242 LILLIE. [Vol. XVII. 



the inner sphere has not necessarily a definite morphological 

 value, in the sense that it is always, probably, a product of 

 growth of the centrosome. But it seems to me probable that 

 the latter is of purely functional significance. 



"To the criticism that the centrosome phases shown in 

 Figs. 24-27, PI. XXV, may be pathological, i.e., due to imperfect 

 extraction of the haematoxylin or other action of the reagents, 

 it may be replied : first, that they are found with different 

 killing fluids ; second, that the changes are perfectly uniform 

 in all cases, so that, knowing the stage of development of the 

 spindle, one can be certain that a definite stage of the centro- 

 some will be found ; third, that inasmuch as the inner aster 

 develops much more rapidly than the outer, the inner cen- 

 trosome passes through these phases more rapidly. Thus 

 one often finds a spindle, e.g.. Figs. 18 and 19, PI. XXV, in 

 which the inner centrosome has already passed through the 

 entire metamorphosis, while the outer is still in the stage of 

 Fig. 24 or 25, PI. XXV." Thus one has a typical centro- 

 some at one end and the supposedly pathological centrosome 

 at the other end of the same spindle ! 



In regard to the formation of the entire central spindle of 

 the second maturation division from the inner sphere (" cen- 

 trosome," MacF.) my results are in complete accord with those 

 of MacFarland on Diaullula ('97). But concerning the origin 

 of the inner sphere ("centrosome," MacF.) of the second 

 maturation spindle I believe my results to be more trust- 

 worthy than those of MacFarland. This author describes the 

 inner sphere as arising around the minute " Centralkorn " by a 

 condensation of the substance of the central spindle at either 

 end. I have sometimes found appearances similar to Mac- 

 Farland's figures in sublimate-acetic preparations which give 

 a poor fixation of the eggs of Unio ; but when the fixation is 

 good I have always found the centrosomes standing out with 

 the greatest clearness, and their differentiation into the inner 

 sphere with the contained centrosome could be plainly traced. 

 The lack of detail in MacFarland's figures of the cytoplasm 

 gives the impression that the fixation of his materials was not 

 of the best, which might explain our differences in regard to 



