492 THOMAS H. BRYCIO. 



tlirougli 180° till the base of the conical sperm head is 

 directed inwards. The rays of the aster now extend widely, 

 and at their centre is a clear ai'ea. Meantime the sperm head 

 becomes converted into a small round nucleus. The move- 

 ment of the sperm head is, at first, radial; then there is a 

 change, and it assumes a new direction towards a point not 

 quite in the centre of the egg; when this change of path is. 

 taken up the egg nucleus begins to move towards the point 

 where the nuclei ultimately meet (Wilson and Giardina), 

 The aster now comes in contact with the egg nucleus, and 

 as the nuclei approach, the clear area at its centre spreads 

 out over its side. The aster theu divides nnd the nuclei 

 conjugate. The radiations now die down during a pause in 

 which the nucleus grows in size (Wilson), to redevelop again 

 focussed at the poles of the nucleus. 



According to Hertwig, Doflein, Erlanger, and Wilson's 

 earlier account, the centrosome corresponds to the whole 

 middle piece, but later Wilson described the middle piece as 

 cast aside, and in the centre of the aster is a sinall dai-kly- 

 staining granule. Boveri (1901) represents the sperm 

 centrosome as a spherical body smaller than the middle piece, 

 and containing within it two centrioles shoi'tly after its 

 entrance into the egg. 



Various other observers have represented a dark-staining 

 grannie at the centre of the aster. My own observations 

 are inconclusive, and do not warrant mo in expressing an 

 opinion.^ 



' Tlie character of the fully-formed ceutrosome iu tlie sea-urchin egg is 

 still bubject to difference of opinion. The form in which I see it iu osmic 

 acid material is that of a largish sphere of very finely alveolar structure. Iu 

 Wilson's papers on magnesium and etherised eggs, "it appears as a well- 

 defined body of considerable size, consisting of intensely stained granules, 

 which often give the centrosome exactly the appearance of a minute nucleus 

 containing a chromatin network." Tliis becomes in the anaphases more homo- 

 geneous, and flattens down into a plate-form, which in the telophases often 

 lies directly on the membrane of the newly-formed nucleus precisely as Boveri 

 (1901) has described for Echinus. Boveri (1901) represents it in several 

 forms. In one set of preparations it is a largish sphere of very finely alveolar 



