HARGITT: PENNARIA TIARELLA AND TUBULARIA CROCEA. 167 
Those which were killed a few hours before their discharge possess 
the germinative vesicle, and those killed after their discharge show the 
small egg nucleus. Since eggs killed immediately before their dis- 
charge from the medusa are the only ones that have shown polar cells 
in process of formation, it may be confidently stated that polar cells 
are usually formed only during this brief period, as Hargitt (:04°) 
has already suggested. While all the maturing eggs of one medusa 
are, as a rule, in approximately the same condition, this is not invari- 
ably the case, for while some are actually forming polar cells, others 
still have large germinative vesicles. ‘This I take to be evidence of 
individual differences in the eggs themselves. 
In the eggs of Pennaria at the end of the growth period the germina- 
tive vesicle is close to the outer surface of the egg, being covered by 
only a very thin layer of cytoplasm; the chromatin, in the form of 
fine grains, is distributed along the reticulum, and shows only a slight . 
affinity for the usual nuclear stains. ‘The nucleolus behaves in the 
way already described, disappearing before the membrane of the 
germinative vesicle does. The time at which the chromatin begins to 
concentrate, preparatory to forming chromosomes, is variable. Fig- 
ure 1 represents an egg killed ten hours before the time when presum- 
ably it would have been discharged, and shows the chromatin more 
concentrated and more deeply stained in one part of the germinative 
vesicle than in other parts. Figure 2 represents the germinative 
vesicle of an egg which is only slightly more advanced than that of 
Figure 1. The chromatin is diffused, there being no sign anywhere 
of deeply staining granules. The shape of the vesicle in this case is 
one often assumed just before the formation of the maturation spindle. 
Figures 3 and 4 show the condition of the germinative vesicles of two 
eggs killed just after the liberation of the medusa; the concentration 
of chromatin into granules along the reticulum is well exhibited. 
The chromatin in Figure 4 is only slightly more concentrated than 
that in Figure 1, although the latter was killed ten hours earlier. ‘The 
same difference in the time of concentration of the chromatin is shown 
in Figures 3 and 7; the former, representing the germinative vesicle, 
has the chromatin much more concentrated than the latter, which 
represents a later stage, in which the maturation spindle is forming. 
Figures 3 and 4 show a wrinkling of the membrane, which seems to 
be the result of a decrease in size of the germinative vesicle. ‘The 
concentration of chromatin is also beginning, and these eggs are 
undoubtedly near to the time of polar-cell formation. ‘The Figures 
also make it plain that the size and staining capacity of the nucleolus 
