THE GENERAL OUTLINE OF MEIOSIS 55 



pressed in the first meiotic division, it is merely 

 postponed until a stage which corresponds to about 

 half-way through prophase. Pachytene can thus be 

 subdivided into a two-strand stage (before splitting), 

 and a four-strand stage (after splitting). 



At the beginning of pachytene the two threads are 

 lying strictly parallel, but they soon begin to wind 

 round one another like the two wires of a piece of 

 electric flex ; thus when splitting takes place it 

 results in four threads two of which are wound round 

 the other two. The reason for this coiling is not 

 quite clear, but it is possibly accompanied by a slight 

 increase in length of the threads. ^^ 



DiPLOTENE 



The force of attraction between homologous genes 

 seems in general to be restricted to two genes at any 

 one level, there being no ' residual attraction '. 

 That is to say that as soon as the pachytene chromo- 

 somes have split the attraction force between the 

 paternal and maternal chromosomes ceases to exist, 

 being replaced by an attraction between the two 

 chromatids of which each chromosome is composed 

 (Fig. 12). In this respect the pachytene chromosome 

 resembles an ordinary mitotic chromosome and 

 differs from a salivary gland chromosome in which 

 as many as 256 chromomeres mutually attract one 

 another in each transverse ' band ' (page 38). As a 

 result of the disappearance of the attraction force 

 between the chromosomes these begin to separate, 

 being repelled by the surface repulsion force which is a 

 general property of all chromosomes, but which has 

 been overcome in zygotene and pachytene by the 

 pairing attraction-force. The moment when the two 

 homologous chromosomes begin to separate marks 

 the transition from pachytene to diplotene. If they 

 were to separate completely we should have another 

 stage similar to leptotene ; actually, however, they do 

 not do so but remain held together at certain points 



