MUSEUM OF COMPARATIVE ZOOLOGY. 353 



rapidity, but have not been known to consume more than two hours. 

 One is not warranted in dividing the phenomena into a number of stages, 

 for the character of regularity is wanting. Aside from these motions 

 which affect the constituents of the nucleus, and are of primary signifi- 

 cance, its whole mass in many cases undergoes a more or less irregular 

 motion of translation through the protoplasm of the cell ; and it may 

 require from three to eight minutes for the nucleus to traverse the diam- 

 eter of one of the larger ones. This motion ceases some time before the 

 division of the karyokinetic mass. Another phenomenon, observed only 

 in the frog, and of secondary importance, is the rotation of the nuclear 

 mass through an arc of 90°. 



During these changes in the nuclear substance the delicate rods and 

 fibres of the cell protoplasm continue their motion and gradually tend 

 toward the centre, where they are believed to undergo an assimilation 

 with the nuclear substance. By this means they become more highly 

 refringent, and therefore indistinguishable from the constituents of the 

 nucleus. 



The division of the nucleus is introduced by the sudden appearance of 

 a parallel arrangement of the nuclear rods, which together form a some- 

 what elliptical cask-like figure. The rods at once divide and the halves 

 move apart rapidly. The immediate destiny of the halves is the consol- 

 idation of their constituent half-rods, and the successive forms which 

 may be assumed in the course of this process are quite as free from regu- 

 larity as are the karyokinetic figures. Between the separating halves of 

 the nucleus one may usually distinguish fine, clear internuclear fibres, 

 which, although composed of fine granules, appear as continuous fila- 

 ments. The author distinguishes from these other fibres or rods which 

 stretch between the two halves, which have nothing to do with the in- 

 ternuclear fibres, and which soon disappear. He has given only isolated 

 descriptions, which seem directly comparable with nuclear division as 

 observed in segmentation spheres. The single observation of a spindle 

 figure with equatorial accumulations of nuclear substance is interpreted 

 as probably an accidental product. A genuine solar figure (which, how- 

 ever, is interrupted for several degrees of arc), whose fine rays stretch 

 from the karyokinetic mass to the periphery of the cell, is distinguished 

 by the author from isolated groups of stouter radial fibres : the latter are 

 identical with the previously mentioned fibres of the cell protoplasm 

 {loc. cit., p. 277) ; the finer, on the contrary, are processes of the karyo- 

 kinetic mass. 



The consolidation of the constituents of the new nuclei advances from 



VOL. VI. — NO. 12. 23 



