376 Naohide Yatsu. 



aster enlarges the centrosome comes into view for the first time, and 

 the mother rays gradnally fade away as the daughter rays grow 

 stronger. When the old rays have entirely disappeared, the moulting 

 of the aster is completed. 



From the above observation on the formation of asters for the 

 second cleavage of C. lacteus it will be seen that : 

 (a) the centrosome does not divide; it being formed separately from 



the beginning. 

 (6) in no stages does a reduction of the centrosome take place, 

 (c) the centriole retains its identity throughout the whole process; 



neither disa]^])earance nor enlargement of the centriole takes 



place. 



2. Types of Centrosome Division. 



Three different views have been held regarding the formation of 

 daughter centrosomes from a preceding one, if we except the direct 

 division of the centrosome, which takes place in some small 

 cells, e. g., the s])ermatocytes and cleavage cells of Ascaris. As we 

 shall see later, none of the views expresses the general mode of the 

 division, but there are actually three different types (Meves, '99, p. 

 499 et seq.). 



A. Formation of daughter centrosomes by reduction (Boveri, 

 '00). It is quite nniversal, Boveri believes, that the centrosome 

 first enlarges (Text Fig. B. II cf. his Text Figs. pp. 102-103), 

 and in it the centriole divides (this may happen long before). Sud- 

 denly the centrosome differentiates into two parts, a central 

 "active" core and an outer disintegrating part. This process he calls 

 a reduction of the centrosome ('00, pp. 97, 101). The reduced cen- 

 trosome now divides into two. Here the division of centrosome 

 actually takes place. The term "reduction" is, I think, not well 

 chosen, since it does not express wdiat really happens. The grown 

 centrosome does not all of a sudden separate into two parts, as 

 Boveri thinks, but thicker centroplasm flows towards, and collects 

 itself around, the centriole, and thus slowly the centrosome of the 

 next genoraticm is foruied within the old. Example, formation of 

 the second cleavage centrosomes in Cerehratvhis marginatus (Coi>, 



