232 LILLIE. [Vol. XVII. 



the fact that the same aster becomes radial after the penetra- 

 tion path is complete. Therefore, it is due in some way to 

 the conditions of the penetration ; these probably affect the 

 form of the aster in two ways : i. It is noticeable that rays 

 form more readily in protoplasm free from yolk-granules ; there 

 is a yolk-free path behind the amphiaster, and the rays would 

 naturally be longer within this area. 2. The mere forward 

 movement of the centrosomes would tend to draw out the rays 

 extending behind and to shorten those in front. 



Shortly before the metaphase of the first maturation spindle 

 the sperm-asters undergo retrogressive metamorphosis, and, as 

 they disappear, the yolk-granules flow in and gradually obliterate 

 all traces of the clear area. Figs, y a, 7 b, 8, and 9 a, PI. XXIV, 

 illustrate progressive phases of this process. Figs. 7 a and 7 b 

 are two sections of the same amphiaster two sections apart. 

 The centrosome nearer the sperm-nucleus (7 a) is united to the 

 tip of the latter by a delicate fiber, and the radiations originally 

 surrounding it have completely disappeared. The more dis- 

 tant centrosome is still surrounded by a well-defined aster, 

 which, however, in the irregularity of the rays and the number 

 and size of the microsomes on them, shows plain signs of 

 degeneration. In PI. XXIV, Fig. 8, we have a more advanced 

 stage of degeneration ; the nearer centrosome can now no 

 longer be distinguished among the microsomes, and the more 

 distant one is barely discernible, the rays having nearly disap- 

 peared ; the yolk-granules have encroached on the clear area. 

 In PI, XXIV, Fig. 9 a, neither centrosome can be distinguished, 

 and the clear area is nearly obliterated. In the last stages of 

 degeneration of the aster the centrosome is distinguishable 

 from a cytomicrosome only by its position. When the aster is 

 entirely gone, one sees in its place simply ordinary vesicular 

 protoplasm with nodal microsomes. There can be no doubt of 

 the resolution of the radiations into the cytoplasm ; and if the 

 centrosomes do not simply become microsomes, they certainly 

 become indistinguishable from them by any visible morphologi- 

 cal character. The view that they persist as definite entities 

 of specific structure and function is not inherently impossible, 

 but there is nothing in the observations to indicate this, and 



