120 THE THEORY OF THE GENE 



The diploid plant has 12 pairs of chromosomes (24 

 chromosomes) which according to Belling and Blakeslee 

 can be arranged in six sizes (Fig. 74), namely, large (L 

 and 1), medium (M and m), and small (S and s), or 

 o (L-j-41+3M+2m+S+s). The formula for the haploid 



1 



* « 



»s«t i * tm 



Tn 



> 



•i « *i 





m 



♦ ' 



a 



Fig. 74. 

 o, Second metaphase chromosome group of diploid Datura stramo- 

 nium with 12 chromosomes (each constricted) ; and b, correspond- 

 ing group of tetraploid with 24 chromosomes. (After Belling and 

 Blakeslee.) 



group is L+4.1+3M+2ni+S+s. These chromosomes, 

 when about to enter the first maturation division (pro- 

 phase), form pairs of rings or else are united by one end 

 (Fig. 75, second column). One conjugant of each pair then 

 moves to one pole and its mate to the opposite pole. Pre- 

 paratory to the second maturation division, each chromo- 

 some constricts, producing the appearance shown in Fig. 

 74b. One constricted half passes to one pole of the spindle, 



