7l8 CALKINS. [Vol. XV. 



is always in contact with the nucleus, sometimes having the 

 form of a sphere, but more often that of an irregular mass with 

 a darker peripheral and a lighter central portion. The latter 

 appearance is caused by the accumulation of microsomes around 

 the periphery, and by their absence in the central portion. In 

 specimens fixed with Hermann's or Flemming's fluid the sphere 

 is seen to have pseudopodia-like processes extending from it 

 into the surrounding cytoplasm and passing imperceptibly into 

 the reticulum (Fig. 9). 



In addition to the microsomes in the sphere other deeply 

 staining, but larger, granules are frequently seen. Sometimes 

 only one of these can be found ; again they are quite numerous, 

 even forming a small group of deeply staining bodies. Similar 

 granules are also found in the cytoplasm, distributed through- 

 out the reticulum. These granules are often strikingly similar 

 to centrosomes and might easily be mistaken for them. Care- 

 ful comparison of many spheres, however, both in sections and 

 in total preparations, shows that they cannot be centrosomes. 

 Their position is often eccentric (Fig. 5), sometimes even in 

 the periphery of the sphere, while the presence of similar gran- 

 ules in the cytoplasm shows that they cannot be " centrosomes " 

 in Heidenhain's sense. It is not improbable that the centro- 

 somes described by Ishikawa, which were found sometimes 

 single, sometimes double, and sometimes in groups, were cyto- 

 plasmic granules of this kind. Ishikawa apparently saw the 

 "sphere " only during division : "The archoplasm, as we have 

 stated, comes to be first seen at the stage a little before the 

 division of the spore-formation " (/. c). My observations, how- 

 ever, leave no doubt that it persists as an extra-nuclear mass 

 throughout all periods. 



C. The Nucleus during Division. 



The phenomena of nuclear division in Noctiluca are so com- 

 plicated and apparently so different from typical mitosis in the 

 Metazoa that the following general summary will help to make 

 the details more clear. 



During the early stages of nuclear activity the sphere divides 

 into two similar halves connected by fibers which here, as in 



