KARYOKINESIS. 57 



the daughter attraction spheres. The relations of the parts of the centrosome, the 



attraction sphere and the aster of Thysanosoon may be indicated as follows : — 



n , f Central Corpuscle 



Centrosome <,,.,.. „ , 



I Medullary Zone \ r . , , /. „ , 

 _. ,. _ " > ol Attraction bphere. 



Cortical Zone I 



Inner Zone I » . , 



Peripheral Zone j 



The most important points in which my observations differ from those of Van 



der Stricht are the following : — 



1. The peripheral boundary of the centrosome (medullary zone) is much denser 

 and more deeply staining than in Tkysanozoon. 



2. During the rest stages in the cleavage the central corpuscle is represented 

 by an enormous number of granules, only two of which form the new centrosomes. 



3. Neither the medullary nor the cortical zones of the attraction sphere ever 

 divide as a whole, but after the origin of the new amphiaster they are slowly dis- 

 solved in the cytoplasm. 



4. The central corpuscle of one generation gives rise to the entire centrosome, 

 i. e., central corpuscle and medullary zone, of the next. 1 This is most plainly seen 

 in the anaphase of the first maturation. 



5. At no time do the astral rays traverse the medullary zone, 2 though new 

 rays which are not part of the old system may arise within that zone around the 

 new centers. 



In Diaulula, according to MacFarland, the centrosome increases greatly in 

 size from the prophase to the anaphase and a single granule appears within it. This 

 granule soon divides into two which move apart and become the new centrosomes. 

 The whole of the old centrosome is transformed into the new centrosomes and cen- 

 tral spindle. The rays are inserted on the centrosome, not on the central granule. 

 Even after the new spindle figure has reached a considerable size the rays continue 

 to be centered on the figure as a whole. 



My observations differ from MacFarland's only in one respect: The whole of 

 the old centrosome is not transformed into the new spindle figure, but the latter 

 arises within the old centrosome. This is plainly true of the maturation stages, 

 corresponding to those which MacFarland has studied, and in the first three cleav- 

 ages, but the case is not so clear in the later cleavages, as a glance at my figures 

 74—76 will show. 



According to Lillie ('98) each centrosome in the prophase of the second matur- 



1 I am not quite certain whether this may not be involved in Van der Stricht's statement that the 

 centrosome becomes differentiated into a central corpuscle and medullary zone. The fact, however, 

 that he maintains a persistence of the attraction sphere leads me to suppose that lie regards each me- 

 dullary zone as derived from a preexisting one. 



2 In this important respect my observations agree entirely with those of Boveri and his pupils, 

 MacFarland and Fiirst, and differ from those of Van Beneden. Indeed, it may be doubted whether 

 the term "medullary zone" should be applied to a structure which shows no radiations. Other con- 

 siderations, however, render it extremely probable that the peripheral layer of the centrosome in gastero- 

 pods and the " medullary zone " of Thysanozoon are homologous. 



8 JOUEN. A. N. S. PHILA., VOL. XII. 



