GENETICS: THE HERITABLE TRANSMISSION OF CATALYSTS 157 



and ovum, and the union of these units as the initial step (concep- 

 tion) in the formation of an individual. Besides the single pair of 

 sex-chromosomes which determine the sex of the individual, there 

 are a number of autosomes ("ordinary chromosomes"), which in 

 man number 46 (23 pairs). In mammals, including man, female 

 cells have additionally two x-chromosomes (female determiners), 

 while male cells have additionally one x- and one y-chromosome. 

 In ordinary cell duplication, termed mitosis, the chromosomes 



MALE FEMALE 



Primary 

 Spermatocyte 



X- Chromosome 



(Tem»\e* . ^ ft 

 Sex- determiner; [£ 



Y- Chromosome 

 / y MMe" . \ 

 ^sex-deterrtunev 



Worn ajermina\ epithelium- 

 Double set ot chromosomes 



Primary 

 Oocytg 



.TV»re<i 

 X-Qvromosome^ 



Other chromosomes (23 pairs 

 inman)arerepresen\e<i here b«| 

 two pairs. These chroTnosomes 

 are «eulr&\ or o? secondary im- 

 portance w Se*-AeYemunat»on. 



REDUCTION DIVISION 



"Paired chromosomes sep- 

 arate, one jjoinci to each cetl 



Number orchromosomes 



TedaccA to '£ that 'in bod»* 

 C*\»S 



EQUATIONS DIVISION 



Chromosomes split lono;- 

 ItudinaUy. Number in 

 daughter ceUs unchained 



X- bearing 



SPERtiATOZOA 



"POLAR eooits 



(Degenerate) 



MATURE OVUM 



FERTILIZATION 



Chromosome number restored 

 bj fusion or Sperm and ovum 



Devetops mto 



MALE 



Develops »nto 



FEMALE 



Figure 18. The mechanism of sex-determination. [Courtesy Journal of Heredity, 



Vol. XXXI, No. 12 (1940)] 



