STUDY OF CHROMOSOMES IN NOTONECTA 135 



pieces are; in figure 102, the three parts in the center are parts 

 of the X Y pair. Figure 103 shows the different appearances of 

 X Y in polar view. In anaphase, X is usually slow in going to 

 the pole, and is found stretched between the two groups of chro- 

 mosomes (fig. 104) . More frequently X appears very much as it 

 did in the first anaphase, as a central mass with smaller pieces 

 attached, often by the merest thread, or with no visible connec- 

 tion (figs. 105 and 106) ; it will be observed that these end pieces 

 may extend either toward or away from Y. Sister groups from 

 an anaphase in polar view show again the number and size rela- 

 tions of the chromosomes, and are practically identical, except 

 for the central component, X or Y (fig. 107). The chromatin 

 content of the two classes of spermatozoa differs more in this 

 species than in any of the others, owing to the unusually large 

 volume of X. The structure of X in telophase is very similar 

 to its previous structure, and it is frequently difficult to discover 

 any connection between its components (figs. 108 and 109). 



In the spermatogonial (figs. 110 to 113) and the oogonial (figs. 

 114 and 115) groups, there are 26 chromosomes. Especially char- 

 acteristic are the long winding chromosomes, single in the male 

 and paired in the female groups; this is of course the X chromo- 

 some. Although X shows such a marked tendency to break up 

 into smaller parts in the spermatocyte divisions, I have found 

 little indication of this in the diploid groups. I have examined 

 many groups, and have always found X represented by a single 

 chromosome of almost equal width throughout its length, with 

 sometimes a slight tendency toward notching at one or iboth 

 ends. In both male and female groups the two large double 

 chromosomes of the spermatocyte divisions are represented by 

 the four next largest chromosomes, and the two small ones by 

 four little ones. . 



b. The X chromosome of N. indica 



The characteristic and unusual appearance of the X chromo- 

 some as described in the preceding section leads one to suspect 

 that it is really compound, and is in the process of splitting off 

 or uniting with smaller chromosomes. Whether these smaller 



