THE DEVELOPMENT OF VERTEBRATA. 295 



parts of the body gave off living particles, winch he called 

 geinmules, which passed into the reproductive cells and in 

 the next generation developed into organs and structures 

 similar to those from which they had been derived in the 

 parents. 



These gemmules were purely imaginary, and there is no 

 evidence in physiology of any living substance passing 

 from the somatic cells or tissues into the reproductive cells. 

 It is, however, now certain that special chemical compounds 

 pass in certain cases from one organ and profoundly affect 

 the functions or development of other parts. It is also 

 certain that all parts of the body give off their waste pro- 

 ducts to the blood. It is possible that these waste pro- 

 ducts include not only carbon dioxide and compounds 

 formed in all living tissues by oxidation, but also specific 

 chemical compounds peculiar to each organ or tissue. 



Since the reproductive cell or gametocyte contains some- 

 thing corresponding to each structure and character of the 

 individual developing from it, and therefore to each part 

 of the soma of the parent in which it is contained, it is a 

 reasonable hypothesis that each part of the soma gives off 

 a hormone which, passing into the blood, acts upon the re- 

 productive cell and affects the corresponding part in it, the 

 corresponding determinant we may say. Thus, although 

 it is true that the " determinants " in the gametes are 

 derived directly from those of the gametes or of the ferti- 

 lised ovum of the previous generation, it may be also true 

 that these determinants are to some degree modified by 

 the hormones of the soma in which the gametes are con- 

 tained. 



Moreover, we have reason to believe that hormones are 

 specially produced by actively growing tissues, and by 

 functional activity of all kinds, since growth and func- 

 tional activity necessarily involve the production of more 

 vvaste products. Hence external stimulation, whether of 

 che nature of mechanical irritation producing growth or 

 of nervous stimulation producing functional activity of 

 muscles, bones, sense-organs, etc., may increase the hor- 

 mones of the parts affected, and so stimulate the " deter- 

 minants" in the gametes, and cause the modification of 



