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- 

 ablv 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 



