XII THE CHROMOSOME THEORY 135* 



halves becomes a complete chromosome, so that all 

 of the resulting cells contain 4 chromosomes like 

 the original zygote. But when the time comes to 

 form gametes, a peculiar process occurs by which 

 the number of the chromosomes in each gamete is 



Maternal 

 Gamete 



i 



ABXab 







Paternal 

 Gamete 



I 







Zygote 

 (AaBb) 



Tetrad 

 Sta^e 



Dyad 

 Stage 



Gametes 



Scheme illustrating the distribution of the maternal and paternal chromosomes 

 in the families formed by the Fi zygote. Further explanation in text. 



reduced to 2. In the~ cells which are destined to 

 form the gametes the corresponding chromosomes 

 pair closely with one another, often giving rise to 

 bodies known as the tetrads. Then follow two 

 rapid divisions by which the chromosome material is 

 first reduced to one half, and next to one quarter of 



