INDIVIDUALITY OF THE GERM-NUCLEI. 253 



the same vertical plane, so that they are all visible in a single 

 section taken parallel to the second cleavage furrow. 



The fourth cleavage furrows combine to form a circular, or 

 more often oval, figure with its longer axis coinciding with the 

 plane of second cleavage. This circular cleavage divides the 

 egg very unequally, cutting off eight micromeres from eight 

 comparatively large macromeres. Preparation for this cleavage 

 involves rotation of the two nuclear vesicles of a single daughter- 

 nucleus of the third cell division from their original position as 

 shown in Fig. 37 to the position shown in Fig. 38. , This latter 

 orientation insures the maintenance, of the segregation of the 

 germ-nuclei in the fourth mitosis (Figs. 39-42). As in the 

 previous divisions, the maternal and paternal chromosome 

 groups have here been traced through the various phases of 

 mitosis to the late anaphase and early telophase, where they 

 are still distinctly separate. The position of the vesicles of the 

 newly-formed daughter-nucleus (Figs. 43 and 44) indicates that 

 one is of maternal origin, the other paternal. The results as 

 regards the orientation of the nuclei up to this point are sum- 

 marized in the diagrams of Plate IX., Fig. 45. 



In the later cleavage divisions there is increasing irregularity 

 in the direction of spindle axes and cleavage furrows, but this 

 does not constitute any serious obstacle to tracing the history of 

 the germ-nuclei since the really important point is the orientation 

 of the nuclear vesicles with respect to the axis of the spindle. 

 As already indicated, this is invariably such as to insure the 

 segregation and equal representation of maternal and paternal 

 chromosome groups in the daughter-nuclei. As the nuclei be- 

 come smaller, it sometimes is difficult to distinguish the two 

 groups of chromosomes in the late phases of mitosis, but com- 

 parison with slightly earlier and later phases leaves no doubt 

 as to the continuity of the respective germ-nuclei. Until an 

 advanced cleavage stage, the double structure of the resting 

 nuclei is almost always clearly demonstrable. As the nuclei 

 become more numerous, it becomes easier to find nuclei cut in 

 such a manner as to show distinctly the double structure. In 

 the more active regions of the egg, the synchronism of division 



