CHROMOSOME RELATIONS IN ONISCUS. 2? 



in the equatorial plate of the first maturation division of the male 

 reproductive cells of the wood-louse, Oniscus, there occur chro- 

 mosomes of three kinds. Some are straight or dumb-bell shaped, 

 others are crescent shaped. In these two forms the component 

 parts of the bivalent chromosomes lie end to end. A third form 

 exists in which the univalent chromosomes are joined not end to 

 end, but side by side. In each type a split may be observed run- 

 ning the length of the chromosomes and the first division is there- 

 fore reducing. In the prophase the third type is represented 

 usually by rings, occasionally by V-shaped structures (Figs. 7 

 and 14), while the crescents appear as curved rods and the re- 

 mainder as straight or dumb-bell shaped rods. The reduced 

 number of chromosomes, as far as could be determined, is sixteen. 

 Of this number two have the ring form, two are crescents and 

 the others are straight or dumb-bell shaped. The chromosomes 

 vary somewhat in size, but in Oniscns the differences are not so 

 great as in some of the insects before mentioned. The largest 

 have the ring form, the smallest are straight (Figs. 1,2,4, ! 7, !&) 

 Sometimes one of the two rings is complete or nearly so, the 

 other incomplete (Figs. I, 11). The rings lie oh opposite sides 

 of the nucleus, sometimes directly opposite, in other cases ap- 

 parently shifted to one side or the other (Figs. I, 2, 3, 8, 9, 1 1, 

 15, 1 6). Since they are the largest and most easily recognized of 

 the nuclear elements they form convenient points of localization. 



The crescent form is the next most easily recognizable. Of 

 this type there are likewise two, one of them sometimes more 

 strongly curved than the other. They also occupy opposite 

 sides of the nucleus and at points between the positions occupied 

 by the rings (Figs. 3, 4, 8, 9, 10, II, 15, 16, 19). 



The relative position of the rings and crescents is more readily 

 determined than that of the rods, because they are not so numer- 

 ous and are also distinguished by their shape. Repeatedly how- 

 ever I have observed an arrangement of three or four straight 

 chromosomes in a row (Figs. 9, 12, 17). One of the smaller of 

 the straight chromosomes, too, is often seen lying not far from 

 one of the crescents (Figs. 8, 13, 17, 18). 



These facts point to an individuality of the chromosomes and 

 also indicate a tendency to localization in the nucleus. The evi- 



