No. 6.] 



THE EGG OF UNIO. 



267 



In the concluding- pJiascs of the telopJiase, the inner chromo- 

 somes and sphere move towards the surface, drawing out into 

 rays the protoplasm of the degenerating aster. In this stage 

 a new set of fibers radiates in a fan-shaped manner from the 

 sphere to the surface. These radiations then disappear entirely 

 and in place of the two centrosomes but one is found within 

 the inner sphere. In some cases the inner sphere may disap- 

 pear, and then the centrosome cannot be distinguished from 



FIG. 3. Prophase of the Second Maturation 

 Spindle in Unio. Origin of the Central 

 Spindle. 



FIG. 4. Prophase of the Second Matura- 

 tion Spindle in Unio. Second Stage in 

 Origin of the Central Spindle. 



cytomicrosomes. This concludes the telophase of the first 

 maturation division. Without any pause begin 



77/6' PropJiases of the Second Maturation Spindle. The for- 

 mation of the second maturation spindle may take place in 

 either of two ways, in both of which, however, the centrosome 

 phenomena are essentially the same. In the first method 

 (Figs. 3-5) the central spindle is formed at the point at which 

 the centrosome remains after the formation of the first polar 

 globule. The central spindle in this method is generally radial 

 in position, though it may lie at any angle. The second method 

 is preceded by the disappearance of the inner sphere, and the 

 horizontally placed central spindle together with the chromo- 

 somes sink in towards the center of the egg. The spindle later 

 rotates into position. 



