THE CENTROSOME 



313 



earlier stages the aster has exactly the structure described by Van 

 Beneden in Ascaris, except that the innermost body {i.e. the " cor- 

 puscule central") is a single minute granule. This is surrounded 

 by typical medullary and cortical zones, through both of which the 



B 





C D 



Fig. 154. — Centrosome and aster in the polar mitoses of Unio. [Lillie.] 



A. Aster of the first polar figure; central granule (centrosome) surrounded by medullary 

 (entosphere) an^cortical (ectosphere) zones. B. Late anaphase of second polar mitosis; radial 

 entosphere bounded by continuous membrane. C. D. Prophases of second mitosis ; formation 

 of central spindle within and from the substance of the old entosphere. 



rays pass (Fig. 152, E, Fig. 154). The inner sphere, consisting of 

 a dense and deeply staining substance, has at first a typical radiate 

 structure and is bounded by a microsome-circle. In later stages (late 

 anaphase) the central granule divides into two and afterward into 

 four or more granules, of which, however, only one or two actually 



