CHAPTER XV 

 REPRODUCTION 



Let us first understand the facts, and then we may seek the cause. 



— Aristotle. 



Reproduction, as we know, is in the final analysis cell division, 

 whether it is binary fission in unicellular forms such as Amoeba 

 and Paramecium, or the setting free by the multicellular organism 

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

 by which they rapidly become transformed into the complex in- 

 dividual, similar to the parent. In most animalc this process is 

 complicated at the start by sexual phenomena: the fusion of 

 two germ cells, the male and female gametes, to form the ferti- 

 lized egg, or zygote. Indeed, sex is fundamentally a physiological 

 difference between gametes, which, however, so profoundly in- 

 fluences the body that we recognize it as male or female. (Figs. 8, 

 31, 168.) 



Disregarding for the time being the actual 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 con- 

 tain germ cells of one kind, and accordingly are either ovaries 

 (egg-producing organs) or testes (sperm-producing organs). 



A. Invertebrates 



In many of the simpler animals, the gonads are merely tempo- 

 rary structures which appear during certain seasons of the year 

 when conditions favor sexual reproduction. In some species 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 accordingly is known as an her- 

 maphrodite. Such frequently is the condition in Hydra where 

 the testes appear as small swellings in the ectoderm a little below 

 the circle of tentacles, and the ovary, which is usually single, is a 

 somewhat larger projection near the opposite end of the animal. 

 Both the testis and the ovary at first appear to be a group of 



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