12 



BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



toward the observer, as sometimes happens (Figs. 237-239). Figure 

 192 shows four rings seen from the edge and two from the side. The 

 four chromosomes of a group may be arranged in a square rather than 

 in a circle (Plate IV. Figs. 175, 176). 



The division of the rings may present different appearances according 

 to the position of the chromosomes with reference to the poles of the 



spindle. The group may be a square, 

 with one side turned toward each pole of 

 the spindle (Diagram 2), or it may be 

 more diamond-shaped (Diagram 1) with 

 an angle directed toward each pole. In 

 either case division takes place as indi- 

 cated by the dotted line, and the chromo- 

 somes a and b go to one pole, c and d to 

 the other. The final result, therefore, is 

 the same as before. But in the first case 

 the chromosomes a and b, still held to- 

 gether by linin threads, move toward the pole maintaining unchanged 

 their relative positions, i. e. the rod with a chromosome at either 

 end remains at right angles to the polar axis of the spindle, and is 

 therefore in proper position for the second division, which follows di- 

 rectly upon the first, and is at right angles with it, a going to one spei"- 

 matid, b to the other. By the second mode the pair a, b starts for the 

 pole, either in a very oblique position or nearly parallel to the polar 

 axis, and with a in advance. It therefore must turn 45° or more so as 

 to be in the proper position for the second division. 



The later stages of the first spermatocyte division are shown in 

 Plate III. Figs. 112, 113, 117, 118, 122, and 123, which are drawn from 

 preparations stained with safranin and victoria-green, or by Henneguy's 

 method. These methods do not bring out the individual chromosomes 

 at this stage. Figures 118 and 122 show the interzonal filaments 

 still bridging over the space between the already separated cells. Some 

 cells at this stage (Figs. 113, 117) have a peculiar appearance, as if 

 the division were amitotic. But the interzonal filaments between the 

 two chromatic masses show it to be a mitotic division. 



The second division of the spermatocytes is shown in Plate III. 

 Fig. 128, and Plate V. Figs. 190, 191, 231. This division is accom- 

 panied by the formation of a typical spindle and centrosomes ; it effects 

 a separation of the constituent chromosomes of each chromatic dumb- 

 bell, and therefore results in giving each spermatid six univalent spher- 

 ical chromosomes, such as are shown in Plate III. Figs. 125, 126. 



