THE NUCLEI OF UROLEPTUS MOBILIS 313 



forming, division. Three to four micronuclei take part in the 

 first; two (very rarely three) nuclei take part in the second; and 

 three or four take part in the third division. Thus of the eight 

 possible products of the first division five or six degenerate. Of 

 the four to six products of the second, two to five degenerate; 

 and of the six or eight possible products of the third division, all 

 but one pair degenerate, and this pair forms the wandering and 

 the stationary pronuclei. 



a. The first maturation mitosis. The formation of the first 

 maturation spindle starts with the swelling of the homogeneous 

 nucleus and the projection of one pole of the nuclear membrane. 

 This projection contains one or more granules of intensely stain- 

 ing material which apparently corresponds with the intranuclear 

 division center of a typical flagellate nucleus of the centronucleus 

 type. As in the latter, a deeply staining fiber connects the pro- 

 jected granule with the nuclear mass (figs. 28 to 34). The pro- 

 jection forms a large vesicular region at one pole of the nucleus 

 and is traversed by fibers running from the pole to the chromatin 

 mass and surrounded by the nuclear membrane. At the outset 

 of nuclear change the micronucleus is ellipsoidal and measures 

 from 2h ij. to 3| m in its longest diameter. The projection forms 

 on the side, not at the end, and when fully developed the nuclei 

 measure from 5 m to 7 m (figs. 28 to 34) . The granular mass in 

 the projection forms one pole of the mitotic figure, while the fibers 

 traversing the projection form the spindle fibers. One entire 

 half of the mitotic figure is thus formed before there is any trace 

 of the chromosomes or of the other pole of the spindle. 



The 'chromosomes' are formed from the disintegrated chroma- 

 tin mass of the micronucleus. At first a thick net-work of chroma- 

 tin surrounds the denser chromatin mass (fig. 28). Later, dis- 

 tinct granules of chromatin appear at nodes on the net-work. 

 Both network and granules are formed at the expense of the 

 chromatin mass, which during this metamorphosis continually 

 grows smaller (figs. 29 to 31). The entire micronuclear complex 

 at this stage has a characteristic and striking appearance which 

 suggests a parachute when examined with relatively low magni- 

 fication (figs. 24, 25, and 26), and I shall refer to this phase here- 



