[VINCENT & THOMPSON] THE ISLETS OF LANGERHANS 281 
appearance. ‘As pointed out by Laguesse, they are resistent to the 
injurious effects of the fixing agents and so are well-preserved. 
The outer row consists of two cells, for the most part faintly 
stained and vacuolated (see Fig. 2d). They are frequently shrunken 
into a sub-globular form. Two kinds of cells are readily distinguish- 
able in this layer. The greater number are of the variety just men- 
tioned, while others are better preserved and stained more deeply, owing 
to the presence of very fine granules. (See Fig. 2 in the lower right 
hand part d). The cells are irregularly cubical, polyhedral or prisma- 
tic in form. 
In Flemming and iron-hematoxylin preparations, the outer row of 
cells appear generally darker than the inner row. This corresponds 
with what Helly describes in the same species, the outer row being called 
by him “ Langerhans’ cells.” The cells of the islets are occasionally 
vacuolated and have here and there darker cells scattered among them 
(see Fig. 2 i). 
In addition to these elements, several elongated oval nuclei can be 
seen which appear to belong to cells of the type of those of the inner 
layer, which are squeezed in between cells of the outer layer. 
As in Scyllium, and even more abundantly, we find solid masses of 
cells in the neighbourhood of the ductules (Fig. 2 i). These consist of 
cells of the same kind as those in the outer layer of the duct epithelium. 
Several of these cells are shrunken within the delicate wall of connective 
tissue which surrounds and separates them. These do not differ in 
any important respect from the corresponding structures in Scyllium. 
Some of them have a shreddy reticular appearance. Among them are 
some examples of the second kind of cell of the outer layer of the duct, 
viz., those whose cytoplasm is well preserved and takes on a dark stain. 
(See Fig. 2 1.) 
There is a large amount of adenoid or splenic tissue in the pancreas 
of mustelus (see Fig. 2, ad.). 
The intimate relation between duct and secreting cavity is of such 
a nature that it is not always possible to draw any hard and fast line 
between the individual cells of the two structures. This has been 
pointed out by Laguesse in the case of the ophidians and the teleostean 
fishes. The developmental and structural relationship between ductules 
and “clear areas,” or islets in elasmobranchs has been fully recognized 
by Diamare, Laguesse, Helly and other observers. The work of 
Laguesse, Dale, and ourselves upon the islets of various groups of 
vertebrates has shown a similar relationship between islets and zymo- 
genous tubules. In elasmobranchs, and especially in mustelus, we see 
exemplified in a very interesting manner a primitive state of affairs in 
