Ookinesis in Cerebratulus Lacteus. 379 



some before the grand-daughter ceutrosonie appears? (Cf. Griffin, 

 '96, Fig. 13.) 



(c) Formation of daughter centrosome by the enlargement of the 

 centriole (Wilson, '95, Conklin, '04, Bonnevie, '06) (Text Fig. B. 

 IV). The centriole enlarges in the centrosome and the two new 

 centrioles are formed de novo within it. The centrosome is, there- 

 fore, a transformed centriole. Xo division of centrosome takes place. 

 This view is characterized by the fact that at the division of the 

 aster the cell passes a stage in which no centriole is present. Wilson 

 expresses a similar view in the case of Toxopneustes ('95, p. 463, 

 '01, p. 584). Boveri suggests this mode as a mere possibility at 

 the formation of the second cleavage aster of Ascaris ('01, pp. 78 

 and 98). Example, CrepiduJa, Cynthia, and Ciena (Conklin, '01, 

 '04, '05), in Enteroxenos (Bonnevie, '06). 



Recapitulating the types : 



(1) Direct division. — The centrosome is bodily divided into two 

 daughter centrosomes (centrosome divides). 



(2) Moulting. 



(a) Division. — A new centrosome formed around the centrioles 

 within the old one is bodily divided into two centrosomes 

 (centrosome divides). 



(6) Separate formation. — The centriole divides in the mother 

 centrosome and around the products the daughter centro- 

 somes appear separately (centrosome does not divide). 



(3) Enlargement of the centriole. — The centriole grows and 

 becomes the centrosome. Two new centrioles appear in it, and en- 

 large into centrosomes (centrosome does not divide). 



3. Cycle of Centrosome. 



As a summary ()f this section the cycle of the centrosome in 

 Cerebratulus lacteus from the sperm-centre as far as the second cleav- 

 age center will be given. The centrosome I, i. e., that of the first gen- 

 eration, makes its ap])earance soon after the throwing-off of the middle 

 piece vesicle. The centriole tlien divides into two, Ceiitrosome I 

 fades away, and new ray systems are formed around the two 



