REPRODUCTION 129 



REPRODUCTION 



Protoplasm has three distinguishing capacities: metabolism, 

 irritability, and reproduction. Protoplasm always occurs in units 

 called cells. A single cell may be a complete organism, or a group 

 of cells may be associated to form an individual. Reproduction 

 can therefore be defined as the capacity, under varied conditions, 

 of certain parts of organisms to detach themselves, and, either alone 

 or after union with protoplasm of another organism of the same 

 kind, to give rise to new individuals in all essential respects Hke 

 the parent or parents. The function of reproduction differs from 

 metabolism and irritability in that it is not necessary for the main- 

 tenance of the life of the individual; it is the capacity upon which 

 depends the continuity of the race or species. 



To understand the physiology of reproduction, one must have a 

 knowledge of the differentiation of the gametes or germ cells, the 

 problems of fertilization or the union of gametes in pairs to form 

 zygotes, the development of zygotes in characteristic ways, and 

 heredity and variation in the new individuals. Much more is 

 known of the morphology than of the physiology of these phases of 

 reproduction. As these topics are considered in the special chap- 

 ter on Reproduction and in the chapters on Development and 

 Genetics, no further discussion of reproduction will be undertaken 

 at this point. 



