XII 



THE CHROMOSOME THEORY i 



JD 



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 



^: ABXab 







Paternal 

 Gamete 







Zygote 

 (AaBb) 



Tetrad 

 Stag-e 



Dyad 

 Stage 



Gametes 



Fig. 39. 



Scheme illustrating the distribution of the maternal and paternal chromosomes 

 in the families formed by the ¥\ 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 



