260 CONK LIN. [VOL. II. 



groups which are closely pressed together but are still separated 

 by a partition wall (Figs. 4, 8, et scg.}, as Ruckert has shown to 

 be the case in Cyclops. Gradually this partition wall dis- 

 appears, being preserved longest on that side of the nucleus 

 nearest the centrosome (Fig. 5). 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 (Figs. 4, 9) ; 

 in others it disappears during the greater part of the resting 

 period, though it may reappear in the following prophase 

 (Figs. 5, 6) ; in all cases, however, the partition wall and groove 

 reappear in the next succeeding telophase, when it is formed 

 again 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 (Figs. 1-16), and in several of 

 the later cleavages up to the 6o-cell stage, and I have no doubt 

 that it is found in all the later cleavages, though it becomes 

 more difficult to see as the nuclei grow smaller. While the 

 halves of these double nuclei occupy similar positions relative to 

 each other at corresponding stages in any cell generation, they 

 occupy different positions at different stages and in different 

 generations ; consequently the position of the groove or par- 

 tition wall which separates the halves of the double nuclei can 

 be satisfactorily studied only in preparations of entire eggs, 

 which may be observed from all sides. All the figures which 

 illustrate this paper are therefore of entire eggs, though many 

 isolated cases of double nuclei have been observed and studied 

 in actual sections. 



On each side of the partition wall which divides these double 

 nuclei there is usually a single small nucleolus ; these two 

 nucleoli persist long after the disappearance of the partition 

 and frequently throughout the whole of the resting period. In 

 most if not all of the early cleavages there are two, and only 

 two, nucleoli present in the telophase (Figs. 3, 4) ; but if this 

 is succeeded by a very long resting period the number may 

 increase to more than two, or all may fuse into a single enor- 

 mously large one. 



It still remains to show that these double nuclei really 



