v] LEPIDOS1REN 65 



contraction, and from this stage onwards the shortening 

 and thickening proceeds rapidly. At the same time the 

 tight mass which was formed in synizesis becomes loosened 

 and the threads are seen to have contracted into relatively 

 short, thick chromosomes, most of which still have the 

 form of rings, in consequence' of their being united by their 

 ends in pairs. In Lepidosiren a process now follows which 

 has not been described in many other animals, but which 

 will possibly be found to be of more general occurrence. 

 The chromosomes as they emerge from the clumping of 

 synizesis are double, consisting of rings made up of two 

 single chromosomes; these rings now break up into their 

 components, giving rise to the somatic number of more or 

 less rod-shaped chromosomes, most of which are contracted 

 in the middle, thus having a somewhat dumb-bell-like shape 

 (PI. IV, //). In the majority of animals this separation of the 

 rings into their single halves has not been described. As 

 the chromosomes become disentangled from the clump of 

 the synizesis stage, the nuclear membrane dissolves and the 

 spindle begins to be formed ; at the time when the membrane 

 disappears the centrosomes have yet not reached opposite 

 sides of the nucleus but lie comparatively near together, 

 and from each a sheaf of fibres runs towards the irregular 

 group of chromosomes. Gradually the centrosomes rotate 

 apart until they come to lie on opposite sides of the chromo- 

 some group, and a definite spindle is formed between them. 

 Meanwhile the chromosomes come together again in pairs, 

 and for the second time form double ring-shaped bodies 

 which take up their position in the equatorial plate of the 

 spindle (/, J). Owing to the constriction in the middle of 

 each single chromosome, these rings generally have the ap- 

 pearance of being four-fold, looking like four rounded bodies 

 in contact with one another, but from the manner of their 

 D.C. 5 



