MUSEUM OF COMPARATIVE ZOOLOGY. 131 



ment of the chromatic substance, as indicated in Figure 4. The effect 

 is most marked upon the nucleoli. Blochmann ('85, p. 482) found only- 

 two nucleoli at this stage, and these were usually situated one at each 

 end of the elliptical nucleus. Where there are several nucleoli, as is 

 usually the case with the nuclei I have studied, there is an approxi- 

 mately equal distribution of them to the daughter nuclei. The nucleoli 

 vary so much in size and shape, that it is impossible to say how precise 

 is the apportionment of chromatin by this method. 



Most nuclei in the elongated condition already show a slight constric- 

 tion, generally more marked on one edge than on the other (Fig. 4). 

 If no further elongation takes place, the constriction becomes deep and 

 narrow, as represented in Figures 5 and 12. This style of division is 

 characteristic of young membranes, and gives rise to daughter nuclei 

 which lie close together, or even in contact (Fig. 13). It is doubtless a 

 more vigorous and rapid type of division than that found in the older 

 membranes, to be described directly. If the nucleus continues to elon- 

 gate while constricting, it assumes the dumb-bell form represented in 

 Figures 6 and 7. The daughter nuclei, at first ovate or pyriform, be- 

 come rounder as the connecting thread becomes tliinuer. Division of 

 this type is almost confined to old membranes; I have rarely found it 

 in those from young embryos. 



The nuclei represented by Figures 6 and 7 show more clearly than 

 usual a peculiar arrangement of the chromatic threads. The filaments 

 have the appearance of a fascicle of slender rods, which lie very close 

 together in the connecting bridge, and thence radiate into both daughter 

 nuclei. They are stainable both with carmine and haematoxylin. Some- 

 times these threads can be resolved into rows of granules (Fig. 7, right- 

 hand daughter nucleus). The later stages also show traces of these 

 longitudinal threads (Figs. 8, 9, 10). In the example represented by 

 Figure 6, the nucleoli partook of the general longitudinal disposition of 

 the chromatic substance, but were probably arranged in this manner at 

 an earlier stage of division, as explained for Figure 4. In the later stages 

 of division, this arrangement of the nucleoli is gradually lost. 



The final stages, represented in Figures 8, 9, 10, may be briefly de- 

 scribed. These stages are far commoner than the early ones ; hence, it 

 must be supposed that they require more time. The constricted por- 

 tion is drawn out into a thin, deeply staining thread. This thread 

 undoubtedly contains chromatin, and in a peculiarly condensed form. 

 In this respect these nuclei differ from the nuclei of the Malpighian 

 vessels of Aphrophora spumaria, as described and figured by Carnoy 



