•56 KARYOKINESIS. 



its substance, and the remainder of the latter passes back into the sphere and ulti- 

 mately into the cytoplasm. It is evident from this description that, as in the case 

 of the nucleus, so also in the centrosome there is a sort of diastole and systole, the 

 phases of the one alternating with those of the other. 



(d.) Comparisons. — I have found centrosomes, similar in all respects to these 

 just described, in three species of Crepidula, and in Urosalpinx, Jllyonassa, Fulgur, 

 Sycotypus, and Aeolis. In structure and history the centrosomes in all these gastero- 

 pods are similar to those which have been observed in Diaulula (MacFarland, '97), 

 Unto (Lillie, '98), Thysanozoon (Van der Stricht, '98), Rhynchelmis (Vejdovsky, 

 '88, and Vejdovsky and Mrazek, '98), Actinosphaeriimi (R. Hertwig, '99), and 

 Haminea (Smallwood, '01), while they bear many resemblances to those which have 

 been observed in Echinus (Boveri, '01), Ascaris (van Beneden, '87, Boveri, '88, '01, 

 Brauer, '93, Furst, '98), Sida (Hacker, '93), Salamandra (Rawitz, '96, Niessing, 

 '99), Salmo (His., '98), Fulgur (McMurrich, '96), Limax (Byrnes, '96 and '99, Lin- 

 ville, 1900). 



Van der Stricht's interpretation of the relations of the centrosome to the 

 attraction sphere in Thysanozoon seems to me most satisfactory, not only because 

 he has for it the approval of Van Beneden and Boveri, but also because by it the 

 various forms of centrosomes present in the animals named above and particularly 

 the remarkable centrosomes of mollusca can be satisfactorily related to one another 

 and to other forms. We need not here concern ourselves with the origin of the 

 centrosomes of the first maturation ; Van der Stricht believes that they arise from 

 the nucleus, and this view is supported and extended by the recent observations of 

 Schockaert (1900). Soon after its appearance the centrosome of Thysanozoon is 

 differentiated into a central corpuscle and a medullar}-' zone (couche niedullaire). 

 These two together constitute the centrosome of Boveri, the central corpuscle being 

 his centriole. The medullary zone is homogeneous in structure, and no astral fibres 

 penetrate it except at the time of origin of the new spindle figure ; it is usually 

 bounded peripherally by a dark line (in reality a sphere). 1 Around this is a clear 

 area traversed at all stages by delicate astral fibres ; this is the cortical zone [couche 

 corticale), and it is possible that it is derived, in part, from the centrosome. Around 

 this is the zone of astral rays, which sometimes may be subdivided into an inner 

 dark and a peripheral clear zone. The centrosome and cortical zone continually 

 enlarge, as the division advances, until they reach a great size. In the division of 

 this centre, the central corpuscle first divides, usually into two, and the central 

 spindle appears between the halves ; the medullary zone then becomes bounded by 

 granules, and this boundary gradually fades from view, though Van der Stricht 

 believes that the entire attraction sphere persists and divides, thus giving rise to 



1 The clear zone which is so generally found around the central corpuscle is believed by many 

 observers to be the result of destaining. Such a zone is produced by destaining yolk-spheres, nucleoli, 

 etc. Fischer ('99) calls this " Spiegelfiirbung." It will be observed that the manner in which these 

 centrosomes stain, which have a dense periphery and clear central area, is the exact reverse of "Spie- 

 gelfiirbung." 



