No. 6.] INDIVIDUALITY OF THE GERM NUCLEI. 263 



derived from corresponding parts of a mother-nucleus, and 

 so on back to the egg and sperm nuclei in the first 

 cleavage. 



Since the descendants of the germ nuclei are halved at every 

 division, it follows that successive divisions of the double nuclei 

 cannot be at right angles to one another, since this would lead 

 to an unequal division of the halves, or even to a division along 

 the plane of contact between the halves. Such an unequal 

 division might be prevented in cleavages which successively 

 alternate in direction by the rotation of the nucleus during the 

 resting period, or by the rotation of the spindle in the early 

 stages of mitosis. 1 . As a matter of fact both of these methods 

 occur in Crepidula. The nucleus usually rotates during the 

 rest through 90, so that although successive nuclear spindles 

 are at right angles to one another the axis of every spindle lies 

 in the same nuclear axis (cf. Figs. 3, 4, 8, 9) ; but in some 

 cases the nuclear spindle does not lie in its definitive position 

 when first formed but undergoes extensive rotation after its 

 formation. While it is not susceptible of absolute proof, since 

 the partition wall is absent during the later stages of the rest, 

 it is highly probable that the plane in which all nuclear spindles 

 lie is the plane of contact between the two halves of every 

 nucleus. 



3. In certain abnormal cleavages the double nuclei are really 

 two entirely separate nuclei lying side by side within a single 

 cell. Such binucleated cells may occasionally be found with 

 the nuclei in the height of the rest, though they are more usual 

 in the telophase or early resting period. There is usually but 

 a single sphere and centrosome in such cells, though in one 

 case of pathological mitosis which I have seen (Fig. 7) there 

 are two mitotic figures side by side ; the chromosomes which 

 have reached the stage of the chromosomal vesicles have not 

 aggregated at the poles of these spindles, but are scattered 

 along their whole length. There are thirty of these 



1 Riickert finds that the nuclei rotate in Cyclops even after the spheres have 

 reached their definitive positions at the poles of the spindle ; I have never observed 

 in Crepidula a rotation of the nuclei, independent of the spindles, at so late a 

 stage in the cell cycle. 



