226 PROFESSOR FRANK J. COLE 
from the second afferent branchial and broke up on the wall of the corresponding 
efferent gill duct. 
I have never found the vascular papille entirely absent, but they may be very 
greatly reduced and almost all of them imperforate, in which case the peribranchial sacs 
contain less blood than usual. On the other hand, in those cases where the papille are 
numerous and well developed, the pleural sacs are always rich in blood. 
Apart from the fact that sections of injected material demonstrate in the clearest 
manner that the vascular papillee may place the cavity of the artery in communication 
with the surrounding “‘ lymph” space, we find in many cases which have not been 
injected a papilla capped by a small blood clot. This clot in favourable cases will be 
found to be directly continuous with the blood coagulum in the interior of the papilla, 
and this in its turn with the contents of the arterial trunk itself On the other hand, 
in many cases it is equally certain that the papille are quite closed, and represent 
merely small evaginations of the arterial wall. Occasionally one finds blood corpuscles 
in the tubules, in their external openings, and just outside those openings. 
In injections the mass always passes readily into the basal cavity of the papilla, and 
often into the proximal ends of the tubules; but it does not always travel along the whole 
length of the tubule, and so to the exterior, or, if it does, the tubule, being elastic, con- 
tracts and empties itself when the pressure is relaxed. When such a papilla is examined 
-as a whole preparation, the uninjected distal portion of the tubule is visible as the 
direct continuation of the injected proximal portion. 
A striking feature is that the tubules leading from the large space at the base of the 
vascular papilla to its apex, and it may be opening there, are lined by a well-marked 
epithelium. This epithelium is confined to the tubule, and its extent is indicated by 
the dots in figs. 2 to 5. Peripherally, it does not extend beyond the external opening 
of the tubule, nor does it line the large blood space at the base of the papilla. The cells 
are shallow, but the nuclei are large and deeply staining, and reach from one extremity 
of the cell to the other. 
The following variations in the structure of the papillae may be recorded :— 
1. The tubules traversing the distal end of a papilla may’ be so complex, as shown 
by i as to almost deserve to be referred to as a rete mirabile. : 
. A papilla may consist of a pear-shaped mass, with its narrow proximal end ex- 
rareet into the usual blood space in free communication with the cavity of the artery. 
At the expanded distal end is a large cavity with very little obvious contents, and 
having no communication with the exterior, but which is connected with the proximal 
blood space by one or more tubules. In other cases the distal cavity was lined by 
epithelium and possessed undoubted openings on to the exterior. 
3. In some of the gelatine injections the papilla was pear-shaped, its enlarged distal 
extremity having a spongy consistency. From the proximal blood space a number of — 
fine anastomosing tubules were given off, traversing the spongy mass, and some of them 
opening by fine pores on to the exterior. 
