858 
GEORGE C. PRICE 
or again, the duct may be continuous and the lumen not, owing to 
the duct having become solid in one or more places. Different sec- 
tions through a head kidney show great variation in both the size 
and shape of the duct. In the youngest specimens studied the duct, 
like the tubules, was lined by a single layer of columnar epithelium, 
and this is true for the most part also in che adult. But in places 
the wall of the duct ma> become greatly thickened by an increase 
in the number of epithelial cells. These change their shape, an d be- 
come much more loosely arranged, so that the tissue loses entirely 
the structure of columnar epithelium. The distribution of this 
cissue along the duct is very variable. It increases somewhat in 
amount with age. 
In all specimens studied, even in the youngest, openings were 
found in the w^all of the duct (fig. 2). These place the lumen of 
the duct in direct communication with the venous diverticulum 
with which the head kidney is connected. These openings are 
natural and not artificial. A single opening may extend through 
as many as a dozen sections, though usually they are much smaller 
and extend through not more than one or two sections. The num- 
ber in a single duct varies from two or three to ten or twelve. The 
fact has already been mentioned that the tubules open on the 
one hand into the pericardial cavity and on the other into the 
duct. Thus it will readily be seen that the tubules, the duct and 
the openings in the wall of the duct form a direct connection be- 
tween the pericardial cavity and the circulatory system. 
Lying along the median side of the head kidney, toward the 
posteiior end, there is a glomerulus, the origin of which has al- 
ready been described. This, like other parts of the head kidney, 
is subject to much variation. Sometimes it is about as long as 
it is wide and does not extend so far posteriorly as tbe rest of the 
organ; and again it may be two or three times as long as it is 
wide and extend beyond the posterior end. Usually it is a single 
body; but it may show indications of having been formed by 
the fusion of tWO or more glomeruli. Occasionally there may be 
two or firee distinct or almost distinct glomeruli lying close 
together, one after the other. In sections it presents the same 
appearance as the glomeruli of the functional kidney, with which 
embryology shows it to be strictly homologous. 
