576 Transactions of the Society. 



between cells a and h, which are already forming the bend towards 

 cell /, that we see so distinctly marked in most of the figures. 



The manner in which the vessel-forming cell vacuolates or 

 hollows itself out so as to form a tube is a question of great 

 importance, which is not yet thoroughly understood, judging from 

 the difi'erent opinions held by observers. This divergence in 

 opinion is, we believe, to be accounted for by the fact that there 

 are several different processes included under the head of vacuola- 

 tion, differing in their course and results, although one process may 

 be often found passing into another. 



When a fixed cell passes into the embryonic form, as in the 

 case of an inflamed epidermic or cartilage cell, or when the cell of 

 embryonic cartilage vacuolates to make way for the development of 

 bone, there is always a plentiful formation of new or young cells 

 within the mother-cell ; but although these processes have certain 

 features in common with the vacuolating vessel-forming cell, they are 

 in other respects unlike it. Again, we have a different process in 

 the pathological vacuolation of cells, as we have shown it, for 

 example, in the sweat-glands in leprosy, in which condition there is 

 no proliferation or formation of new cells, but a cavity is formed 

 between the nucleus and the cell protoj)lasm, which increases until 

 it bursts. This vacuolation seems to have the effect of separating 

 the nucleus from the rest of the cell substance, and thus leads to 

 death of the individual cell. In such cases, the nucleus may be 

 either compressed against the cell protoplasm forming the wall of 

 the vacuole, appearing like the seal of a signet ring when viewed 

 edgeways, and almost normal in shape when viewed from the front, 

 or it may appear distorted and floating loosely in the fluid of the 

 vacuole. 



With neither of the above processes does vacuolation of the 

 vessel-forming cell appear to be identical, although we have some- 

 times appearances shown apparently analogous with both. Thus 

 in g. Fig. 12> the nucleus appears to be separated from the cell 

 protoplasm, and floating loosely hke a distorted blood-corpuscle 

 within the fluid of the vacuole. On the other hand, in a, Fig. 3, 

 we have several bodies floating within the fluid of the vacuole, but 

 they are far too minute to be mistaken for blood-corj)nscles. It 

 has also been suggested that vacuolation is merely the formation of 

 fat within cells ; but this is certainly not the case with the vessel- 

 forming cells, or indeed with any other vacuolating cell we are 

 acquainted with. Apart from the fact which we have shown, that 

 osmic acid blackens the fat formed in cells and leaves the vacuolar 

 fluid transparent, we have also ascertained that in a developing fat- 

 cel] the nucleus is always surrounded by the protoplasm, however 

 thin the layer may be. The fat is therefore formed in the proto- 

 plasmic substance itself, and not between it and the nucleus, which, 



