126 ETHEL NICHOLSON BROWNE 



•In the telophase of the first division, which is also the prophase 

 of the second, the chromosomes become somewhat crowded, but 

 it is often possible to distinguish the individual ones. The large 

 compound chromosome retains its structure of the anaphase, 

 and can in most cases be followed with ease at this time, and 

 this is also true of the next largest one (fig. 35) ; both these chro- 

 mosomes, probably on account of their bulk, usually lag behind 

 the others in division. When on the second spindle (fig. 36), 

 the line of separation between the two curved bars falls along 

 the equatorial plane. In the case of the largest chromosome, 

 therefore, the line of separation in the second division is already 

 marked out in the metaphase of the first, and is in evidence in 

 all the following stages. A side view of the second division 

 shows a characteristic XY pair in the center of the spindle (figs. 

 36 and 37 B). Owing to the fact that X and Y divide as sepa- 

 rate chromosomes in the first division, and as a single chromo- 

 some in the second, the number is reduced by one. Consequently, 

 in serial sections of a complete spindle, there are twelve chro- 

 mosomes, including the XY dyad in the center, and on the periph- 

 eral fibers the large compound chromosome, the next largest 

 similar in appearance, and the two small chromosomes, one of 

 which was in the center in the first division (fig. 37 A, B, C). 

 Frequently, the X and Y chromosomes do not take up a linear 

 arrangement, but lie side by side in the metaphase, on different 

 spindle fibers, a peculiarity found in all the species of Notonecta 

 and comparable with the non-linear arrangement of the com- 

 ponents of the small central dyad in the first division; this ar- 

 rangement seems to occur only in those chromosomes whose 

 components normally conjugate late. An entire spindle in 

 which this arrangement of X and Y occurs, is shown in figure 

 38 A, B, C. Polar views show the same arrangement and size 

 relations of the chromosomes as side views, and sometimes X 

 and Y lie side by side at the same focus (figs. 39 and 40) . Whether 

 the two large chromosomes appear double ffig. 39) or single (fig. 

 40), depends of course, upon whether both sister elements are 

 in view or only one of them. The difference in the amount of 

 chromatin contained in the large chromosomes and the small 



