228 KANSAS UNIVERSITY SCIENCE BULLETIN. 



is possible to determine that separation of the chromosomes 

 takes place along the plane which marked the longitudinal divi- 

 sion of the prophase thread in such a way that the chromatids 

 show no clear interspaces. The individual chromosome near the 

 end of its division has the same form as that with which it 

 started, except for the difference that the chromatids are now 

 in contact for the greater part of their length along the plane 

 of their cross-division. As the daughter chromosomes separate, 

 this line of division comes into evidence through the springing 

 apart of the two chromatids now composing each chromosome. 

 The result is the formation of two V-shaped chromosomes with 

 mantle fibers attached to their apices. The accessory chromo- 

 some does not participate in this division, but passes unchanged 

 to one pole of the spindle. 



8. By reason of the action of the accessory chromosome in 

 the first spermatocyte mitosis, there are produced two numer- 

 ically equal classes of second spermatocytes — (a) those con- 

 taining sixteen dyad chromosomes and an undivided accessory 

 chromosome, and (b) those with merely the sixteen dyad ele- 

 ments. In both cases the mitotic figure quickly reforms with- 

 out an intervening rest stage in which the chromosomes lose 

 their identity. There is a loosening up of the chromomeres in 

 all the elements except the accessory chromosome, so that they 

 have a structure and staining reaction similar to that of the 

 first spermatocyte chromosomes just before they enter the meta- 

 phase. The dyads of the first spermatocyte telophase, and of 

 the succeeding and greatly abbreviated second spermatocyte 

 prophase, are quite as definite structures as are the chromo- 

 somes of the first spermatocyte prophase. 



9. All the chromosomes of the second spermatocyte are 

 paired structures and divide in a similar way. The spindle is 

 small and weak as compared with that of the first spermato- 

 cyte, and the chromosomes arrange themselves radially on its 

 periphery in such a way that the pairs lie in the plane of the 

 spindle axis with their joined ends inward. The space between 

 the chromatids represents the line of cross-division observable 

 in the prophase segments of the first spermatocyte, and their 

 separation accordingly represents a reduction division. The 

 accessory chromosome, on the contrary, divides along the plane 

 marking the longitudinal cleft of the spermatogonia! spireme. 



