No. 3.] THALASSEMA AND ZIRPHAEA. 585 



In accordance with the above, the problem to be first attacked 

 was that of the centrosome, — its morphology, its physiological 

 significance, and its behavior towards fixing agents ; on all of 

 which points the accounts of previous investigators are, in 

 a greater or less degree, conflicting. It was especially hoped 

 that some positive evidence might be obtained respecting the 

 continuity of the centrosome, a question which has of late 

 seriously engaged the attention of cytologists. In opposition 

 to the earlier view of van Beneden and Boveri, a considerable 

 number of later authors {e.g., Burger, '92 ; Watase, '94 and '97 ; 

 Farmer, '96 ; Foot, '97 ; Carnoy, '97 ; and numerous botanical 

 investigators) regarded the centrosome as a more or less tran- 

 sient structure, arising by the modification of preexisting cyto- 

 plasmic elements {e.g., microsomes, Watas6, '97), persisting for 

 a shorter or longer period, and being dissolved and reformed 

 according to the varying conditions of mitotic activity — the 

 expression rather than the cause of the aster-formation (Foot, 

 '97). On this point my results are, I think, clear and convin- 

 cing as far as they go, though they certainly do not bring the 

 question to a conclusion. In both Thalassema and Zirphaea 

 the centrosome (sperm-centrosome in Thalassema) can be 

 traced uninterruptedly from the first appearance of the aster 

 throughout all intermediate stages into the cleavages, and 

 at no time disappears from view. My results here agree 

 entirely with those of van Beneden and Boveri [Ascaris), Mead 

 {Chaetopterus), Wheeler {Myzostoma), Kostanecki and Wier- 

 zejski {P/iysa), and others. Neither in Thalassema nor in 

 Zirphaea could the centrosomes be demonstrated during the 

 growth period of the ovum, but it must be borne in mind 

 that in the absence of rays or characteristic envelopes, a 

 granule so minute as the centrosome would be indistinguish- 

 able from numerous cytoplasmic and yolk granules which fill 

 the egg. 



A third and no less important question touches the deriva- 

 tion of the cleavage-centrosomes, on which point my observa- 

 tions in the case of Thalassema have, I think, no room for 

 doubt, while in the case of Zirphaea the facts are too equiv- 

 ocal to justify a positive conclusion. 



