CHAPTER XV 

 REPRODUCTION IN ANIMALS 



So careful of the type . . . 



So careless of the single life. Tennyson. 



IN addition to the organs devoted to the life of the individ- 

 ual animal, the Vertebrates in common with all forms of life 

 necessarily are provided with means for the continuation of 

 the life of the race. Reproduction, it will be recalled, is, 

 in the last analysis, division; the setting free by the organism 

 of cells with the power of going through a complex series of 

 changes, involving cell division and differentiation, by which 

 the relatively simple germ cell becomes transformed into the 

 obviously complex individual, similar to the parent. In most 

 plants and animals this process is complicated at the start by 

 the fusion of two germ cells, the male and female gametes, 

 to form the fertilized egg, or zygote. Disregarding for the 

 time being the ultimate origin of the germ cells in the body, 

 we find in the Metazoa special organs in which the germ cells 

 reside and undergo changes preparatory to their liberation. 

 Such reproductive organs, or GONADS, ordinarily contain germ 

 cells of one kind, and accordingly are either OVARIES (egg- 

 producing organs) or TESTES (sperm-producing organs) . 



In many of the simpler animals, the gonads are merely 

 temporary structures which appear during certain seasons of 

 the year when conditions favor sexual reproduction. Fre- 

 quently also the same individual produces both eggs and 

 sperm, in which case the sexuality of the germ cells is not 

 reflected back, so to speak, to the organism as a whole, which 



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