928 PROFESSOR W. C. M‘INTOSH AND MR E. E. PRINCE ON 
The urinary vesicle presents a series of boldly marked folds superiorly, and its walls 
are in contrast with the massive sheath of the rectum. The segmental tubes are thus 
not developed in the pronephros, but have advanced considerably forward from the 
metanephros. In the pronephros the coils of the duct amidst the cellular mass seem to 
take the place of the tubes behind. Like the interior of the chamber, the duct leaving 
the urinary vesicle (Pl. XXYV. fig. 6) is lined with columnar epithelium, and it opens on a 
special papilla. The differences between the anterior region of the urinary vesicle before 
and after the development of the segmental tubes is clearly shown by contrasting figs, 5 
and 7 of Pl. XXV. The development of the pigment on the wall of the organ (urinary 
vesicle) is also a noteworthy feature. 
Body-Cawity.—The form and capacity of this chamber varies very much in post-larval 
stages. It may constitute, as pointed out on a prior page, a huge depending sac, especially 
well seen in Pleuronectids. In the flounder, ;%; inch long, it is a thin-walled protruding 
pouch, anteriorly occupied by the bulky liver, while posteriorly the capacious intestine 
mainly fills up its cavity. Precisely the reverse condition obtains in the post-larval 
Clupeoid, ;°; inch long, the body-cavity appearing merely as a slit-like space between the 
lengthened intestine and the peritoneum in front, the liver passing beneath the alimentary 
canal as well as the large swim-bladder behind, and reducing the space very much. In 
the goby, too, at 3% inch, the body-cavity diminishes posteriorly so much as to form 
an interval barely perceptible between the intestine and the body-wall. Its anterior 
end may, as in a Gadoid, ;; inch long, pass beneath the pericardium, so that in section 
the ventricle of the heart occupies a position superior to the compact fore-end of the 
liver. This sub-pericardial protrusion of the body-cavity exhibits a thick muscular mass 
in its wall upon each side. The viscera in the post-larval wolf-fish and salmon appear to 
fill up the limited peritoneal chamber more completely than in the spacious and prominent 
cavity characteristic of such forms as the Pleuronectids and the ling. 
