16 KARYOKINESIS. 



The central corpuscle, which is shown in figs. 13 and 14 as a faintly staining, 

 hollow sphere, soon becomes elliptical in shape, fig. 15. At the poles of this ellipse 

 its walls grow thicker and stain deeply. These thickened points become the centro- 

 somes of the second maturation spindle, while the remainder of the ellipse forms 

 the central spindle 1 (cf. figs. 14-16<z, and text fig. Ill, g, k). The daughter centro- 

 somes and central spindle lie within the mother centrosome ; the outlines of the 

 mother centrosome then disappear and the new amphiaster lies free in the granu- 

 ular remains of the sphere. 



During this metamorphosis the centrosome undergoes great changes in its stain- 

 ing qualities ; in the prophase and rnetaphase it stains deeply with hematoxylin ; as 

 it enlarges, however, the peripheral portion only takes liEematoxylin, while the cen- 

 tral part takes plasma stains ; finally, in the late anaphase, even the peripheral por- 

 tion takes plasma stains. 



At no time during this metamorphosis do the astral radiations penetrate the 

 centrosome. As long as they can be seen they remain attached to its surface, and 

 even after the new amphiaster has arisen within the mother centrosome, the rays 

 are still centered on the figure as a whole, figs. 25-28, and text fig. Ill, g and h. 

 The new radiations Avhich appear around the daughter centrosomes develop de 

 novo, as MacFarland ('97) and Griffin ('99) maintain. 



Up to the time when the second polar body is cut off, the history of the centro- 

 somes during the second maturation is similar to that already described for the first; 

 at the beginning of division they are minute granules, as the division advances they 

 become larger, and in the anaphase are large hollow spheres. 



After the second polar body has been formed, however, the centrosome which 

 remains in the egg becomes a very large sphere filled with many coarse granules and 

 with a boundary layer of close-set granules, from which, in some cases, polar fibres 

 proceed, figs. 34-36. I have never seen a peculiarly large granule which might be 

 . regarded as a centriole within this centrosome, nor have I seen the formation of a 

 central spindle as at the close of the first maturation. On the other hand, the for- 

 mation of a large number of granules within the centrosome is a phenomenon 

 which occurs in the telophase of every cleavage (text fig. IV), and seems, there- 

 fore, to be the more usual process. It seems probable, therefore, that the par- 

 ticular manner in which the daughter centrosomes and central spindle arise within 

 the mother centrosome at the close of the first maturation, is a modification of the 

 more typical process shown in the cleavage, clue, perhaps, to the entire omission of 

 a resting stage between the two maturation divisions. In the maturation, there- 



1 In view of the remarkable resemblance of this stage to a corresponding stage in the division of the 

 " centrosome " in Diaulula (see MacFarland's figs. 36, 37, et seq.) it may be supposed that the vesicular 

 body which I have called the "central corpuscle" is really the centrosome and that the surrounding 

 body is only the inner zone of the sphere. Fortunately, however, the outlines of the centrosome are so 

 perfectly distinct and its history, as shown in my preparations, so continuous, that there can be no 

 question as to its identity in this case. The outlines of the old centrosome remain until after the central 

 corpuscle has given rise to a perfect spindle within it ; so that in Crepidula, and several other gastero- 

 pods which I have studied, the new centers and central spindle arise from the central corpuscle and not 

 from the entire centrosome as in Diaulula. 



