450 WESLEY R. COE, 



centrosomes free in the cytoplasm. The aster-fibres become much 

 fainter in their inner portions, and tend to radiate from the area 

 between the centrosomes rather than from the centrosomes themselves. 

 The radiations become still more irregular and stain much less deeply. 

 The centrosomes themselves are no longer to be de- 

 tected (Fig. 28, 29) ^), although from the course of development at 

 a later period it is reasonable to suppose that they still retain their 

 identity. It is possible that they remain near their former places, but 

 since they are no longer at the foci of radiations it is impossible to 

 distinguish them from other deeply stained granules of the cytoplasm. 

 After the disappearance of the centrosomes, and some- 

 times before, the aster-fibres begin to degenerate. 

 This disintegration begins at the end of the fibre which was previously 

 attached to the centrosphere, and having once begun the fibre breaks 

 up throughout its whole length at about the same moment (Figs. 28, 

 30). The degenerating fibre first loses its staining capacity somewhat, 

 decreases in size, and finally breaks up into a row of granules which 

 retains its original position for a considerable period of time, so that 

 the place which the fibre has occupied can be distinguished long 

 after the fibre itself has broken up. Indeed, these rows of darkly 

 stained granules often persist even after the germ-nuclei have fused, 

 and the cleavage asters have attained a considerable size (Figs. 31, 

 32). One of the most remarkable features of these eggs is the per- 

 sistence of their aster-fibres after being once formed ^). This is 

 noticeable in the aster of the polar spindles, in the fibres about the 

 central spindle during the division of the original sperm-aster, still 

 more pronounced in the degenerating sperm-asters just noticed, and, 

 as we shall see later, is likewise characteristic of the degenerating 

 cleavage asters after the division of the egg into two cells. 



Gradually the rows of granules of the degenerating fibres become 

 scattered among the yolk-globules and are lost in the cytoplasmic 

 recticulum. In other words, they probably resume their original 

 functions as parts of the general network of the cytoplasm. An egg 

 very similar to that of Fig. 26 is shown in Fig. 28, at a later stage 

 of development. Two sets of degenerating radiations of unequal size 



1) That is to say , in the vast majority of eggs in this stage I 

 have been unable to find them. That they are sometimes present 

 at this stage or one a trifle later will be shown below. 



2) In Micrura caeca and in Cerebratulus leidyi the persistence 

 of the fibres of the sperm-asters is not so pronounced. 



