No. 2.] ORGANIZATION OF THE EGG OF UNIO. 23 1 



There is nothing specially worthy of note in the further 

 behavior of the sperm-nucleus during its resting period ; appar- 

 ently it undergoes no further change, and certainly makes no 

 other movements until after the formation of the second polar 

 body. Then it begins to enlarge again by the accumulation of 

 caryolymph in its interior, keeping pace with the growth of 

 the egg-nucleus ; and both nuclei begin to move inward at 

 the same time. The details of the growth and movements 

 of the germ-nuclei are better considered together, and will be 

 described thus after the section on the maturation of the egsr. 



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2. TJie Sperm-Aster and Amphiaster : Origin and 

 Disappea ra nee . 



a. Observations. — I have not been able to determine the 

 exact origin of the sperm-centrosome in the Qgg of Unio, 

 and have nothing to contribute to this subject. The aster 

 is comet-like in form during the penetration, and possesses 

 a small apical centrosome which soon divides ; separation of 

 the halves produces a typical amphiaster. The amphiaster 

 frequently precedes the sperm-nucleus, with its axis in the line 

 of movement and its long rays trailing behind (PI. XXIV, 

 Figs. I and 2) and losing themselves in the cytoplasmic retic- 

 ulum. But the centrosomes may separate in a direction at any 

 angle to the penetration path up to a right angle {PI. XXIV, 

 Fig. 4). It seems probable that the latter is the case, when 

 the division of the centrosome is delayed until the penetra- 

 tion path is nearly completed. Frequently the rays of the 

 amphiaster engage with those of the inner aster of the matura- 

 tion spindle, and may become continuous ivith them. In one 

 case I found an apparently perfect spindle, one pole of which 

 was formed by the inner centrosome of the maturation spindle, 

 and the other by one of the sperm-centrosomes. This would 

 seem to indicate development of the fibers from a common 

 reticulum or foam-structure. 



The comet-like form of the aster in the tgg of Unio seems 

 to be quite unique. That it is not due to any peculiarity 

 in the structure of the protoplasm of this Qgg is shown by 



