THE CARRIERS OF THE HERITAGE 27 



of chromosomes as the parent, since they are simply 

 fragments of the parent. 



Professor Loeb, by the use of certain chemicals, 

 has succeeded in doing artificially what apparently 

 is never accomplished in nature, namely, making an 

 egg that normally requires fertilization develop par- 

 thenogenetically. 



10. THE HEREDITARY BRIDGE 



Whatever may ultimately prove to be deter- 

 miners 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 generation. 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 mater- 

 nal contributions to the offspring. Although there 

 is abundant evidence that half of the filial char- 

 acteristics come from the male parent, the only 

 actual fragment of the paternal organism given over 

 to the new individual 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 re- 

 spective parents and, in all probability, into the 

 nuclei of these germ-cells, since the nuclei are ap- 



