THE GENERAL OUTLINE OF MEIOSIS 61 



chromatid have fused together (the point of genetical 

 crossing-over) does not move — all that shifts is the 

 visible chiasma. This process of shifting may happen 

 in chromosomes with one or many chiasmata ; it 

 may be only slight or may result (as in Fig. 14c) in 

 all the chiasmata moving to the extreme ends of the 

 bivalent. In the latter case they appear never to 

 slip off the ends, but a completely terminal chiasma 

 has entirely lost the appearance of a cross ; it 

 merely consists of four chromatids which are in 

 contact, end to end (Fig. 14c). 3i» ^^ 



Both the second and the third processes (' rotation ' 

 and ' terminalization ') clearly depend in part on the 

 general repulsion force (represented by a hollow 

 arrow in Fig. 14). This leads in the case of rotation 

 to the loops and ' free arms ' (which may be considered 

 as forming half- loops) taking up positions farther 

 away from one another in space. In the case of 

 terminalization the effect of the repulsion force will 

 obviously be greater inside a closed loop than in a 

 * half-loop ' and will lead to an expansion of the loop, 

 or loops if there are several, at the expense of the 

 terminal half-loops. In some organisms at any rate 

 there seems to be a special repulsion between the two 

 spindle attachments of a bivalent so that if these lie 

 in a closed loop, that loop will expand at the expense 

 of any adjacent loops which may be present. Natur- 

 ally as this process of terminalization takes place it 

 involves a change in the pairing relationships of the 

 chromatids. Whereas at first, points of crossing -over 

 and chiasmata coincide and paternal chromatids are 

 only paired with paternal, and maternal with maternal 

 (Fig. 14a), as they diverge paternal and maternal 

 chromatids come to lie paired for a certain distance 

 (between the point of crossing-over and the new 

 position of the chiasma). This shows that it is not a 

 failure of the paternal and maternal chromatids to 

 attract one another which prevents them remaining 

 paired after the split has occurred in pachytene — the 



