444 



LECTURE XVIII. 



changes from the division of the nucleolus to the final 

 division of the cell. If an epithelial cell acquires two 

 nuclei, divides and this process is repeated, a long series 

 of developmental changes may, by means of a continual 

 repetition, be produced. If the skin becomes irritated 

 in consequence of continued friction, and the irritation 

 is increased to a certain point, the epithelium will 

 thicken, and if the proliferation is very energetic, it may 

 lead to the production of tolerably large tumour-like 

 formations. The same mode of development which is 

 presented by layers of epithelium, we meet with also in 

 the interior of organs. In cartilage, for example, where 

 the individual cellular elements are inclosed in an inter- 

 cellular substance, the place of each of them is at 

 last occupied by an accumulation of numerous cells, 

 the whole group, like the cell from which it proceeded, 

 being shut off from its neighbors by the intercellular 

 substance. This mode of development, therefore, is 

 one which, though very simple in itself, may, since it 

 originates in dissimilar parts, produce very different 

 results. 



But we have besides another class of new-formations 



Fig. 125. Endogenous new formation ; cells containing vesicles (physaliphores). 



A. From the thymus gland of a new-bom infant together with epithelioid cells: in 

 the interior of a vesicle which has a double contour (more distinctly marked in G) 

 and is besides surrounded by a cell-like border, lies a perfect nucleated cell. 



B. C. Cancer-cells (Cf. Archiv. fur pathol. Anatomie, Vol. I., PI. II., and Vol. III. 

 PL II.) B, one with two nuclei ; <7, one with a physalid which nearly fills tne 

 whole cell and another, where the physalid (brood-cavity) again encloses a perfect 

 nucleated cell. 300 diameters. 



