170 bulletin: museum of comparative zoology. 



polar cell would soon have been formed in this egg. After the first 

 polar cell is detached, the chromosomes at the deep end of the spindle 

 seem to lose their identity, and form a more or less typical resting 

 nucleus, a condition which will be described more at length a little 

 later (p. 171). 



The actual formation of the second maturation spindle I have not 

 observed, the stage represented in Figure 13 being the earliest con- 

 dition subsequent to that of Figure 12 which I have found. The 

 first polar cell (Fig. 13) is shown, still connected to the egg by inter- 

 zonal filaments; the chromosomes have already lost their individuality 

 and their substance is loosely distributed through the outer end of 

 the polar cell, though not constituting a definitely formed nucleus. 

 The second maturation seems to represent an advanced stage of 

 the spindle. In the outer half the fibres are more prominent than 

 those within the egg. The latter converge more or less sharply toward 

 a centre within the egg, while the outer fibres converge less definitely 

 toward the distal end of a somewhat conical protuberance of the sur- 

 face of the egg, which one naturally interprets as the prospective 

 second polar cell. Within the egg faint astral radiations appear in 

 the region of the apparent pole of the spindle, but no such radiations 

 are found at the end of the peripheral fibres. At neither pole is there a 

 body resembling a centrosome. As regards the chromatin of the sec- 

 ond polar cell, the Figure suggests two possibilities. Either (1) the 

 chromatin may be distributed along the fibres of the peripheral half 

 of the spindle, in which case these fibres represent true spindle fibres 

 with some chromatin, and the inner half of the fibres represent inter- 

 zonal filaments. This would, perhaps, account for the prominence 

 of the fibres in the polar cell, and would not be greatly unlike the 

 condition in the adjacent first polar cell. Or (2) the chromatin that 

 normally belongs to the polar cell may be represented in the two large 

 chromatic masses near the equator of the spindle, which have not been 

 drawn into the polar cell, and apparently would have been left in the 

 egg had not the constriction, already started, been interrupted by the 

 killing of the egg. This latter view gains some weight, perhaps, from 

 the aberrant condition of part of the peripheral group of chromosomes 

 seen at a slightly earlier stage in the first maturation spindle (Fig. 12). 

 It may be that part of the chromatin of the polar cell is in the two 

 chromatin masses, and that another part of it is distributed along 

 the peripheral fibres. Whatever the view regarding the chromatin, 

 it seems fairly certain that the thickenings in the equator of the 

 spindle represent the interzonal bodies of ordinary cell division, and 



