No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 485 
evidences of cell degeneration or of a formation of new cells, so 
that we must conclude that each of these cells continues to 
functionate periodically during the whole time of the existence 
of the leech. 
We may now describe in succession the prophase and the 
metaphase of the structural changes. 
Prophasis (Figs. 178-196). — In the smallest cells found in 
the youngest leech examined no trace of secretion is present 
(Figs. 178-180). In these the nucleus is usually central in 
position, with a delicate chromatin network, and with a single, 
most frequently oval, nucleolus, in which one or a few small vac¬ 
uoles are commonly present. Around the nucleus, and filling 
the cell duct, is a somewhat dense cytoplasm, which becomes 
more vacuolar at the periphery of the cell. The chief cyto¬ 
plasmic changes from now on are as follows (I have not fig¬ 
ured these changes, since they may be briefly characterized) : 
that portion of the cytoplasm close to the nucleus gradually 
becomes more dense and begins to stain differently from the 
rest, and then becomes quite homogeneous ; most frequently 
there is a layer of this homogeneous substance between the 
nucleus and the cell duct, only that portion of the cytoplasm at 
the proximal end of the cell, as well as a thin layer around 
the homogeneous substance, retaining its primitive appearance. 
Next, this homogeneous mass gradually breaks up into the 
numerous secretion corpuscles (Fig. 181, Seer.), the shape of 
the latter being ovoid after fixation in corrosive sublimate, but 
more spherical after the action of Flemming’s fluid. These 
secretion corpuscles stain at first just like the homogeneous 
substance, but gradually commence to stain otherwise, and in 
the functionally mature cell stain differently from the primitive 
cytoplasm, as well as from the homogeneous substance from 
which they were derived. The whole cell thus gradually 
becomes filled with these small corpuscles, until finally no 
trace of the original cytoplasm is to be seen, except a few 
faintly staining fibers. The cytoplasm which fills the duct 
undergoes the same morphological changes as that of the cell 
body just described, so that the first secretion corpuscles in it 
are the derivatives of its own substance ; the cytoplasm of the 
