CHAPTER XIII 



ORIGIN AND MORPHOLOGICAL AND PHYSIOLOGICAL CONDITION 

 OF THE GAMETES IN PLANTS AND ANIMALS 



THE THEORETICAL SIGNIFICANCE OF GAMETIC ORIGIN 



The question of the origin of the gametes or sex cells derives its 

 chief importance from the germ-plasm theory, first advanced by 

 Galton ('72) and Jager ('77) and later developed by Weismann 

 ('85, '92) , which postulates the continuous existence of a germ plasm 

 independent of the soma^ — that is, of other parts of the organism — 

 except for nutrition, and giving rise to the gametes. If such a 

 germ plasm exists and is continuous from generation to generation 

 we should expect to find in at least some organisms indications of 

 the separate existence of germ plasm and soma, even in early stages 

 of development. An early segregation of the germ plasm from the 

 somatic cells has been recorded for various animals and these facts 

 have commonly been regarded as affording support to the germ- 

 plasm hypothesis. Other facts, such as the formation of gametes 

 and the occurrence of regeneration from apparently differentiated 

 cells in some animals and in plants, forced Weismann to assume the 

 existence of a " supplementary germ plasm" which was supposed to 

 exist in the nuclei of many differentiated cells and which might be 

 "activated" under the proper conditions and give rise to new 

 embryonic cells, or even to gametes. The existence of this supple- 

 mentary germ plasm may be assumed wherever it is necessary for 

 the theory, so that a vicious circle is established. 



But when we consider the facts apart from theoretical considera- 

 tions, we find that the gametes appear to be integral parts of the 

 organism when they arise, that they become highly specialized and 

 differentiated cells, and that fertilization, whatever the nature of 

 its mechanism, initiates a process of dedifferentiation and rejuve- 

 nescence which is followed by another period of differentiation and 

 senescence. This and the two following chapters are concerned 

 with the development of this point of view. 



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