SPECIAL PROBLEMS OF MEIOSIS 



83 



quency of multivalent formation. Another factor of 

 major importance in this connexion is whether the 

 polyploid in question is an auto- or an alio -polyploid ; 

 allo-polyploids form far fewer multivalents than auto- 

 polyploids. Thus if we consider an allo-tetraploid 

 with four chromosomes Aj, A 2, A3' and A 4' (Ai and 

 A 2 being derived from one parent species and A3' 

 and A 4' from the other) it will probably form two 



B,B,B3 



A.A^A^jA^ 



a Two-Strand Pachytene b Diplotene 



Fig. 18. — Diagrams of meiosis in a male tetraploid organism 

 which has a somatic chromosome set of twelve autosomes 

 and two X-chromosomes. One quadrivalent (A1A2A3A4) 

 one trivalent (BiBgBg), two bivalents (CiCg and C3C4), 

 and a univalent (B4) are formed among the autosomes. 

 The two X-chromosomes are held together in close 

 association as a result of a pairing-attraction, but do not 

 form any chiasmata on account of their strong positive 

 heteropycnosis. In a the positions where the chiasmata 

 will subsequently arise are indicated by arrows. S.A. = 

 spindle attachment, ch — chiasmata. 



bivalents (Aj, A 2) and (A3', A4') since although all 

 the chromosomes are partly homologous those derived 

 from the same parent are completely so. An idea 

 of the range of variation as regards multivalent 

 formation can be got from Table VII which shows 

 the frequency with which quadrivalents are formed 

 in auto-tetraploids. 



The general history of multivalents at meiosis 

 (chiasma-formation, shifting of chiasmata, &c.) 



