No. 2.] THE SEA-URCHIN EGG. 453 



from the periphery of which the rays extend out into the 

 cytoplasm {Phototype 5, Text-fig. IV). 



D. Formation of the Cleavage- Amphiaster. — The origin of 

 the achromatic figure or cleavage-amphiaster may be followed 

 with the greatest clearness, and shows especially well the 

 formation of the spindle-fibres. As the egg prepares for 

 division the astral rays begin to extend themselves again in all 

 directions from the central mass, still retaining very distinctly 

 their granular structure and never fully resuming the stiff 

 fibrous appearance characteristic of the sperm-aster. Mean- 

 while the nuclear membrane fades away at the two poles 

 nearest the asters and from the latter spindle-fibres apparently 

 grow into the nucleus, and thus give rise to a distinct spindle 

 which lies inside the nuclear membrane (Phototype 6, Text- 

 fig. V). 



Further study of the developing spindle-fibres leads to the 

 conclusion that they do not really grow into the nucleus from 

 without, but are differentiated in situ out of the achromatic 

 nuclear network. The relation of the spindle-fibres to this net- 

 work is, in other words, precisely the same as the relation of 

 the astral rays to the cytoplasmic network. The growing 

 spindle-fibres may be seen in the clearest manner to branch 

 out at their ends into the nuclear network, and the latter dis- 

 appears pari passu, with the development of the spindle. In 

 some cases the boundary of the spindle exactly coincides with 

 the nuclear membrane, and in this case the membrane-sub- 

 stance also seems to be converted into the spindle-substance. 

 In other cases, however, the membrane may be separated from 

 the spindle by a considerable space, and fades away in this 

 position without relation to the spindle. In the completely 

 formed amphiaster the spindle-fibres are closely similar to the 

 astral rays, but are more closely crowded (so that the spindle 

 appears somewhat bluer) and their granular character is not 

 apparent until a later period. 



All these facts point to the conclusion that the achromatic 

 nuclear substance is fundamentally of the same character as 

 the cytoplasmic reticulum, and it will be shown hereafter that 

 in the closing phases of karyokinesis, both astral rays and 



