134 THE THEORY OF THE GENE 



somes (n=12) (Fig. 81a). The triploid has 36 chromo- 

 somes (Fig. 81b). The haploid group is composed of 1 

 extra large (L), 4 large (1), 3 large medium (M), 2 small 

 medium (m), 1 small (S), and 1 extra small (s) chromo- 

 somes. The diploid group is therefore 2 (L+41+3M-J- 

 2m+lS+ls) and the triploid has three of each kind. 



The maturation divisions have been studied by Belling 

 and Blakeslee. The reduced groups consist of 12 sets 

 of three each, united as in Fig. 81b. These trivalents have 

 the same size relations as have the bivalents in the dip- 

 loid group, i.e., they are formed by the union of like 

 chromosomes only, which are united in various ways as 

 seen in the figures. Two may be united at both ends and 

 the third joined on at one end only, etc. 



At the first division two of each triplo-set pass*to one 

 pole and one to the other pole of the spindle (Fig. 75, 

 third column), and since the assortment takes place at 

 random in the different triplets several combinations of 

 chromosomes are realized. The numbers found in one 

 count of 84 pollen mother cells are recorded below in 

 Table I. The results are in close agreement with the ex- 

 pectation for random assortment. 



table I 



Assortment of Chromosomes in 84 Pollen Mother Cells of Triploid 



Datura, 19729(1) 



Metaphase of Second Division. 



Assortment of Chromosomes 



Nos. of double groups 1 



Calculated on random orienta- 

 tion of trivalents 0.04 0.5 2.7 9.0 20.3 32.5 19.0 



Rarely the first division of the triploid may be omitted. 

 This is favored by transient cold. At the second division 



