64 KRISTINE BONNEVIE. 



somes with four single arms, all of the same length and usually- 

 extending to one and the same side of their connecting point 

 (chrom. 3, 8, p. 62). 



Another kind of cross — double-armed ones — may also be 

 derived from the original tetrad, its four elements being arranged 

 in pairs, but combined in a different way at each end of the tetrad 

 (Fig. 1 , f. At the upper end the elements x + y are diverg- 

 ing from z + u, at the other end x -f- z remain parallel, diverging 

 from y -\- it.) Through a flattening down of a figure, formed in 

 this way we get a cross-shaped chromosome (Fig. 1, g), whose 

 arms appear longitudinally split, and in which always two arms, 

 lying opposite to each other, are of the same length. Sometimes 

 this may be the case with all four arms, but more often we find 

 a considerable difference between the two pairs. In many chromo- 

 somes this difference is so great that the short arms of the cross 

 appear only like a pair of lateral projections on the middle of a 

 longitudinally split ribbon ; and from those forms there is a very 

 short step to the chromosomes represented in Fig. 1, b and c. 



Finally we may often find chromosomes consisting of two appar- 

 ently separate halves (Fig. 1, e ; chrom. 7, p. 62). Here also the 

 origin of the chromosome may be traced back to a tetrad, and 

 most easily through a transition form like that in Fig. 1, b, the 

 arms of such a chromosome being divided along their longitudi- 

 nal split. 



A comparison of the whole chromosome group in a number 

 of nuclei shows that these different forms are not characteristic 

 of special chromosomes. I have found nuclei, in which all the 

 chromosomes were cross-shaped, others in which two or three 

 rings were present among the crosses, and again others, in which 

 one or both of these forms were mingled with the more irregu- 

 larly formed chromosomes. 



Nor did I find any evidence in favor of the view, that the dif- 

 ferent forms of the chromosomes should represent different stages 

 in their development. It seems more probable that the rings, 

 the two kinds of crosses, the rodlike chromosomes, etc., arise 

 simultaneously from the original tetrads and that their special 

 shape is more a result of chance — possibly of their conditions 

 within the nucleus — than of any individual character of the 



