No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 455 



and material which has been more advantageously fixed and 

 stained.^ 



10. Poly dor a. 

 (Plate 28, Figs. 249-281.) 



The ^g'g cells of this form, as those of most Polychaeta, are 

 derived from the peritoneal cells of the body cavity, the latter 

 cells building pseudoepithelia around the intestine, as well as 

 occurring free in the body cavity. Those in the pseudoepi- 

 thelia (Fig. 249) are more or less flattened, disc-shaped, while 

 the free cells (Figs. 250-254) are oval in shape, with more 

 regular outlines. Their cytoplasm is not dense, and one or several 

 large vacuoles are frequently found at the periphery of the cell ; 

 a delicate cell membrane is present. The cytoplasm of these 

 sexually indifferent cells does not stain with haematoxylin. The 

 nucleus is small, irregular in outline, and contains a fewchromatin 

 granules ; very frequently the greater part of this substance lies 

 close to the nuclear membrane. I have never found more than 

 one minute nucleolus, and this is almost always close to, or in 

 actual contact with, the nuclear membrane (Figs. 251, 252, 254) ; 

 in many nuclei I failed to find nucleoli, though in these cases they 

 may have been obscured by the chromatin. I found one divi- 

 sion stage of a nucleus (Fig. 249) ; there were two daughter- 

 nuclei of the same size and form lying close together ; the 

 nucleolus of each was somewhat elongate in form (in all others of 

 these cells examined it is spherical), which might show that the 

 nucleoli had been produced by the division of a single one in the 

 mother-nucleus. In many of the smaller free peritoneal cells 

 a peculiar body often occurs (A^. P. Fig. 253). This is always 

 smaller than the nucleus, more or less spherical, often homo- 

 geneous in appearance, and it may stain either deeply red with 

 eosin or faintly with haematoxylin, or in other cases it may not 

 stain at all, but appear as a light yellowish, refractive mass. 

 From the comparative study of a large number of cells contain- 

 ing these bodies it may bq determined that they are degener- 

 ated nuclei or portions of nuclei. Thus in Fig. 250, which 



1 For other observations on nucleoli of Siphonophora, c/., besides the paper by 

 Conklin and Brooks, the review of O. Hertwig ('78b). 



