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 {¥\gs. 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 



