224 Papers from the Department of Marine Biology. 



from those cases where the whole nuclear content has been piled up in a 

 broken mass. Some of the best workers on the structures of the cyto- 

 plasm have left us figures that give no true conception of the structures 

 of the nucleus. Much careful work remains to be done upon it before 

 we possess an adequate picture of its organization. 



The plasmosome is one of the most prominent contents of the nucleus. 

 It is large, being usually about 7 microns in diameter when single, and 

 is an almost perfect sphere. Its most prominent physical property is 

 a dense refractiveness which makes it easily visible in the living cell 

 even under a wide opening of the condenser diaphragm. As the dia- 

 phragm opens, the outline of the plasmosome lasts a trifle longer than 

 that of the nuclear membrane, and when one considers the greater 

 radius of the nuclear membrane this means that the plasmosome is 

 the most refractive body in the cell, excepting perhaps the karyosomes. 

 No accurate measures have been taken to measure the actual refrangi- 

 bility of the various organs of this cell by accurate physical means, 

 but it is hoped that this will be done at another time. 



In perhaps a majority of cases the plasmosome is a single body, but 

 this factor varies in the different individual fishes examined. I think 

 it may be safely stated that in the oldest and largest specimens the 

 plasmosome is most apt to be single, while in younger specimens it is 

 most apt to be a multiple body. Also a distinct tendency to vary in 

 the distribution of the plasmosome was observed in individuals of the 

 same size and sex. When multiple, it is most usually found as a single 

 large body with one or more smaller masses in some other (usually 

 distant) part of the nucleus. Or in some cases it may be found as two 

 fairly large spheres of approximately equal bulk. In one Torpedo 

 ocellata of 15 cm. each nucleus appeared to contain its plasmosome as 

 four or more bodies of equal size, any one of which was much smaller 

 than the usual size of 7 microns. In all cases of multiple plasmosome 

 it appeared that the total volume of the various bodies was greater than 

 that estimated in the nuclei where a single body appeared. It may be 

 said here that in the large American Tetronarce occidentalis a single 

 plasmosome is almost the invariable rule. 



The plasmosome is by no means a homogeneous body. In the large 

 and typical forms at least three substances may be distinctly seen in it. 

 One is a granular, non-refractive, and non-staining content of the 

 various "vacuoles" found in this structure. The principal vacuole 

 occupies a large part of the exact center or, in numerous cases, a more or 

 less eccentric position in the body of the plasmosome. The usual 

 diameter of this vacuole is a little more than half of that of the whole 

 plasmosome, or 4 microns. It is always round and "bubble-like" and 

 in some very few cases is so close to the periphery of the plasmosome 

 that it has broken through and is open to the other contents of the 

 nucleus. The edges of the plasmosome thus broken are rounded up as 



