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 (PI. XXV. 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-Cavity. — 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 ^ 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, T 5 ¥ 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. 



