No. 3] THALASSEMA AND ZIRPHAEA. 619 



group of dyads (PI. XXXIV, Fig. 54), rotates from its paratan- 

 gential into a radial position, and rises to the surface extremely 

 near the point at which the first polar spindle underwent division. 

 The divergence of the daughter-chromosomes now follows, 

 during which the inner centrosome does not seem to undergo 

 any noticeable change until telophase, when it divides and 

 appears as two black granules surrounded by a common grayish 

 envelope (PI. XXXIV, Fig. 53). The rays, while diminishing 

 somewhat in distinctness, still persist with sufficient clearness 

 to enable the aster to be easily recognized as such. During 

 the formation and fusion of the vesicles, the rays still persist 

 and focus to one or two or more granules. In many prepara- 

 tions it is extremely difficult to make sure which of these are 

 the true centrosomes, as the astral center is often somewhat 

 disturbed, whereby other and undoubled cytoplasmic granules 

 become thrust into it. In one very favorable preparation, dur- 

 ing the fusing of the vesicles, there are seen at the focus of the 

 persisting egg-aster two black granules, each invested by its 

 own grayish envelope, and exactly similar to the centrosomes 

 of the second polar telophase (PL XXXIV, Fig. 56). Little 

 doubt can exist that these are the centrosomes. Remains of 

 the rays are still to be seen after completion of the egg-nucleus, 

 and sometimes the centrosomes show as well (PI. XXXIV, 



Fig. 57). 



During second polar anaphase and constitution of the egg- 

 nucleus, the sperm-head becomes vesicular and develops an 

 aster (PI. XXXIV, Fig. 53) focused about a distinct centro- 

 some. An amphiaster, such as Lillie ('97) finds in Unio, never 

 arises, and a later division of the centrosome could not be deter- 

 mined with certainty by reason of the numerous cytoplasmic 

 granules surrounding it. 



Approach of the germ-nuclei increases the difficulty of mak- 

 ing out the behavior of the astral systems, although the indica- 

 tions are that one (either sperm or egg) degenerates, while the 

 other gives rise to the cleavage-amphiaster. This difficulty 

 arises from the fact that renewed activity sets in only after the 

 nuclei have so near approached as to make it impossible to 

 decide to which the centrosomes belong. In some cases the 



