THE GENTROSOME 



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 



oiA'o"j.-:\ii-.A '^ 



B 



Fig. 154. — Centrosome and aster in 



A. Aster of the first polar figure ; central 

 (entosphere) and cortical (ectosphere) zones, 

 entosphere bounded by continuous membrane, 

 of central spindle within and from the substance 



D 



the polar mitoses of Unio. [LiLLIE.] 



granule (centrosome) surrounded by medullary 

 B. Late anaphase of second polar mitosis ; radial 

 C. D. Prophases of second mitosis ; formation 

 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 



