No. 2.] ORGANIZATION OF THE EGG OF UNIO. 239 



nally surrounded the sphere is still in process of degeneration, 

 and the yolk-granules are beginning to intrude on it (PI. XXIV, 

 Fig. 14). The fibers that reached from the sphere to the 

 periphery of the Q.gg have completely disappeared. This is a 

 period of inactivity. 



Figs. 15-20, PI. XXV, illustrate the development of the 

 second maturation spindle ; they are drawn from eggs in which 

 the spindle formed in its definitive radial position. But in other 

 eggs of the same set the spindle was found forming at various 

 angles with the axis of the ^gg, later swinging into a radial 

 position. It will be noticed that there is no inward migration 

 of the chromosomes and sphere prior to the spindle formation, 

 as in the eggs of some other animals, and as in those eggs of 

 Unio in which the " division-apparatus " of the first matura- 

 tion division disappears. 



The first step in the formation of the new spindle is the 

 enlargement of the sphere ; new radiations appear around it 

 (PI. XXV, Fig. 15); the centrosomes diverge, frequently in a 

 radial direction, as has been already mentioned, and a very 

 minute central spindle is seen between them ; the boundary of 

 the sphere still persists. As ivill be shown later, the entire 

 inner- sphere is the product of a single centrosome ; hence in 

 a certain sense " Centrosoma tmd Centralspindel bilden ein 

 Games'' (Heidenhain). 



By elongation of the central spindle the centrosomes are 

 brought in contact with opposite sides of the wall of the 

 sphere, and, pressing against the latter, cause it to elongate 

 (PL XXV, Fig. 16). The central spindle, filling the entire 

 interior of the sphere, stretches between the centrosomes, 

 which have become larger and more subdivided, and stain 

 intensely black in iron-haematoxylin. The fibers of the cen- 

 tral spindle have been formed out of the ground-substance of 

 the sphere, either by direct conversion of its substance or by 

 growth of the few original fibers at the expense of the ground- 

 substance. The radiations which have arisen anew (Fig. 16) 

 are related to the sphere as a whole, and not to the centro- 

 somes as independent centers. 



As the spindle elongates still more, the wall of the sphere 



