No. 2.] ORGANIZATION OF THE EGG OF UNIO. 243 



this point ; for it seems to me unlikely that where the agree- 

 ments are so many such important differences should also 

 exist. 



I have already said that the sphere and centrosomes remain- 

 ing within the Qg% after the first maturation division may dis- 

 appear prior to the formation of the second polar spindle. 

 When this happens, the process of formation of the second 

 maturation spindle is (naturally) somewhat different from that 

 already described. There next follows a stage with a single 

 centrosome, whether one of the old ones or not I cannot 

 say ; this lies just within and partly surrounded by the chro- 

 mosomes, and is first recognizable as the center of a small 

 aster. The regular arrangement of the microsomes on the 

 rays of the latter gives the appearance of a regular sphere, 

 analogous, I believe, rather to the outer than to the inner 

 sphere already described. The centrosome divides in two, 

 and the whole combination of chromosomes and aster moves 

 in towards the center of the egg. A minute central spindle 

 can be seen between the two centrosomes, which develops as 

 the latter move apart. The developing spindle takes up a 

 horizontal position central to the chromosomes and a little 

 above the center of the egg (PI. XXV, Fig. 30). The sphere 

 has dilated and continues to include the entire spindle, although 

 it forms no part of the latter, as in the former method ; and 

 the rays of the aster are related to the sphere and do not pene- 

 trate within it. 



The centrosomes at either end of the spindle now divide 

 repeatedly, forming a group of centrosome granules at either 

 end ; and the chromosomes are gradually drawn around the 

 periphery of the spindle at its center. Asters begin to de- 

 velop at either end, and the whole structure revolves into a 

 radial position (PL XXV, Fig. 31), moving towards the surface 

 as it does so. The centrosomes at both ends then simul- 

 taneously go through exactly the same process of development 

 as in the former method. 



This mode of formation of the second maturation spindle 

 was found in all of the eggs, which were preserved at this 

 stage, of one female ; the modification was undoubtedly due 



