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 forbesii, and 

 that preserving produced unequal contraction in the chromatin and plastin, 



FIG. ii. 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 n 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 



