No. 6.] 



THE EGG OF UNIO. 



269 



the inner aster is occupied by an extraordinarily minute centro- 

 some lying in a sphere. In the metaphase the outer aster 

 has exactly the same structure (Fig. ?E}. Later yet the inner 

 sphere of both asters is bounded by a membrane (Figs. *]F and 

 6) formed in precisely the same way as the homologous struc- 

 ture of the first maturation spindle. In this stage the centro- 

 some is much larger than at the metaphase. What, now, has 

 become of the compound centrosomcs of earlier stages, and ivJiat 

 is tJie origin of the concentric spheres? 



Figures jA to F illustrate the changes undergone ; *]A to E 

 are drawn from horizontal sections of the outer aster, and 



FIG. 5. Later Stage of the Second 

 Maturation Spindle in Unio. 



FIG. 6. Telophase of the Second Matura- 

 tion Division in Unio. 



and 6 from radial sections of the inner aster in the metaphase 

 and telophase of the spindle, respectively. The condition seen 

 in Fig. jA has been reached by growth and division of a single 

 small centrosome ; this figure represents a horizontal section 

 through the outer centrosome of the stage of Fig. 5. The 

 next four figures carry us to the beginning of the metaphase. 

 They show two processes taking place : (i) The subdivision of 

 the relatively large centrosome granules and their distribu- 

 tion in the form of a sphere ; and (2) the increase of the red- 

 staining substance in which the granules are imbedded. The 

 peripherally distributed granules become the stratum of micro- 

 somes bounding the inner sphere. One of the granules remains 

 behind as the centrosome of the new inner sphere ; which one 

 of them is, apparently, determined entirely by position. The 

 outer sphere has developed during this process. 



