228 GENETICS 



V 



9. THE HEREDITARY BRIDGE 



Whatever may ultimately prove to be determiners 

 of the hereditary characters which appear in successive 

 generations, it is obvious that, in any event, such 

 determiners must be located in the zygote, that is, in 

 the fertilized egg. This single cell is the actual bridge 

 of continuity between any parental and filial genera- 

 tion. Moreover, it is the only bridge. 



In the majority of animals the egg develops en- 

 tirely 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 indi- 

 vidual is the single sperm-cell, which unites with the egg 

 in fertilization, and the whole of this even is not usually 

 concerned in the process of fertilization. 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. To the new 

 individual developing by mitosis from the fertilized egg 

 into an independent organism, the factors of environ- 

 ment and response referred in to Figure 1 are subse- 

 quently added. 



When it is remembered that the human egg-cell 

 is only about Vizsth of an inch in diameter, a gigantic 

 size as compared with that of the human sperm-cell, 



