smallwood: maturation of haminea solitaria. 295 



somes, or bodies to which the term " centrosome " is applicable : Mark 

 ('81) ill Limax, Boveri ('90) in Heteropods, McMurrich ('96) in Fulger, 

 MacFarlaud ('97) in Diaulula, Murray ('98) in Pulrnonates, Couklin 

 ('98, :01) in Crepidula, Byrne.s ('99) in Limax, Linville (:00) in Limax 

 and Linuea, and Lillie (:01) in Unio. The centrosome in Physa 

 (Kostanecki und Wierzejski, '96) is probably the same as in other 

 Mollusca. 



The fact that the centrosome may pass through a cycle of changes has 

 been recognized only recently. If the various observers cited had all 

 had this possibility in mind when carrying on their studies, it is probable 

 that there would have been more agreement in their results. Large 

 centrosomes have been observed in other phyla by the following 

 workers : Korschelt ('95) in Ophryotrocha, Wilcox ('95) in Caloptenus, 

 Kliuckonstrom ('97) in Prosthecerpeus, Van der Stricht ('98) in 

 Thysauozoon, Van Name ('99) in Plauarians, Gathy (:00) in Annelids, 

 and Boveri (:0l) in Ascaris and Echinus. Wilson (:00), in summariz- 

 ing the results of various investigators, is inclined to favor the restricted 

 use of the word "centrosome," limiting it to the body termed centriole in 

 this paper. On page 314, he says : " By following Boveri's terminology, 

 therefore, MacFarland is driven to the strange conclusion that the second 

 polar spindle is nothing other than an enormously enlarged * centro- 

 some,' — a result little short of a redudio ad absurdum when we consider 

 that in Ascaris the polar spindle arises by a direct transformation of 

 the germinal vesicle." Notwithstanding this criticism, I believe that 

 the term " centrosome " as defined by Boveri should be accepted ; and the 

 absurdity disappears when we recognize the growth and metamorphosis 

 which this body exhibits. 



In a previous part of this paper the progressive changes in 

 the growth of the centrosome in Haminea have been mentioned, 

 and the manner in which the centroplasm was probably formed 

 has been described. We may now discuss the derivation of the cen- 

 trosomes of the second mitotic figure from the deep centrosome of the 

 first maturation figure. MacFarland believes that, in the formation 

 of the second polar spindle (Diaulula), the centrosome divides and that 

 the new centrosome results from a new arrangement of the parts. The 

 inner sphere (the centroplasm in Haminea) in his opinion is formed by 

 a process of condensation around the " Centralkorn " (centriole) and per- 

 sists in the new centrosome. Boveri (:0l), on the contrary, maintains 

 that the whole centrosome divides, its division being initiated by the 

 division of the centrioles. He further believes that all centrosomes may 



