v] ORIGIN OF BIVALENTS 71 



some have maintained, that the strepsitene rings consist of 

 double chromosomes bent round on themselves like a split 

 key-ring, a view which is not easy to reconcile with the later 

 stages but which seemed at one time a natural corollary to 

 the side-to-side pairing hypothesis. 



After the formation of the ring- or loop-shaped double 

 chromosomes which undoubtedly emerge from the syni- 

 zesis or contraction phase, whatever view is taken with 

 regard to their origin, there is certainly a real difference in 

 their behaviour in the next stage ("diakinesis") in different 

 animals. In Lepidosiren, as has been seen above, the 

 chromosomes emerge in the reduced number (half the soma- 

 tic number) of double chromosomes, and then break up into 

 their component parts, giving rise to the somatic number 

 of single chromosomes. Almost immediately they then pair 

 for the second time, giving the reduced number of doubles 

 (sometimes called gemini or bivalents), arrange themselves 

 on the spindle, and separate in such a way that the two 

 members of each pair go to opposite poles of the spindle. 

 This separation and reunion of the members of the pairs is, 

 however, probably exceptional; more frequently the double 

 chromosomes (gemini) which emerge from synizesis con- 

 tract to form compact ring-, loop- or X-shaped chromosomes 

 which then arrange themselves directly on the spindle and 

 separate into their component halves in the heterotype 

 division. When, as in Lepidosiren, a separation and fresh 

 pairing takes place, it is comparatively easy to see that 

 complete single chromosomes segregate from each other in 

 the heterotype division, but when no such second pairing 

 takes place it is less easy to be certain of this. It has been 

 maintained that in some animals the first spermatocyte 

 division does not separate complete chromosomes, but that 

 it consists in a longitudinal splitting of double chromosomes 



