[VINCENT & THOMPSON] THE ISLETS OF LANGERHANS 283 
Many of the light islets are distinctly marked out by their wide 
capillary network (Figs. 3 and 4, cap. bld. c) in the peripheral zone, 
and by the absence of zymogenous granules. A large number of the 
islets, and a distinctly larger proportion than is common in other elas- 
mobranchs, appear to be independent of, or, at any rate, separated from 
the ductules. 
In some preparations (Flemming’s fluid and iron-hæmatoxylin) 
the islets and ductules stand out fairly prominently (Fig. 4), but the 
former are of a mean darker tint than that of the zymogenous tubules, 
with the exception of the zymogenous granules themselves which appear 
-almost jet black (See Fig. 4, zym. gran.). The islets brought out in 
this way (Fig. 4 i) very strongly resemble the corresponding structures 
in the ophidians (q. v.). They consist of solid masses of irregular, 
spindle-shaped, polyhedral, or ovoidal cells, whose nuclei are larger than 
those of the surrounding zymogenous tissue and whose protoplasm is 
stained a pinkish sepia colour (Fig. 47). This last is almost homo- 
geneous in appearance, or dotted with exceedingly fine granules. Some 
cells are stained more deeply than others. (Fig. 4 i.) 
_ Many of the islets, as above stated, do not show the same intimate 
relationship to ductules as is the case in other elasmobranchs. The 
islets in Rhina squatina, indeed, more strongly resemble the islets of 
higher vertebrates than do those in any other kind of cartilaginous fish 
we have examined. They have diverged further from the primitive 
type. 
Some capillaries are seen to penetrate the interior of the larger, 
more solid islets. (See Fig. 3, cap, bld. c.) 
The double row of cells in the ducts is not so common as, ¢.g., in 
scyllium and other species. Perhaps the most beautiful islet picture 
is shown in Flemming and safranin preparations and reveals a higher 
type of islet than in other species we have examined (see Fig. 3 i). 
It is subdivided into imperfect lobules by a rich capillary plexus (Fig. 
3, cap., bid. c). 
In Flemming and safranin, just as in Flemming and iron-hemat- 
oxylin preparations we obtain a more uniform tint of the protoplasm 
in islet than in zymogenous tissue (Fig. 3, i). 
Some of the islets consist almost entirely of the darkly-staining 
cells, others are largely made up of the lightly-staining cells. These 
last most closely resemble the islets of higher vertebrata (see Fig 3 i). 
In this species, as in Mustelus iaevis, in Flemming preparations 
one frequently sees the duct epithelium stained lightly and the islet cells 
darkly (Fig.4 d, i). There appear to be several varieties of cells in 
the islets of Rhina squatina and some of these appear to be transitional 
