274 



RECENT CYTOLOGY 



every egg (as well as every polar body) contains an 

 idiochromosome . 



In fertilization some of the eggs become impregnated 

 by spermatozoa containing the heterotropic chromo- 

 some. Such eggs invariably develop into females 

 having a pair of idiochromosomes in each somatic 



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ov, ovum ; /J, polar body ; sp, spermatozoa (a. and b the two kinds). 



cell. Other eggs are fertilized by spermatozoa lacking 

 the heterotropic chromosome, and these become males, 

 their somatic cells containing only the single hetero- 

 tropic chromosome derived from the egg. The ac- 

 companying diagram illustrates the behaviour of the 

 chromosomes during these processes. The hetero- 

 tropic chromosomes are represented as black, whilst 



