KARYOKINESIS. 



15 



d 



clear. Within the central area a faintly staining body becomes visible, which, in its 

 \\/ turn, becomes a hollow sphere (text 



fig. Ill, e and/) ; this is the "corpus- 

 cle central" of Van Beneden ('87), 

 or the "centriole" of Boveri ('95). 1 



Up to this stage the centrosomes 

 at the two poles of the maturation 

 spindle are identical in size and struc- 

 ture. When, however, the outer 

 pole of the spindle comes into contact 

 with the egg membrane, the sphere 

 and centrosome at this pole become 

 flattened, figs. 14-16, though the cen- 

 trosome still shows its dark periph- 

 ery, its central clear area and its cen- 

 tral corpuscle. 2 In the late anaphase 

 the outlines of the centrosome at the 

 outer pole are marked by a layer of 

 granules, while within the central 

 clear area is the elongated central 

 corpuscle, fig. 16<2. Finally, after the 

 complete separation of the first polar 

 body, the granular outlines of this 

 centrosome disappear, though the 

 central corpuscle, or rather the cen- 

 trosomes and central spindle to 

 which it gives rise, are still found 

 within it (Plate II, figs. 22, 28a, 29, g 

 34, 36). 



The centrosome at the inner 

 pole of the spindle continues to en- 

 large until it reaches a truly aston- 

 ishing size, becoming fully 4 (z in 

 diameter. Its peripheral layer is at 

 first a solid zone of deeply staining 

 material. In later stages this zone 

 breaks up into plates, figs. 16-25 

 and text fig. Ill, and still later it ap- 

 pears as a ring of close set granules. 



« 



/ 4 



,<,/. 



f*\ 



o m 



h 



Fig. III.— Centrosomal Cycle i 

 the Maturation of Crepiduls 



Fig. IV. Centrosomal Cycle 

 the Cleavage of Crepidula, 



1 Boveri ('01) denies the general homology of his "centriole" with the "corpuscle central" of 

 Van Beneden. There can be no doubt that the structure in question in Crepidula is homologous with 

 Boveri's "centriole." 



2 Van der Stricht ('98) has observed a similar flattening of the sphere and centrosome against 

 the egg membrane in the first maturation of Thysanozoon. 



