The maturatiou and fertilization of the egg of Cerebratulus. 451 



^re showD, one on either side of the fusing gerni-nuclei. Although 

 these radiations may still be followed to a more or less definite focus 

 on each side, yet no centrosome is to be found. As the fibres break 

 up still more, as is shown in Figs. 29 and 30, they are directed to 

 no definite foci but radiate in all directions from the area about the 

 germ nuclei, much as the degenerating fibres of the aster remaining 

 in the egg after the formation of the second polar body radiate from 

 the region of the fusing chromosomal vesicles which are to form the 

 egg-nucleus. In Figs. 31 and 32, the fibres (or, rather, the granules 

 which composed them) can be seen in the lower hemisphere of the 

 egg only, and at a slightly later stage they are not to be found at all. 



We may say, then, that after the germ-nuclei are nearly in con- 

 tact the sperm-asters have served the purpose for which they were 

 intended ^) and completely disappear in a manner quite similar to that 

 of the aster remaining in the egg after the formation of the second 

 polar body ^). Even the centrosomes are lost from sight in most 

 cases. Is it because the eggs are insufficiently well preserved that 

 we cannot find the centrosomes? In all other stages — even those 

 immediately preceding the first cleavage — the centrosomes come out 

 wonderfully clearly and distinctly. Have they disintegrated and become 

 non-existent, or have they merely disappeared for the time being, and 

 are they destined to reappear in a new role in the cleavage spindle? 

 Because of the fact that a pair of centrosomes appear later in exactly 

 the position occupied by the sperm-centrosomes before their disap- 

 pearance, and also because of the fact that they may occasionally 

 be found in this "critical" stage, and, thirdly because in other eggs 

 (Echinoderms, Thalassema, Chaetopterus, and many others) the sperm- 

 asters are known (?) to form the cleavage asters directly — for 

 these reasons rather than because of the testimony of the majority 

 of eggs which I have examined, I am forced to believe that the dis- 

 appearance of the sperm-centrosomes is apparent only, and that they 

 later reappear to form the centres of the cleavage asters. 



In the few instances in which the centrosomes may be found 

 during the fusion of the germ-nuclei they lie close beside the nuclei 



1) It is by no means clear to my mind what their actual function 

 may be. The germ-nuclei are certainly brought together in other eggs 

 (Ascaris, Myzostoma etc.) without the aid of sperm-asters. 



2) It will be remembered that these sperm-asters were found to 

 disappear in the living egg of Cerebratulus leidyi, and of Micrura 

 caeca, as was described above. 



