MUSEUM OF COMPARATIVE ZOOLOGY. 311 



When the products of the division cells are of unequal size (as in 

 Unio), the " Mutterkern " takes an eccentric position, so that the plane 

 of division is equidistant from the new nuclei. If oue side of the spindle 

 lie nearer the surface of the cell than the other, the constriction of the 

 protoplasm begins earlier on that side of the cell. 



Strasburger criticises 0. Hertwig's description of the nuclear spindle 

 as a band, since it is really cylindrical, and is shown to be so by Hert- 

 wig's own observations. He in turn admits the justice of Hertwig's crit- 

 icism when the latter claims that the nuclei do not arise in Phallusia (as 

 Strasburger first reported) at the centre of the asters. Both Auerbach 

 and 0. 'Hertwig have, he says, overlooked the homogeneous portion of 

 the young nucleus, which reaches to the centre of the sun."^ The inter- 

 zonal filaments, like the nuclei, are susceptible of being nourished, and 

 are thereby often considerably increased in mass and numbers, especially 

 in the equatorial plane. 



The general and theoretical considerations announced by Strasburger 

 will be considered in connection with the review of his studies on plant 

 cells. 



Zeller (76, p. 258, Taf. XVIII. Figs. 26-31) describes for Polysto- 

 mum integerrimum remarkable changes in the nuclei during segmenta- 

 tion. After the two nuclear structures (pronuclei) have united, the 

 resulting body disappears, and at two opposite poles of the egg there 

 arise near the surface two clusters of vesicular nuclei with nucleoli. 

 These nuclei increase in number and in size. Approaching each other 

 closely, the components of each cluster are at length dissolved into a 

 homogeneous mass. The cell now changes its form, elongating in the 

 direction of the axis which joins the two clear poles, and finally from the 

 more pointed end there grows out a bud which is ultimately pinched off. 

 The single cell is thus divided into two cells of unequal size. 



The most natural interpretation to be given the clusters of nuclei is 

 that they are the same as the clusters seen by Biitschli and other ob- 

 servers to result from the metamorphosis of the " nuclear plate " of a 

 spindle, although nothing like a spindle figure was seen by Zeller. 



* In Limax I have seen the nucleus deviate considerably from the spherical form, 

 and appear as though drawn out toward the centre of the aster (Fig. 80% compare 

 also the pronuclei, Fig. 68) ; but there is nothing which gives support to Stras- 

 burger's view that a homogeneous portion of the nucleus reaches to the centre of the 

 sun. I am even inclined to think that Strasburger may have fallen into an error in 

 the case of Unio, by mistaking granules at the centre of the sun for evidence of a 

 continuation of the outline of the nucleus to that point. 



