No. 2.] ORGANIZATION OF THE EGG OF UNIO. 24 1 



teristic of the metaphase. These figures represent tangential 

 sections of the outer aster and centrosome in stages corre- 

 sponding to those of the figures immediately above them, i.e., 

 to Figs. 17-20. To quote from my earlier paper on "Cen- 

 trosome and Sphere in the Egg of Unio " ('98) : " These 

 figures show two processes taking place : (i) the subdivision of 

 the relatively large centrosome granules and their distribution 

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

 substance in which the granules are imbedded. The peripher- 

 ally distributed granules become the stratum of microsomes 

 bounding the inner sphere. One of the granules remains 

 behind as the ' centrosome ' of the newer sphere ; which one 

 is, apparently, determined entirely by position. The outer 

 sphere has developed during this process. 



" The black granules in the inner sphere of Fig. 27 are 

 plainly much less in bulk than those of Fig. 24. There is no 

 doubt that a large part of the centrosome granules has been 

 changed into the red-staining substance of the sphere, which is 

 identical in all noticeable respects with the substance from 

 which the central spindle was formed. In a later stage the 

 fibers of the latter are dotted with large, deeply staining micro- 

 somes." Here is evidence of the lack of persistence of the 

 microsomes as definite morphological elements. 



" From this description it would seem to follow that the 

 centrosome of one cell generation becomes the inner sphere 

 of the next ; and this is undoubtedly true at times. But I 

 do not believe that the inner sphere has necessarily any such 

 definite morphological value as this would seem to imply. 

 For it may disappear between the first and second maturation 

 divisions, and is then re-formed as the first step in the pro- 

 phase of the second maturation spindle from the cytoplasm. 

 The same method of formation may also be observed in other 

 places {e.g., formation of the first cleavage spindle). 



" Both Van Beneden's and Boveri's conceptions of this 

 structure appear as phases in the history of the mitosis, 

 though Boveri's 'centrosome' is really inner sphere, and his 

 ' Centralkorn ' or ' centriole ' really the centrosome." 



/ am 710W, however, inclined to modify the qualification that 



