No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 
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 a11 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 
