104 BULLETIN : MUSEUxM OF COMPARATIVE ZOOLOGY. 



That the structures often act as a unit during the stages of active 

 division cannot be disputed, but neither can it be denied that at other 

 times ceutrosome and archoplasm are entirely distinct structures. Dur- 

 ing the division of the chromosomes and during cytoplasmic cleavage 

 in Scolopendra the ceutrosome and aster do indeed appear to act as a 

 unit, but that their origin is quite different is shown by a study of the 

 prophase stages, when it is seen that the ceutrosphere does not arise 

 by the enlargement of the centrosome, a new ceutrosome being formed 

 endogenously, but that the sphere and aster arise by the transformation 

 of the already existing archoplasmic masses. Thus, while aster, ceutro- 

 sphere and centrosome are a unit in one sense (in their behavior during 

 active mitosis), the aster is not formed by the growth or direct trans- 

 formation of the centrosome as described by MacFarland, although it 

 may have its origin from the substance of the ceutrosphere (Lillie). 

 But in this case the centrosphere is not of direct centrosomic origin, 

 but is obtained from the previously existing archoplasm or by the differ- 

 entiation of the cytoplasm. 



From these considerations relative to the morphology of the centro- 

 some, I believe the conclusion may be drawn that there are possibly 

 three, but probably only two, general classes of structures which form 

 the centre of the astral systems : that described by Wilson (Toxopneustes) 

 and Conklin (Crepidula), and those reported by Boveri (Ascaris), Lillie 

 (Unio), Mead (Chaetopterus), etc. I am convinced that no really 

 essential difference exists between the centres of the asters as described 

 by Boveri and MacFarland on the one hand and those reported by Mead, 

 Lillie, and Linville, and observed by myself in Scolopendra on the other. 

 It is evident that the discrepancies which appear to exist are due largely 

 to differences of interpretation. 



The Centrosome as a Cell Organ. — The question as to whether the 

 centrosome shall be considered a cell organ or not seemed at one time 

 very likely to be answered in the affirmative ; but it is now in a very 

 unsettled condition. While many observations have been accumulated 

 which prove beyond dispute that in some cells at least, and possibly in 

 a majority, the centrosome is one of the permanent morphological ele- 

 ments of the cell, and therefore should be considered an organ of per- 

 haps equal rank with the nucleus, in other cells this is apparently not the 

 case. In these at certain stages no structure can be found which answers 

 to the requirements of a centrosome. This has led many to the belief 

 that the centrosome is not a true organ of the cell, but merely a transitory 

 appearance caused by the activities of other parts of the cell. In con- 



