CHAPTER V 



OOGENESIS AND SPERMATOGENESIS. REDUCTION OF THE 



CHROMOSOMES 



" Es konimt also in der Generationenreihe der Keimzelle irgendwo zu einer Reduktion 

 der ursprunglich vorhandenen Chromosomenzahl auf die Halfte, und diese Zr?///^';;. reduk- 

 tion ist demnach nicht etwa nur ein theoretisches Postulat, sondern eine Thatsache." 



BOVKKI.I 



Van Beneden's epoch-making discovery that the nuclei of the con- 

 jugating germ-cells contain each one-half the number of chromosomes 

 characteristic of the body-cells has now been extended to so many 

 plants and animals that it may probably be regarded as a universal 

 law of development. The process by which the reduction in number 

 is effected, forms the most essential part of the phenomena of matura- 

 tion by which the germ-cells are prepared for their union. No phe- 

 nomena of cell-life possess a higher theoretical interest than these. 

 For, on the one hand, nowhere in the history of the cell do we find so 

 unmistakable and striking an adaptation of means to ends or one of 

 so marked a prophetic character, since maturation looks not to the 

 present but to the future of the germ-cells. On the other hand, the 

 chromatin-reduction suggests questions relating to the morphological 

 constitution of nucleus and chromatin, which have an important 

 bearing on all theories of the ultimate structure of living matter and 

 now stand in the foreground of scientific discussion among the most 

 debatable and interesting of biological problems. 



Two fundamentally different views have been held of the manner 

 in which the reduction is effected. The earlier and simpler view, 

 which was suggested by Van Beneden and adopted in the earlier 

 works of Weismann, Boveri, and others, assumed an actual degenera- 

 tion or casting out of half of the chromosomes during the growth 

 of the germ-cells — a simple and easily intelligible process. Later 

 researches conclusively showed, however, that this view cannot be 

 sustained, and that reduction is effected by a rearrauiienient and redis- 

 tribution of the 7iuciear siibstance without loss of an\' of its essential 

 constituents. It is true that a lars^e amount of chromatin is lost dur- 

 ing the growth of the Qggr- It is nevertheless certain that this loss is 

 not directly connected with the process of reduction ; for, as Hertwig 



1 Zellenstiidien, IH., p. 62. '^ Cf. Figs. 97, 116. 



