i\o. 4.] BULLA SOLITARIA. 147 



trace them nearly to the egg membrane. From now on, 

 there does not appear to be any appreciable change in the 

 mitotic figure, until after the egg has been laid, when it begins 

 to migrate to the periphery of the egg, where it assumes a 

 radial position. There is no difference in the character of the 

 two poles of the spindle. During this movement of the spin- 

 dle the chromosomes pass into the metaphase, and the cen- 

 trosome becomes differentiated into a central corpuscle and 

 a medullary zone which is limited by the walls of the old 

 central corpuscle. This is the first time that we have a struc- 

 ture to which we can apply the term "centrosome" in the sense 

 that I purpose to use the term. The cortical zone has enlarged 

 and become much fainter. The chromosomes do not divide at 

 once ; the activity is centered in the centrosomes. While the 

 chromosomes are still in the equatorial plate, the central cor- 

 puscle in each centrosome divides, having the dumbbell form. 

 The centrosome rapidly increases in size, the periphery is 

 limited by a definite line, which gradually becomes thinner. 

 The medullary zone does not take a plasma stain, as it did in 

 the previous stage (Fig. 9). The centrosome now begins to 

 assume an elliptical form and at the same time to rotate. 

 This rotation continues until the elongated centrosome, which 

 encloses the second polar spindle, lies radially and in the same 

 position that the first polar spindle did. The central cor- 

 puscles, connected by a central spindle, are so influenced by 

 this elongation of the walls of the centrosome that they come 

 to lie near the ends --at the foci of the ellipse. As the outer 

 pole of the second polar spindle nears the periphery of the 

 egg, the rays extend to the chromosomes, and they are pulled 

 into the spindle to form the equatorial plate of the second 

 polar spindle. It will be seen that in the main my results 

 corroborate those of MacFarland, 1 Lillie, 2 and Conklin. 3 



The changes which take place in the centrosome during this 

 stage are very interesting. The centrosome is so large and 

 comes out with such perfect clearness that I have been able to 



1 " Cellulare Studien an Mollusken-Eiern," /.ool.Jahrb. 1897. 



- " Centrosome and Sphere in the Egg of Unio," Zool. Bull. Vol. i, No. 6. 1898. 



3 Science, March, 1898. 



