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assimilated yolk spherules. Thus, as we find in the cytoplasm 
first yolk balls, then their fragments, and finally the mature yolk 
spherules, so in the nucleus the first generation of nucleoli 
are assimilated yolk balls and their fragments, while the small 
ones of the second generation are derived from the only yolk 
elements then present in the cytoplasm, namely, yolk spherules. 
The nucleoli of the first generation also differ from those of 
the second, at the time of the first appearance of both, in 
their manner of staining ; so that they would seem to differ 
chemically from each other. 
Nuclear structures of problematical significance . — In only 
one out of the three individuals of this worm studied were the 
following remarkable structures to be observed, though the fixa¬ 
tion method of both of the other individuals was exactly the 
same. These bodies first appear in ova of the second nucleolar 
stage, but here show always the same typical structure, so that 
I can say nothing as to the manner of their first formation. In 
preparations stained with haematoxylin and eosin they are 
colored by the former stain a little more deeply than the 
nuclear chromosomes, so that they stand out sharply in the 
nuclear substance (N. Bd., Figs. 122-139). The smaller ones, 
i.e.y those of the younger germinal vesicles (Figs. 122-126), 
are finely granular, though whether they each consist of a mass 
of fine granules or of homogeneous ground substance in which 
granules are distributed, I cannot determine. In the larger 
nuclei they often appeared wholly homogeneous (Fig. 132). 
In shape they are usually nearly spherical, with a sharp outline, 
which may or may not represent a limiting membrane; the 
larger ones are often more irregular in form (Figs. 132, 133, 
139). In the smaller nuclei they are as a rule, but not always, 
smaller than in the larger ones ; in the smallest nuclei in which 
I have found them there is only one of these bodies to a 
nucleus ; while in the larger nuclei they are not only larger, 
but also there may be from one to four of them in each nucleus. 
In only one small nucleus were three of them present (Fig. 
128). In two cases, both larger nuclei, I found division stages 
of these bodies : in the one case (Fig. 131) the body was ovoid 
in outline, with a shallow constriction at right angles to its 
