64 REPRODUCTION 



male. It is believed by many students of the subject 

 that they are carried by the chromosomes which are so 

 equally shared by the male and female in fertilization. 



7. The Narrow Bridge. Genetics is the name of a divi- 

 sion of biological science; it is the experimental study of 

 evolution and heredity; its aim is to ascertain the knowl- 

 edge that is necessary for the improvement of inherited 

 characters; it is a division of science which has already led 

 to great improvements in the races of domesticated 

 animals and cultivated plants and it holds the promise of 

 great results in the improvement of the human race. In 

 his very interesting book on that subject*, Professor 

 Walter has a paragraph which reads about as follows: 



"Whatever may ultimately prove to be the determiners 

 of the hereditary characters which appear in successive 

 generations, it is evident that they are located in the 

 fertilized egg. This single cell is the actual bridge of 

 continuity between the generations. Moreover, it is the 

 only bridge. 



"For the majority of animals the Qgg develops entirely 

 outside of and independent of the mother, thus limiting to 

 the egg-cell itself all possible maternal contributions to 

 the offspring. Although there is abundant evidence that 

 half of the filial characteristics come from the male parent, 

 the only actual fragment of the paternal organism given 

 over to the new individual is the single sperm-cell. Thus 

 the entire factor of heritage is packed into the two germ- 

 cells derived from the respective parents and, in all prob- 

 ability, into the nuclei of these germ-cells, since the nuclei 

 are apparently the only portions of these cells that 

 invariably take part in fertilization. 



"When it is remembered that the human egg-cell is 

 only about 1-125 of an inch in diameter, and that this is a 

 gigantic size as compared with that of the human sperm 

 cell, and, furthermore, when one passes in rapid review 



"Genetics, H. E. Walter, The Macmillan Company. 



