514 PROCEEDINGS OF THE AMERICAN ACADEMY. 



mantle fibres extend between the two masses of daughter chromosomes, 

 presenting the peculiar appearance shown in Figure 33. The nuclear 

 substance may still be distinguished from that of the ordinary cyto- 

 plasm by its more transparent appearance. With the constriction of 

 the cell, however, the distinction between nuclear-sap and cell-sap is 

 finally lost, although a disc of more transparent material still remains 

 in the plane of constriction until cell division is nearly complete 

 (Figure 34). 



By the constriction of the cell, the interzonal filaments are grouped 

 together in a sheaf-like bundle. This is much shorter and broader 

 than in Scolopendra, and persists but a short time. At the plane of 

 constriction each fibre is interrupted by a small enlargement. These 

 bodies at first occur only in the superficial fibres, where, by their fusion, 

 they form a darkly stained ring about the sheaf of spindle remnants 

 (Figure 35). Later, however, as constriction proceeds still farther, the 

 interzonal filaments entirely disappear, while the ring surrounding them 

 at a former stage has been converted into a darkly stained plate. 

 This is continuous with the constricted cell membrane, and closes the 

 opening between the two daughter cells. In sections it appears as a 

 black band, and extends through a considerable depth of focus. 



The second spermatocyte mitosis follows the first very closely, there 

 being no distinction between the telophase of the first and the prophase 

 of the second division (Figure 35). The centrosomes do not leave their 

 position upon the cell membrane, but in separating move along this 

 structure until they arrive at opposite sides of the cell. The nuclear 

 membrane then breaks down, and the spindle is formed. This spindle, 

 just as in the first spermatocyte division, is truncate at the poles and 

 remains entirely distinct from the astral systems. 



The succeeding changes in the second mitosis are so similar to those 

 in the first spermatocyte as to render separate description unnecessary. 

 The dividing cells in the second spermatocyte may, however, always be 

 readily distinguished from those of the first by several characteristics. 

 The cells are much smaller, the spindle is more attenuated, and the 

 chromosomes are of quite a different shape. The changes in the sper- 

 matid during its development into the spermatozoon will be the subject 

 of a later paper. 



Summary. 



Pseudo-reduction is accomplished by an end-to-end union in pairs 

 of the chromosomes during the telophase of the last spermatogonia! 

 division. All of the chromosomes take part in this except the ac- 

 cessory chromosomes. 



