No. 6.] THE EGG OF UNIO. 273 



ceraeus show a similar vesicular inner sphere ("centrosome" K.). 

 There are generally figured within the sphere a number of 

 granules ; but in regard to this point one feels that celloidin 

 sections 15/11 thick and stained in borax carmine are inadequate 

 evidence. However, in some eggs in an earlier stage he speaks 

 of a minute "centriole" within the "centrosome." 



To the criticism that the centrosome phases shown in Fig. 

 ?A-F are pathological, i.e., due to imperfect extraction of the 

 haematoxylin or other action of the reagents, it may be re- 

 plied : 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 centrosome will be found ; 

 third, that, inasmuch as the inner aster develops much more 

 rapidly than the outer, the inner centrosome passes through 

 these phases much more rapidly than the outer. Thus one often 

 finds a spindle in which the inner centrosome has already 

 reached the condition of Fig. jE, while the outer is in the 

 stage of 7 'A (v. Fig. 5). Now, jE is a perfectly normal and 

 typical centrosome ; hence, if one uses the pathological argu- 

 ment, one must be prepared to assert that one end of a 

 spindle may be pathological and the other normal in tJic same 

 section. 



Those who oppose the centrosome theory are often met with 

 the reply, " one positive observation is worth a great many 

 negative ones " ; the implication being that one observation in 

 favor of the permanency of the centrosome is worth a great 

 many against it. I think that we are all convinced of the diffi- 

 culty of proving a negative proposition. Nevertheless, it is a 

 confession of weakness to dismiss all observations incompatible 

 with a theory with the above caption. The observations against 

 the permanency of the centrosome as a unique organ of the 

 cell are now so many that they demand attention. One has a 

 right to ask in detail how the theory is to be upheld against 

 the foregoing observations and against others, such as Mead's, 1 

 that the two centers of the first maturation spindle are selected, 



1 Mead, A. D.,"The Origin of the Egg Centrosomes, " Jeitrn. of Morph., vol. 

 xii, No. 2, 1897. 



