MUSEUM OF COMPARATIVE ZOOLOGY. 537 



protoplasm. Their centrifugal growth is not proof of it, but simply 

 makes it probable that the influence exerted from the centre of the 

 aster is increasing at the time of their formation, and that the substance 

 of which they are composed is affected in a manner similar to that of 

 the vitelline rays. An objection to this supposition is found in the 

 more common descriptions of the nuclear metamorphosis, in which the 

 asters are located at the boundary of the nucleus and vitellus, the former 

 being elongated into a more or less spindle shape. There is nothing in 

 this to suggest an incursion of vitelline substance ; besides, the spindle 

 fibres have been shown, especially by 0. Hertwig, to be formed within 

 the nucleus when its lateral walls were still complete. At the ends of 

 the spindle-shaped nucleus, however, in all such cases, the nuclear boun- 

 dary has ceased to be visible, — the substances of nucleus and vitellus 

 are in contact. It therefore does not appear entirely impossible that 

 an invasion of slender threads of vitelline protoplasm might take place 

 at these points. But if they were just like the rays running through 

 the yolk, one would expect to find them traversing the whole space of 

 the nucleus, and not limited, as they have been shown to be in some 

 cases, to an axial portion. 



It appears significant that the aster is never found to lie wholly 

 within the nucleus, but has been found wholly outside that structure. 

 Since, then, in some instances, there is no motive for ascribing to a 

 portion of the rays of the aster, in its early stages, a condition different 

 from the rest, I am led to the conclusion that the rays which stretch 

 through the nucleus are invasions of delicate filaments of protoplasm 

 about which the nuclear substance is progressively accumulated. This 

 may terminate in an intimate fusion of the two substances, or the latter 

 may exist as an investment of the former. I believe that the deport- 

 ment of the fibres at the time the nuclear disk divides offers some 

 support to the latter view. 



The peculiar movements of the nuclear substance are perplexing. No 

 entirely satisfactory explanation of them has been given. With the 

 formation of the spindle fibres there is unquestionably a transfer of this 

 substance, not only to definite tracts indicated by the course of the fibres, 

 but also, and principally, toward the equatorial plane. Do these move- 

 ments occur in response to a force operating from the centres of the 

 asters *? In view of the first appearance of the fibres at their astral ends, 

 this seems a reasonable assumption. The force must, then, be one of 

 repulsion for the nuclear substance. But that is not readily reconcilable 

 with the subsequent division of the nuclear disk and the approach of 



