" HETEROTYPICAL MITOSIS IN NEREIS LIMBATA. 73 



of attachment, their plane of division, their typical changes of 

 form during the separation of the daughter chromosomes and 

 the longitudinal split of these chromosomes — these are char- 

 acters common to all the divisions, taking place within fifteen 

 hours after fertilization. 



And yet, every division (or group of divisions) has its char- 

 acteristic appearance, each representing one step in a series of 

 transformations following the stage of the conjugation of the 

 chromosomes. 



The first maturation division is characterized by a great vari- 

 ability in the shape of the chromosomes and also in the time of 

 the separation of the daughter chromosomes. No stage in this 

 division can be considered as the metaphase of the whole mitotic 

 figure, each chromosome passing through its characteristic stages 

 independently of all the others, and without being placed in a 

 typical equatorial plate. Characteristic also is a peculiarity in 

 the (chemical or physical ?) structure of the chromosomes, mak- 

 ing them appear less stiff and consistent than in other divisions. 1 

 Thus the daughter chromosomes are by their separation very 

 often sharply bent, their ends forming right angles with each other 

 (chrom. 18-25, Is t ma t. div., p. 63), and there is also a marked 

 tendency towards a spherical shape of the free ends of the chro- 

 mosomes. 



Taking the first maturation division as a starting point, all the 

 changes in the mitotic figures going on within 15-16 hours after 

 fertilization may be looked upon as a gradual return from these 

 irregularities to the normal mitosis. 2 



The chromosomes regain slowly their original more rigid 

 structure; and their cross-shape during the separation of the 

 daughter chromosomes becomes less conspicuous to the same 

 degree as the rigidity increases. 



Of great interest are also the gradual changes of the chromo- 



1 Meves (1897) and Kingsbury (1902) have drawn quite similar conclusions from 

 the form of the daughter chromosomes in the first maturation mitosis of amphibians. 

 2 The second maturation division does not form a good link in this series of trans- 

 formations, its whole appearance being more like the later cleavage-divisions than the 

 earlier ones. This is, however, a natural consequence of the lack of a resting stage 

 before this division, the changes of the chromosomes within the nucleus being of im- 

 portance for their appearance during the mitosis. 



