10 Papers from the Marine Biological Laboratory at Tortugas. 
vacuoles as the result of a contraction of the nucleolus due to the action of 
the preserving fluid, and a subsequent pushing of the hardened nucleolus 
from its original position by the microtome knife. Further study and com- 
parison with pictures like that presented in figure 13, where the “ vacuole” 
is replaced by a body with a coarse chromatic reticulum, and the observation 
that the chromatin mass is always to one side, but not entirely outside of the 
“vacuole,” compels the conclusion that one is dealing with a double nucle- 
olus, i. e., with a chromatin nucleolus and a true nucleolus or plasmosome. 
It may still be true that the plasmosome serves merely as a plastin 
ground-substance for the chromatin nucleolus, as in Asterias forbesu, and 
that preserving produced unequal contraction in the chromatin and plastin, 
II 
Fic. 11.—Ovum of Ophiocoma at culmination of growth-period; nucleolus still chromatic 
and with definite contour and partially surrounded by a vacuole through which extends 
a very pale and delicate reticulum; chromosomes of various shapes and sizes are scat- 
tered through the finely granular nucleoplasm. The cytoplasm has a reticular structure 
and contains very many yolk-spherules and microsomes. X 1500. 
and a subsequent shifting, either artificial or natural, brought them into the 
relation seen in figures 11 and 13. But the fact that the plastin-nucleolus 
is not always present in the same section with the chromatin nucleolus (and 
where the latter is of the same size as where a plasmosome is present), as in 
figures 9, 10, and 12, indicates that there is here a plasmosome and a 
chromatin nucleolus. That the latter is not of the nature of an accessory 
chromosome is-seen by the fact that it fades out in the later stages prior to 
