No. 3.J THALASSEMA AND ZIRPHAEA. 597 



Such a mitosis, however, never occurs ; for shortly after the 

 chromosomes remaining within the egg have passed into ves- 

 icles, the centrosomes, with all trace of rays, disappear com- 

 pletely. The reconstructed egg-nucleus, which results from 

 the close aggregation, if not complete fusion, of the vesicles 

 (PI. XXXII, Fig. 28), now advances, unaccompanied by any 

 sort of aster or rays, to meet the sperm. 



c. The Sperm-Amphiaster. 



The sperm-aster first appears about the time of second polar 

 telophase, at the edge of the clear area surrounding the sperm- 

 head, and, roughly speaking, on the side nearer the center of 

 the egg. It contains a single focal centrosome to which the 

 rays converge directly (PI. XXXII, Fig. 19). In many cases a 

 process staining the same as the sperm-head extends from the 

 latter through the clear area to the center of the aster {cf. 

 Michaelis, '96, and Kostanecki, '96, both of whom have described 

 a similar phenomenon). When no such process can be demon- 

 strated, the aster appears independent of the sperm-head. The 

 early division and divergence of the centrosomes give rise to 

 a minute amphiaster already some distance in advance of the 

 sperm-head (PI. XXXII, Figs. 26, 27). This distance varies 

 considerably (cf. MacFarland, '97), as do also the relative time 

 and amount of the divergence, which may occur before or after 

 the sperm-head has become vesicular. The daughter-centro- 

 somes occupy the apices of an elliptical clear area outside 

 of which the rays are grouped, those at the apices being 

 focused at the respective centrosomes, while the intermediate 

 rays still converge to the previous center, a phenomenon not 

 uncommon in other forms. In some preparations a cloudy, rod- 

 like structure, which may be called the centrodesmus (primary 

 Centrodesmus of Heidenhain, '94), was seen extending between 

 the daughter-centrosomes (PI. XXXII, Fig. 27), but this bears 

 no relation to the definitive spindle, since it soon disappears 

 and leaves the asters independent. The further development is 

 marked by steady increase in the number, length, and strength 

 of the rays of these asters and the approach of the now hemi- 



