[VINCENT & THOMPSON] THE ISLETS OF LANGERHANS 279 
The appearance of the light or clear areas in the pancreas of elasmo- 
branchs is certainly at first sight very different from that of the islets 
cf Langerhans in any other group of vertebrates, and it is not surprising 
that Diamare was at first unwilling to admit their homology with the 
mammalian islets. But from a study of Diamare’s own drawings and 
the observations of Laguesse and ourselves, it seems clear that the groups 
of lightly stained cells arranged here and there round the smaller ducts 
of the elasmobranch pancreas do, as a matter of fact, represent the 
primitive type of islet structure in vertebrates. This is rendered more 
probable by the embryological work on mammals by Laguesse and Helly, 
though Helly, in our point of view, does not appear to have seen the 
full significance of his own observations. 
Scyliium canicula. 
In preparations fixed in a saturated solution of corrosive sublimate 
and stained with Delafield’s hematoxylin and eosin, or by Heidenhain’s 
iron-hematoxylin method, one readily sees large numbers of ducts of 
various sizes and numerous “ clear areas”’ in more or less intimate rela- 
tion to the ducts. It is noteworthy that it is much more difficult to 
see these structures in preparations fixed in Flemming’s fluid and 
stained with iron-hematoxylin, and by this method the “ clear areas ” 
are often as deeply tinted as the zymogenous tissue, and even sometimes 
more deeply.! The result in this respect varies somewhat according 
to the degree of differentiation in the iron-alum solution. 
The larger and medium-sized ducts have a fibrous coat of consider- 
able but varying thickness, and are lined with a layer of columnar cells 
of unequal height,? and having large oval nuclei. In the large ducts 
the outlines of the epithelium cells are not so distinct as in the smaller. 
The protoplasm has a homogeneous, “ ground-glass ” appearance. 
The smaller ducts (see Fig. 1) can usually be seen to consist 
roughly of two layers of cells, and, occasionally, for a part of the 
circumference, of three or more. But the cells appear often irregu- 
larly disposed and it is difficult to distinguish definitely the two rows. 
There is frequently little or no difference between the different cells 
forming the walls of the small ducts, though sometimes one can see 
here and there individual more darkly stained cells in the outer layer, 
1Compare with what has been said on the subject of the Ophidians 
(Supra), 
1 Laguesse refers to this appearance and quotes several other authors on 
the subject. Some of these attribute a definitely secretory function to the 
epithelium cells of the larger ducts. 
Sec IV., 1907. 17. 
