42 



INBBEEDING AND OUTBREEDING 



It is clear from this short description of gametogenesis 

 and fertilization that the processes in plants and in ani- 

 mals are identical in what we deem to be the essential 

 features, the behavior of the chromosomes. If one visual- 

 izes the behavior of hereditary characters in crosses in 

 which the parents differ as the result of the operation of 

 potential factors carried by these bodies, he can correlate 



f- 



n ---I 

 a 



A 



'"- c 



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FIG. 13. Entrance of the spermatozoon through the membrane of the egg of a star- 

 fish giving an idea of the difference in size between the sperm and the egg. (Wilson's " The 

 Cell." Courtesy Macmillan Co.) 



every fact thus far discovered, with the exception of a 

 few isolated cases found in plants where particular char- 

 acteristics appear to be distributed by the cytoplasm lying 

 outside of the nucleus. Not only can the distribution of 

 ordinary characters be interpreted as functions of the 

 chromosomes, but the distribution of the sexes as well. 

 There is reason to think the behavior of the sex-control- 

 ling chromosomes may perhaps occasionally be influenced 

 by external conditions, but sex itself is determined by the 



