314 SOME PROBLEMS OF CELL-ORGANIZATION 



persist. The inner sphere is now bounded by a definite membrane, 

 and its radiate structure becomes obscure, the astral rays extending 

 only to the boundary of the sphere, though a few rays persist within 

 it (Fig. 154, B). It is clear from this that the inner sphere and 

 central granule pass through phases that bridge the gap between 

 Van Beneden's and Boveri's descriptions. Lillie's observations fully 

 sustain the conclusion that the cent ml granule i^' centriole'' of Boveri) 

 corresponds to the '' corpnscnle central" of Van Beneden, ajid the inner 

 sphere {mednllary zone) to Boveri's '' centrosomeT A comparison of 

 the polar aster of Unio with that of Thysanozoon, as described by 

 Van der Stricht ('98), leaves hardly room for doubt that the cortical 

 zone represents Boveri's " Heller Hof " ; for in both forms the rays 

 of the cortical zone are much thinner and hghter than the more 

 peripheral portions, thus giving a clear zone, which in Unio is bounded 

 by only a fairly definite microsome-circle and in Thysanozoon by none. 

 Lastly, we must recognize the justice of the view urged by Kos- 

 tanecki, Grififin, Mead, Lillie, Coe, and others, that the term centra- 

 some should be applied to the central granule and not to the sphere 

 surrounding it (medullary zone), despite the fact that historically the 

 word was first applied by Boveri to the latter structure. For in both 

 Dianlula (MacFarland) and Unio (Lillie) the second polar spindle 

 arises from the substance of the inner sphere, while the central 

 granule, becoming double, gives rise to the centrosomes at its poles. 

 By following Boveri's terminology, therefore, MacFarland is driven to 

 the strange conclusion that the second polar spindle is nothing other 

 than an enormously enlarged " centrosome " — a result little short of 

 a redjictio ad absurdnin when we consider that in Ascaris the polar 

 spindle arises by a direct transformation of the germinal vesicle 

 (p. 277). The obvious interpretation is that the central granule is 

 the only structure that should be called a centrosome, the surround- 

 ing sphere being a part of the aster, or rather of the attraction-sphere. 

 Thus regarded, the origin of the spindle in Dianlula presents nothing 

 anomalous and a similar interpretation may be placed on the polar 

 spindles of Ascaris as described by Fiirst ('98).^ 



1 In echinoderms the concurrent results of Reinke ('95), Boveri ('95), myself ('96-'97), 

 show that the " centrosome " is a well-defined sphere containing a large group (ten to twenty) 

 of irregularly scattered, deeply staining granules. I have shown in this case that in the early 

 prophases there is but one such granule, which then becomes double and finally multiple, 

 forming a pluricorpuscular centrum (Fig. 52) not unlike that described by Heidenhain 

 in giant-cells. Kostanecki, who asserts that the centrosome of echinoderms is a single 

 granule ('96, i, '96, 2, p. 248), has not sufficiently studied the later phases of mitosis. 

 Cf. also Erlanger ('98). The centrosomes described in nerve cells by Lenhossek ('95) are 

 apparently of somewhat similar type. Until the facts are more fully known the exact nature 

 of these "centrosomes" remains an open question. Lillie's observations on Unio show 

 that here, too (first polar spindle), the centrosome divides to form a considerable number of 



