282 ROYAL SOCIETY OF CANADA 
which ductule, islet and zymogenous tubule reveal themselves as por- 
tions, variably modified, of the same morphological entity. Thus, the 
connection between “clear areas” or islets and ductules is obvious, or 
at any rate, very readily perceived, as pointed out above. The external 
layer of cells of the ductules form the primitive islets. The ductules 
can be seen to be gradually merged into the zymogenous tubules and 
occasionally typical zymogenous cells can be seen scattered among the 
cells of the ductules. Further, transitions are frequent here, as in the 
higher vertebrates, between the clear islets and the zymogenous ¢ells. 
These transitions occur naturally in the region where ductule with its 
masses of islet cells is becoming continuous with or merged into the 
zymogenous tubule. 
It is possible that the clear areas in elasmobranchs are strictly 
homologous with Laguesse’s primary islets. This would account for 
their comparative inconstancy. In some specimens they appear, in fact, 
to have disappeared, as do the original primary islets in mammals. 
Acanthias vulgaris. 
The most striking characteristic of the specimens of Acanthias we 
have examined is the very large proportion of splenic or hemolymph 
tissue which they present. 
The same general appearance as regards ductules and islets are 
presented as in other elasmobranchs. In this species, however, the 
abundant capillary supply to the clear areas is more obvious. 
Rhina squatina. 
Our material derived from this species has yielded remarkably 
beautiful preparations. In marked contrast with other elasmobranchs 
some of the smaller ductules show only a single row of columnar epithe- 
lium cells. Others, however, have a double row. As in other carti- 
laginous fishes the ductules and “clear areas” are more readily seen 
in preparations fixed in corrosive sublimate and stained with hematoxy- 
lin and eosin, than in Flemming preparations, though in the case of 
the latter the details are often weil brought out (see Figs. 3 and 4). 
In the ductules with the double row of celis some of the outer 
row stain deeply as in other elasmobranchs. Still others are apparently 
transitions to zymogenous cells. In this species also as in mustelus we 
can occasionally detect transitions between the clear areas (islets) out- 
side the ductules and the zymogenous tubules (see Fig. 3). In some 
cases certain of the cells in a zymogenous tubule are of the islet type 
(Fig. 4). 
