CHAPTER XVII 

 KEPRODUCTION 



Introduction 



The capacity for reproduction is one of the principal 

 characteristics of hving matter. In the simplest forms 

 of hfe it occurs in two ways. One consists merely in the 

 division of the single-celled organism into two and the 

 subsequent growth of these until they resemble the parent. 

 By the repetition of this process several times a large 

 number of individuals is formed. But sooner or later 

 this process comes to an end, the capacity for division 

 undergoing decay. Further propagation can only occur 

 by a second method which consists in the fusion of two 

 cells. By this process the reproductive function is re- 

 stored, the new cell undergoing division with great vigour. 



In higher animals reproduction is effected by essentially 

 the same two processes. The repeated division which the 

 fertihsed ovum undergoes are exactly comparable with the 

 division of protozoa, the only difference being that in 

 the former the cells formed, instead of becoming inde- 

 pendent, remain bound together to form the multicellular 

 animal, the process culminating in the formation of a new 

 individual. 



The capacity for division comes to an end at different 

 periods according to the nature of the tissue. In nerve- 

 cells new-formation probably never occurs after birth, 

 while in other tissues it persists throughout hfe. In the 

 latter case it may be continually occurring, as in the cells 



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