462 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
becomes narrower and higher, and the cells at the same time lose their great height 
and their cilia; just behind the gill swelling the lumen is subcircular or horizontally 
oval. Its walls are composed of a single layer of cells but 5 pi high. Still farther 
back the lumen becomes vertically oval, and the cells of the floor lose their columnar 
nature and multiply so that a thickened floor is formed. The change in shape of the 
lumen (159 and 160) is due to a longitudinal groove, shallow in front, becoming deeper 
behind, abruptly stopping still farther back just in front of the hind gut. Behind this 
place the lumen is again contracted. This ventral groove and thickening of the floor 
is the rudiment of the liver. The region through which this groove extends is quite 
extensive, reaching through 19 sections of the 85, making up the entire length of the 
intestine, or through almost the entire region from the ciliated gullet to the hind gut 
(fig. 162). A short distance behind the pronephros another thickening appears in the 
walls of the intestine, this time in the roof, which is at the same time extended later- 
ally (fig. 160). This upper thickening is not so extensive, reaching through 10 sec- 
tions and extending in the early condition as far back as the lower mass. This upper 
thickening is the rudiment of the air bladder. The mesoderm surrounding the ali- 
mentary tract in these regions is also considerably thicker than in other regions. At 
the beginning of the ventral groove the lumen of the intestine has a diameter of 45//, 
the cells a height of 9 //, At the end of the groove the lumen has a diameter of 68 ju, 
and the cells lining it have a height of 13//. Just behind the groove the height of 
the lumen is again reduced to 45/./. From this point the intestine rapidly widens till 
it reaches a diameter of 160// (fig. 161). The cells lining this portion are 22// high. 
Just in front of the anus the lumen becomes a vertical slit and the inner layer of the 
ectoderm becomes reinforced along the ventral line to form a keel. For four sections 
the anus is a vertical slit, the sides of which flare outward below. Behind the slit it 
is continued as a median groove with equally flaring sides, on the crests of which the 
nephridial ducts empty. On either side of the vertical portion of the intestine, just 
in front of the anus, the splanchnic and the somatic mesoblast are united in a solid 
mass in which the sex cells are embedded. Behind the anus a short, solid cord of 
hypoblast extends into the tail, between the aorta and the caudal vein. In older 
larvae the ridges behind the anus meet in the median line and the segmental ducts 
thus come to empty in the median line. The relations of the liver to the air-bladder 
in older larvae may be gathered from figs. 162 and 163. Their further development in a 
larva 2v5 mm. long is shown in fig. 165, where the gall-bladder has begun to develop. 
In larvae a little over 3 mm. long, the thyroid gland aud the hypophysis are well 
along in their development. These larvae measured 3-2 mm. after hardening, and 
were probably longer in life. The intestine (figs. 166 to 174) has not changed much 
except in the regions of the gills. The lumen extends forward to in front of the 
thyroid gland. Anterior to this point the two layers of hypoblast cells are still in 
contact with each other. The mouth has, however, become evident laterally, as will 
be seen from fig. 169. The anterior opening of the intestine is still the hyobranchial 
gill-slit. The ciliated gullet has now reached its full development. In the entire gill 
region the intestine is depressed; its width decreases in this region from 181// to 
112. Behind the gullet it dwindles to a diameter of but 22//; in the hind gut it 
reaches 136. In the anterior gill region the walls of the enteron are less than 4// 
thick aud consist of a layer of pavement cells unquestionably of hypoblast origin and 
of a layer of very thin cells (fig. 172). This thinner layer I take to be ectodermal cells 
