326 



SOME PROBLEMS OF CELL-ORGANIZATION 



matocyte-divisions of the salamander. In leucocytes (Fig. 49) the 

 large persistent aster has at its centre a well-marked radial sphere 

 bounded by a circle of microsomes, as described by Van Beneden, but 

 without division into cortical and medullary zones. The astral rays, 



however, show indications of other 

 circles of microsomes lying outside 

 the centrosphere. Druner found 

 that a whole series of such concen- 

 tric circles might exist (in the cell 

 shown in Fig. 1 56 no less than nine), 

 but that the innermost two are often 

 especially distinct, so as to mark off 

 a centrosphere composed of a medul- 

 lary and a cortical zone precisely as 

 described by Van Beneden. These 

 observations show conclusively that 

 the centrosphere of the radial type 

 is merely the innermost portion of 

 the aster, which acquires a boundary 

 through the especial development 

 of a ring of microsomes, or other- 

 wise, and which often further 

 acquires an intense staining-capac- 

 ity so as to appear like a centrosome 

 (p. 313). In TJiysanozoon(V?c[\ der 

 Stricht) only a single ring of micro- 

 somes exists, and this lies at the 

 boundary between the medullary 

 and cortical zones (Fig. 152, D\ the latter differing from the outer 

 region only in the greater delicacy of the rays and their lack of 

 staining-capacity, thus producing a " Heller Hof." In other cases, no 

 " microsome-circles " exist; but even here a clear zone often surrounds 

 the centrosome (e.g. in Physa, t. Kostanecki and Wierzejski), like that 

 seen in the cortical zone of Thysanozoon. 



There are some observations indicating that the entosphere (medul- 

 lary zone) may be directly derived from the centrosome (central 

 granule). This is the conclusion reached by Lillie in the case of Unio 

 referred to above, where, during the prophases of the second polar 

 spindle, the central granule enlarges and breaks up into a group of 

 granules from which the new entosphere is formed. Van der Stricht 

 ('98) reaches a similar conclusion in case of the first polar spindle of 

 Thysanozoon. We may perhaps give the same interpretation to the 

 large pluricorpuscular centrum of echinoderms (p. 314). This obser- 

 vation may be used in support of the probability that the astral rays 



Fig. 156. Spermatogonium of salaman- 

 der. [DRUNER.] 



The nucleus lies below. Above is the enor- 

 mous aster, the centrosome at its centre, its 

 rays showing indications of nine concentric 

 circles of microsomes. The area within the 

 second circle probably represents the " attrac- 

 tion-sphere " of Van Beneden. 



