No. 2.] ORGANIZATION OF THE EGG OF UNIO. 237 



in appearance, staining uniformly in Bordeaux red, which is 

 traversed by fine radiations centering in the centrosome. A 

 short distance from the centrosome, and at the periphery of 

 the homogeneous substance surrounding it, each ray possesses 

 a distinct microsome ; these microsomes together bound a 

 definite body, which I have called tJie injier sphere, and the 

 next row of microsomes on the rays bounds a fairly definite 

 outer sphere. Beyond this the radiations pass out into the 

 general cytoplasm for a considerable distance. From the cen- 

 trosome to the outer sphere each ray is a thread or fiber, but 

 beyond the spheres the rays can plainly be seen to be united 

 by more or less regular anastomoses ; and farther out the branch- 

 ing of the rays and the increase in number of the anastomoses 

 cause the rays, as such, to disappear in the general cytoplasmic 

 network. The whole appearance of the aster is such as to 

 give the impression that it has arisen by a central strain in the 

 cytoplasmic network ; and the history of the origin of the 

 aster in this and other places in this Qg% is such as to make 

 this conclusion a practical certainty. I am inclined to believe 

 that the cytoplasmic groundwork in this Qgg is primarily an 

 alveolar structure, which may become reticular or filar (as in 

 the center of the aster) by the breaking down of alveolar walls. 

 In this opinion I agree with Wilson's conclusions in his recent 

 work (1900) on the structure of protoplasm. 



The existence of double rays is a characteristic appearance 

 in this stage of karyokinesis (PI. XXIV, Fig. 10, and others). 

 However, I do not interpret these as due to " splitting" of rays 

 (Kostanecki, '97), but to approximation of independent rays. 

 One would naturally expect this to occur if the aster arises in 

 the manner indicated. 



During the anaphase the centrosome divides into two at each 

 end of the spindle (PI. XXIV, Fig. 11); and, as the proto- 

 plasm begins to protrude for the formation of the first polar 

 body, each one of these four centrosomes becomes double or 

 quadruple (PI. XXIV, Fig. 12). Another process, which has 

 been going on during the anaphase, is the peripheral accumu- 

 lation of the ground-substance of the inner sphere and the 

 disappearance in this substance of the bounding row of micro- 



