" heterotypical" mitosis in nereis limbata. 67 



the axis of the spindle) so as to form a more or less nearly closed 

 ring (chrom. 18, 22). 



The vertical arms of the cross on the other hand, are at first 

 short, and their elongation evidently takes place at the cost of the 

 horizontal arms. They are in most cases thinner than the hori- 

 zontal arms (except their endpiece, if this is lying beyond the 

 point of attachment of the fibers), and very often no longitudinal 

 split can be seen in this part of the chromosomes (chrom. 19/21, 

 24, 27). It is clear, however, from their earlier as well as 

 from their later stages, that a doubleness is present even here ; 

 and when it is not visible, it may be due to the stretching of 

 the vertical arms of the chromosomes. 



Metaphase rings seem to be formed mostly from horseshoe- 

 shaped chromosomes (chrom. 9, 16, 28). They are divided 

 into two half rings. I have never observed a longitu- 

 . dinal split in these half rings ; and an examination of a great 

 number of chromosomes shows that the metaphase-rings of Nereis 

 are to be directly compared with the cross-shaped chromosomes 

 of the same stage, the space between the two branches of each 

 half ring being identical with the longitudinal split in the vertical 

 arms of the cross. Very often we find this space in the rings 

 filled by an achromatic substance (like the " Zwischensubstanz " 

 of the chromosomes of Enteroxenos) ; l and we find, indeed, all 

 transition stages between the open rings and the thin vertically 

 stretched arms of the crosses. 



1 (Added on the proof-sheet, June, 1907.) The existence of such a substance in 

 the chromosomes of Enteroxenos is absolutely denied by A. and K. E. Schreiner 

 (1907). They say (p. 12) : 



" Wir haben an unseren Praparaten vergebens nach dieser sehr eigenthiimlichen 

 Substanz gesucht, und es scheint uns unzweifelhaft, dass sich Bonnevie in diesem 

 Punkt vollkommen getauscht hat, indem sie bald die Spalte zwischen den Kompo- 

 nenten eines Doppelchromosoms, bald die Langslichtung in den Komponenten 

 selbst, bald aber auch achromatische Verbindungen zwischen zwei oder mehreren 

 Chromosomen als eine Kittmasse gedeutet hat." 



In the face of this sweeping criticism I can only repeat that in Enteroxenos, as 

 well as in Nereis, Thalassema and Doris, I have found the chromosomes of the 

 maturation divisions, and especially those of the first, containing an achromatic sub- 

 stance which can be stretched out to a considerable width between the branches of 

 the chromosomes. 



In this respect the chromosomes of the maturation differ from those of other di- 

 visions in the same material. 



If this substance has not been visible in Schreiner's preparations, it must depend 

 upon its being dissolved or contracted. 



