INDIVIDUALITY OF THE GERM-NUCLEI. 257 



which are closely pressed together but still separated by a parti- 

 tion wall, as Riickert has shown to be the case in Cyclops. 

 Gradually this partition wall disappears, being preserved longest 

 on that side of the nucleus nearest the centrosome. Here a 

 groove is formed on one side of the nucleus which marks the line 

 of contact between the two halves. In some cleavage cells this 

 groove is visible throughout most of the resting period; in others 

 it disappears during the greater part of the resting period, though 

 it may reappear in the following prophase; in all cases, however, 

 the partition wall and groove reappear in the next succeeding 

 telophase, when it is again formed in the manner described above. 

 I have observed the double character of the nucleus in the 

 telophase of every cleavage up to the 29-cell stage, and in several 

 of the later cleavages up to the 6o-cell stage, though it becomes 

 more difficult to see as the nuclei grow smaller. ... It still 

 remains to show that these double nuclei really represent the egg 

 and sperm nuclei which have not yet lost their individuality. 

 This cannot be demonstrated in Crepidula, for the reason that 

 this double character is not apparent at every stage in the nuclear 

 cycle, but it is extremely probable" (Conklin, 1901). Additional 

 observations recorded by Conklin in support of his interpretation 

 may be summarized as follows: (i) In the metaphase of the 

 first cleavage division the maternal and paternal germ-nuclei 

 are represented by separate groups of chromosomes; in the 

 early anaphase these groups of chromosomes can no longer be 

 distinguished, but the nuclei are clearly double in the immediately 

 following late anaphase and telophase, and the position of the 

 partition wall in these double nuclei corresponds to the plane of 

 contact between the germ-nuclei. (2) The groove which is found 

 on one side of the nucleus in the telophase of the first cleavage 

 mitosis persists well into the resting stage, and a corresponding 

 groove is found in the same position in the prophase of the second 

 mitosis. The central spindle for the second cleavage mitosis 

 lies in this groove, and thus the amphiaster actually lies in the 

 only plane in which it would be possible to halve the two parts 

 of the double nuclei. Although cleavage divisions successively 

 alternate in direction, unequal division of the double nuclei is 



