No. 2.] 



THE SEA-URCHIN EGG. 



451 



During the fusion of the nuclei the cap-shaped central body 

 of the aster draws apart in the middle and finally gives rise to 

 two completely separate halves that place themselves at opposite 

 poles of the cleavage-nucleus. Pari passu with this process 

 the astral rays become divided into two systems centred in the 

 respective halves of the central mass. This process does not 

 involve a splitting of the rays, but is caused by their division 

 into two groups through the divergence of their points of 

 attachment. As this takes place, the outer portion of the 

 rays, opposite the equator of the nucleus, are seen to cross one 

 another at an angle, at first very slight, but finally increasing 

 to 90° or even more, — a fact which indicates either that the 

 peripheral portions of the rays must change their position or 

 that new rays are developed in the space between the two 

 half-asters. This crossing of the rays seems very difficult to 

 explain unless they are actual fibres, and not, as Eismond 

 maintains,^ optical sections of the lamellae of an alveolar 

 structure. At the close of division each daughter aster has 

 essentially the same structure as that of the original, the rays 

 still reaching nearly to the periphery of the ^gg. 



C. The ''Paused — The division of the sperm-aster is 

 followed by a long pause (at a temperature of 27 C, about 15 

 or 20 minutes) during which remarkable changes occur both in 



. . i . : I 



vC^\\^■^■i':^^'■7.■■ 



"''•^yX^^ . ' 



/ • //MM \ • 



Fig. IV. — Cleavage-nucleus during the "pause," showing the first appearance of the centriole 

 (visible only on one side), the differentiation of the nuclear network, and the beginning of the 

 spindle-formation, [t/ Phototypes 5 and 6.] (25 minutes.) 



the central mass and in the astral rays. The latter only will 

 be considered at this point. 



Immediately after the division of the sperm-aster the astral 

 rays still extend nearly to the periphery of the Qgg and are 



1 Anat. Anz., X, 7, 1894. 



