338 WILSON AND MATHEWS. [Vol. X. 



and always advance from this point to meet the egg-nucleus. 

 This is, perhaps, the reason why in Asterias the first cleavage 

 is constant, passing through the polar globules, or very near 

 them, although the sperm enters the Q.gg at any point. If this 

 is so, it obviates the apparent contradiction between Asterias 

 and Toxopneiistes, that in the former the egg-axis coincides 

 with the first cleavage-axis, while in the latter it may or may 

 not. The two axes in Asterias coincide because the axis of 

 conjugation of the two nuclei is very near the egg-axis. 



The egg-nucleus re-formed, advances to meet the sperm- 

 nucleus, and as it moves the radiations about it disappear. 

 The front sperm-aster comes in contact with the egg-nucleus 

 first (see Fig. 6, C), moves to one side, and the two nuclei 

 come into apposition. The axis of the sperm-amphiaster now 

 traverses the plane of junction of the two nuclei and its posi- 

 tion corresponds with that of the first cleavage-amphiaster (see 

 Fig. 6, D). 



The division of tJie sperm-aster previous to the meeting of the 

 two nuclei is an important point of diffei'ence from the procedure 

 in Toxopneiistes and Arbacia, and gives additional reason for 

 doubting the occitrrence of any quadrille S7tch as is figured by 

 Fol. By a division of the asters before conj ligation the fot'ina- 

 tion of the first segnientation-amphiaster directly and wholly 

 from the sperm-amphiaster is clearly seen. If there is a qua- 

 drille it is necessary to suppose that the egg-centers have no 

 radiations about them, in which case no observer could identify 

 them ; that they are so small as not to be distinguished after 

 the closest search ; that before conjugation the egg-centers 

 divide and take up a proper position on the egg-nucleus to 

 meet the sperm-asters, which have already assumed their defi- 

 nite position ; and that the egg-centers after conjugation are no 

 longer to be discerned in the segmentation-asters , since there is 

 in these no discoverable trace of the two centrosomes as figured 

 by Fol. 



Having come into apposition, the two nuclei, now occupying 

 a slightly eccentric position in the ^g%,, fuse completely and 

 indistinguishably to form the segmentation-nucleus. The large 

 radiations about the asters become more difficult of demonstra- 



