WHITE SUCKER 
167 
having stronger muscular walls, but sections show that there are no glands anywhere 
in the digestive tract except the liver and pancreas. The entire tract is lined 
with a rugose mucosa consisting of simple columnar epithelium. These rugae are 
coarse and high {}/i millimeter) at the beginning of the tract and reduce so as to be 
barely visible in the rectal region. They follow a zigzag course, running about 3 
millimeters in each oblique direction. 
Sections at any level are similar to those of any other region except for measure- 
ments. In the straight anterior region there are about 25 rugae, each about one-half 
a millimeter high. Below the epithelium is a densely cellular connective tissue. 
About one-tenth of a millimeter outward from the bottom of the rug* lies the hyaline 
stratum compactum, 20 microns in thickness. This is a dense nonnuclear layer of 
connective tissue fibers peculiar to the intestine of fishes. Immediately outside of 
it is the circular muscle layer, one-sixth millimeter thick, and outside of this is the 
longitudinal muscle layer, which is only one-quarter as thick as the layer of circular 
fibers. 
By studying a series of young fish from 9 to 25 millimeters in length, which had 
been rendered transparent by slow clearing in xylol, the changes in the form of the 
intestine could be seen with surprising clearness. (Figs. 40 to 47.) From hatching 
up to 17 millimeters the intestine is straight and occupies a median ventral position 
back of the level of the liver. 
It is at the critical period (17 millimeters), when the mouth is becoming inferior 
and the diet is changing, that the first loop of the intestine is formed. It is located 
