THE INDIVIDUAL AND THE SPECIES 179 



and become greater in number, but remain unchanged 

 in other respects. These cells become the essential 

 reproductive organs, or gonads, of the adult animal— 

 that is, the ovaries of the female and the testes of the 

 male. In the females of the higher animals (the 

 mammals, and perhaps some of the Arthropods) 

 these cells only divide and grow during the early 

 stages of development, and long before the beginning 

 of adult life the number of ova in the gonads has 

 become fixed. In all males, and in the females of 

 most animals, however, the reproductive cells appear 

 to be capable of unlimited multiplication. 



The essential cells of the gonads, the ovarian 

 mother-cells or the sperm mother-cells, constitute the 

 germ-plasm. In modern, speculative, biological litera- 

 ture the term germ-plasm is, however, restricted 

 to the chromatic material in the nuclei of the repro- 

 ductive cells, the cytoplasm being regarded as non- 

 essential for the transmission of the hereditary qualities 

 of the organism. It seems clear, however, that this 

 distinction between the cytoplasm and the chromatic 

 matter of the nucleus is not always a valid one, so 

 that it is best to speak of the whole cell as constituting 

 the germ-plasm. The embryonic cells, therefore, have 

 different fates : some of them become transformed 

 during development into the body or soma, and others 

 remain unmodified throughout life as the germ. The 

 soma enters into intimate relationships with the 

 environment ; it is affected by the vicissitudes of 

 the latter; and it may actively respond to them. 

 The germ-cells may possibly migrate through the body, 

 perhaps, it has been suggested, developing fatally and 

 irresponsibly into the mysterious, malignant tumours 

 of adult life. Normally, however, they remain segre- 

 gated in the reproductive glands, secluded from the 



