KARYOKINESIS. 49 



centrosome and forms the extra-nuclear portion of the spindle. The aster also 

 stains more deeply after this outflow than before and very like the chromatic 

 nuclear sap. 



The shape of the spindle depends in part upon the degree and stage of this 

 nuclear outflow. The spindle is at first as wide at the equator as the entire mother 

 nucleus, but as the flow of nuclear substance toward the poles continues it "tows 

 longer and slenderer, the centrosomes at the same time moving farther and farther 

 apart, until in tlie late anaphase almost the whole of the interfilar substance has 

 moved out of the spindles into the spheres. 



Whether or not the spindle fibres and linin threads exist as such in the living 

 cell or are artefacts must still be left an open question. It can scarcely be doubted, 

 however, that they do represent substances which are diflferent from the surround- 

 ing materials of the cell, and this is after all the important thing. That the spindle- 

 fibres and especially the connective fibres sometimes show considerable elasticity 

 and rigidity has been pointed out repeatedly by those who hold that the centrosomes 

 are pushed apart by their activity. Nowhere is this better shown than in the first 

 maturation of Crepidula, where in the shortening of the spindle the fibres are bent 

 and kinked and the chromosomes at the outer pole are pushed clear through the 

 polar bodj' into contact with the opposite cell wall. In spite of this, however, it 

 seems to me very questionable whether the spindle fibres are anything other than a 

 fluid more viscid than the surrounding cell substance. 



I agree with those authors (Butschli, Fischer, Ehumbler, Wilson), who hold 

 that the astral rays represent diifusion streams in the cj^toplasm, rather than a stable 

 system of fibres. There are certain evidences that the astral rays are composed in 

 the main of cytoplasmic material, principally hyaloplasm or interalveolar substance ; 

 chief among these is the fact that the aster is alwa^^s proportional in size to the 

 extent of the cytoplasmic area which comes within its influence, a fact which Wilson 

 ('96) emphasized and which I also ('94) pointed out and have since had abundant 

 opportunity to verify. But while the aster and astral rays are in the main composed 

 of hyaloplasm it is probable that in normal mitoses certain nuclear substances enter 

 into their formation. In the mollusks which I have studied there can be no doubt 

 that certain achromatic substances from the nucleus and spindle flow into the aster 

 and at the same time the central area of the aster as well as its rays stain more 

 deeply than the hyaloplasm throughout the cell body. Whether there may not be 

 a centrifugal movement of escaped nuclear substance along the astral rays as well as 

 a centripetal movement of the hyaloplasm must be left an open question. 



In this connection I must refer to one of the first observations ever made on 

 indirect nuclear division, — that of Auerbach ('74) on the living eggs of certain 

 nematodes. He observed tlie double suns (asters) with their connecting stalk 

 (spindle) and supposed that they were formed by the collapse of the nucleus and 

 the passing out of nuclear sap into the cytoplasm, " the astral radiations being 

 merely the expression of the paths along which fine streams of nuclear sap pass 

 out into the protoplasm." Butschli ('76) also observed the passage of nuclear sap 



7 JOUEN. A. N. S. PHILA., VOL. XII. 



