40 



THE MATURATION OF THE EGG OF THE MOUSE. 



In some of the eggs from a mouse killed about 33 hours p.p. and 

 not inseminated, there appear a few cytoplasmic radiations at the spindle 

 poles, from which the circumpolar bodies have vanished. Also in the 

 egg illustrated in fig. 29a, the cytoplasmic granules around the inner 

 end of the spindle are oriented with their long axes in a radial direction ; 

 but otherwise the evidence of cytoplasmic radiations about the poles 

 of the second maturation spindle has been lacking. 



As already stated regarding the first spindle, the clear region around 

 the spindle exists at the same time with the circumpolar bodies, except 

 that it may appear a little before them (fig. 19), and sometimes persists 

 longer (fig. 30). It can usually be found surrounding the spindles of 

 eggs which have been only a short time in the oviduct (figs. 20, 21, 22, 

 23, 24), but in most eggs in which the circumpolar bodies have vanished, 

 it has likewise disappeared (figs. 25, 26, 27). It becomes quite faint 

 (figs. 29, 30), or is altogether gone, after the chromosomes have divided. 



5. Position and Orientation of second maturation spindle. 



The second maturation spindle always lies near the surface of the 

 egg — in fact, sometimes so near that its flatness (fig. 20) is apparently due 

 to pressure. There is no satisfactory evidence that it moves through 

 the cytoplasm, although it is found at different distances from the first 

 polar cell when that is present. This topic will be taken up later (pp. 44 

 and 63). 



There is less variation in the orientation of the second spindle 

 than in that of the first. Very rarely, indeed, can it be found perpendic- 

 ular to the surface; occasionally it is oblique, but in the majority of 

 cases it is parallel to the surface. It is parallel in all instances in which 

 the daughter chromosomes have separated and have reached, or nearly 

 reached, the poles before the abstriction of the second polar cell begins; 

 but in those in which the abstriction has begun (figs. 29a, 296) it is 



oblique ; and in the stage of the telophase 

 of the formation of the second polar 

 cell (fig. 30) the interzonal filaments are 

 usually almost , if not quite , perpendicular. 



Fig. 29a 



6. Abstriction of second Polar cell. 



The process resulting in the forma- 

 tion of the second polar cell is precisely 

 like that by which the first polar cell is 

 Fig. /. produced. The beginning of the process 



is illustrated by an egg shown in part 

 in figs. 29a and 296 and in fig. /. The last is a diagrammatic, imaginary 

 section of the egg, in a plane parallel with the axis of the spindle, but 

 perpendicular to the actual sections shown in figs. 29a and 296. The 

 daughter chromosomes have virtually reached the poles of the spindle 

 and have lost their identity by being merged together; the " Zwischenkor- 



