26 THE MATURATION OF THE EGG OF THE MOUSE. 



Stage II. — -Formation of First Maturation Spindle. 



The passage from the preceding stage to this one is rapid. The 

 germinative vesicle has shrunk still more and is surrounded by a narrow, 

 clearer region, in which, however, there are cytoplasmic granules (figs. 

 2 and 3). Its contents are no longer clear and homogeneous, but show 

 a granular condition, much like that of the immediately surrounding 

 cytoplasm. A few achromatic threads are still visible. The nucleolus 

 and chromatic spherules have disappeared (compare fig. 1 with figs. 2 

 and 3), and instead there is a group of chromosome bodies, which is 

 usually located at one side, rather than in the middle, of the nucleus. 



Figs. 2, 3, 4, and 5 show the first steps in the formation of the first 

 maturation spindle. The fundaments of the chromosomes differ greatly 

 in form. Some are masses of irregular shape, which it is hard to dis- 

 tinguish from the large granules; some are ring-like; a few are elongated 

 and show a simple or a compound curve; still others show divisions into 

 two or four parts (figs. 2 and 3). Later (figs. 4 and 5), these all become 

 completely differentiated and assume more definite and characteristic 

 forms, some in advance of others. As they assume more precise forms 

 they become more separated from one another. Their number is at 

 first uncertain, but by the time they have reached the condition seen 

 in figs. 4 and 5 it is clearly 20 (see table 2, Stage II, p. 14). 



At an early stage in their development the fundaments of the chro- 

 mosomes lie in a group at one side of a homogeneous portion of the 

 karyoplasm which is denser than the surrounding nuclear contents 

 (figs. 3 and 4). This denser portion, at first indefinite in form (fig. 4), 

 increases in size and develops into the first maturation spindle. As it 

 grows the chromosomes move apart and all come to lie at its surface. 

 At length it becomes elliptical in outline (fig. 5), and then shows delicate 

 fibrillations extending from pole to pole. At the same time the sub- 

 stance of the spindle becomes less homogeneous, showing granules dis- 

 tributed through it, so that, except for the fibrillations, it becomes in 

 appearance more like the rest of the karyoplasm. Meanwhile, the clear 

 zone around the nuclear membrane disappears (figs. 2 to 5), and at the 

 same time the general contents of the germinative vesicle assume more 

 nearly the appearance of the surrounding cytoplasm; the nuclear mem- 

 brane, which meanwhile has shrunk little, if any, more, is gradually 

 dissolved (fig. 5), vanishing more quickly in some regions than in others. 

 Its disappearance may begin in some parts very early (fig. 36). 



Stages III to V. — Development and Division of First Maturation Spindle. 



Stage III (plate 1, figs. 6, 7, and 7a). — With the complete disap- 

 pearance of the membrane of the germinative vesicle the spindle is 

 left free in the midst of the cytoplasm. It is broadly elliptical (fig. 6) 

 and shows, not only on its surface but in the interior as well, very fine 

 fibrillations, which conform in direction to its shape. As in Stage II, 



