564 
ME. E. C. BABEE ON THE MINUTE ANATOMY 
such an extent that they are only separated from the cavity of the vesicle by a thin mem- 
brane (figs. 12, c 8c 9, e). They lie, as it were, in a cup formed by the flattened epithelial 
cells. 
4. In other cases the parenchymatous cells are seen lying in a cup or hollow formed 
by the epithelial cells, and in which the flattened epithelial cells, or the membrane, sepa- 
rating the parenchymatous cells from the cavity of the vesicle have given way, thus placing 
the parenchymatous cells in direct communication with the contents of the vesicle 
(figs. 12,5, 13,5, & 14, b). In this, as in all other situations described, the parenchy- 
matous cells may be single or several together, apparently in a common cavity. 
Lastly, rounded cavities or cups, similar to those shown in figs. 12, 13, & 14, whose 
walls are formed by flattened epithelial cells, and which communicate freely with the 
vesicle, may be frequently seen in an empty condition. 
The simple and, as it appears to me, the only reasonable interpretation of the above 
facts is this : — The parenchymatous cells, either in groups or singly, lie originally in the 
intervesicular tissue, outside the epithelial walls of the vesicles. They begin to make 
their way into a neighbouring vesicle by pressing on the epithelial wall of one with which 
they are in contact. This pressure exerted by a parenchymatous cell causes the epi- 
thelial cells to lose their quadrilateral outline, and to present a more or less triangular 
or crescentic appearance. The pressure continuing, the epithelial cells become flattened, 
and finally either separate at once (figs. 13, 5 & 14, 5) or first become reduced to a thin 
membrane, which then gives way (fig. 12, 5). In either case the result is the same. The 
parenchymatous cell is placed in direct communication with the contents of the vesicle, 
and can escape into the cavity of the same. And that it does so is shown by the fact 
that these cups or hollows formed by flattened epithelial cells are frequently seen in an 
empty condition, having lost their parenchymatous cells. 
To sum up this process : it appears that the cells forming the parenchyma of the 
thyroid gland of the dog originate in the intervesicular tissue external to the walls of the 
vesicles ; from some cause ( inherent movement , or growth of cells behind them ) they exert 
pressure on the wall of an adjoining vesicle ; the cells of this become flattened out or 
absorbed , and finally give way , allowing the parenchymatous cells to escape into the cavity , 
and to form a constituent of the contents , of the vesicle ; but what part these cells take 
in the formation of the contents of the vesicles I am unable to state. 
The distribution of parenchyma through the gland appears tolerably uniform, though 
probably its cells are more numerous between the small than between the large vesicles. 
As above mentioned, in some places they are numerous and form groups, whereas in 
others they appear only thinly scattered on the walls of the vesicles. As I have 
observed them in all thyroid glands of the dog that I have carefully examined, they 
must be regarded as normal and constant structures. 
The parenchymatous cells can usually without difficulty be distinguished from the 
small vesicles, which they frequently resemble in size. The former are less highly 
refractive, and possess a nucleus as above described. In fig. 15 the two are seen repre- 
sented side by side. 
