HEREDITY 



233 



not to occur at random (if it did so the whole hypothesis would 

 be spoiled). We have, before maturation, ABCDabcd = N, 

 and after the coupling we might have A B, a C, D d, b c = ^ N. 

 But there is some evidence that the pairing (synapsis) has tendency 

 — that " homologous " chromosomes come together, thus : A a, 

 B b, C c, D d = ^ N, and there is also some evidence that the 

 next process, that of disjunction, is also a tendential one. At this 

 phase the gonidial cell-nucleus contains the 4 haploid chromo- 

 somes, A a, B b, C c, D d, each of them formed by the joining of 

 homologues. Either now, or even earlier, each diploid chromo- 



... , A a, B b, C c, D d, , , 



some splits mto 2, thus — =-7- 7; — 7^— r so that what we 



A a, B 0, L c, D d, 



really have are 4 " tetrad " chromosomes. We visualize the 



nucleus as in i, Fig. 30. 



Fig. 30. — Diagram of the Maturation Divisions. 



I, Gonidial cell about to divide heterotypically to form (2 and 3), the dyads. 



What follows is called the " heterotypic " division, 

 tetrad divides into two parts : 



Aa , A , a 



-TT- becomes — and -, 

 Ala A a 



Each 



and so on. 



Bib , B ^ b 



-p-fr becomes - and - 

 B\b B b 



X 



