No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 477 



with chromatin granules or with fibers of chromatin, which 

 pass between it and the nuclear membrane ; there is never a 

 clear space around the nucleolus, but it seems to be held in 

 position by the chromatin. 



16. Muscle Cells of Piscicola rapax (Verr.). 

 (Plate 29, Figs. 325-337-) 



(The nuclei of the longitudinal muscle layer of the body wall 

 were studied. For the examination of the different stages of 

 these nuclei worms of different sizes must be studied ; I exam- 

 ined the nuclei of leeches of about 6 mm. in length, where the 

 cells and their nuclei are smallest, as well as of larger and fully 

 mature individuals, where these cells and their nuclei attain 

 their maximum dimensions.) 



In the smallest nuclei (Fig. 325) a single nucleolus is inva- 

 riably present and lies centrally ; it is of medium size, more or 

 less oval in outline, and contains a varying number of small 

 vacuoles. In larger nuclei it becomes larger and more elongate 

 in form, lying in the longitudinal axis of the nucleus (Figs. 328 

 and 331) ; at the end of this stage its greatest dimensions are 

 reached. Next commences a process of fragmentation of this 

 original nucleolus into a number of smaller nucleoli, which are 

 of different sizes. There appears to be little uniformity in the 

 mode of this nucleolar division (Figs. 327, 329, 332, 333) : the 

 nucleolus may become dumbbell shaped and then divide into 

 two larger pieces ; or when much elongated it usually breaks 

 simultaneously into a number of consecutive portions ; or buds 

 of nucleolar substance may be divided off from its surface. This 

 segmentation is not strictly dependent upon the size of the 

 nucleus, nor upon the size or form of the nucleolus. The frag- 

 mentation continues, the larger daughter-nucleoli also dividing, 

 until in the largest nuclei (those of the mature worm) as many 

 as twelve small nucleoli may be present, which are irregularly 

 distributed through the nucleus (Figs. 335-337). In all these 

 stages at least some of the nucleoli contain vacuoles, though 

 they have not been reproduced in all the corresponding figures. 



All the nucleoli of the largest nuclei are thus produced by a 



