger both of starving and 

 of suffocation. To offset this, recourse is had to a process 

 called fission, which effects at the same time a relief from the 

 physiological difficulty and a multiplication of the individual. 

 In its simplest form this reproduction by fission, as it is 

 termed, is inaugurated by (i) a lengthening of the nucleus; 

 (2) a contraction of its middle portion, producing a form 



FIG. i. Simple fission. Diagrams based on the infusorian Paramcecium. 



In all the figures the macronucleus is on the left, the micronucleus on the 

 right. The division of the micronucleus is effected by mitosis, that of the macro- 

 nucleus is direct. 



like an hour-glass, and (3) a separation of the two halves, 

 forming two independent nuclei, each half of the original 

 size. A similar subdivision of the body of the cell follows, 

 the arrangement being such that each piece becomes supplied 

 with one of the two nuclei, and is capable of beginning an 

 independent existence. In certain other cases the proceeding 

 is more complicated. The organism surrounds itself with a 

 shell or cyst, secreted by the protoplasm, and after a quiescent 

 period, breaks up, not into two, but a larger number, usually 

 four, eight or sixteen, which become released by the bursting 

 of the cyst and swim out into the water, each in its turn to 

 assimilate foreign matter until of the size for another encyst- 

 ment. 



It is but natural to refer to the undivided organism as the 



