456 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
embryo. The lumen in this region appears when the embryo is 3-2 mm. long and has 
about 22 somites. In the formation of the gill the hypoblast extends out to the epi 
blast without any invagination of the latter. The intestine in the anterior portion is 
formed as I have described it for Gymatogaster. Towards the middle the intestine is 
said to be formed in a different manner, the hypoblast being raised in the middle line 
to form a canal bounded below by the periblast. 
McIntosh and Prince find that in the embryo whose optic vesicles are in process 
of formation the hypoblast is a thin sheet over the entire ventral surface save at 
the posterior extremity. Later it becomes a massive cylinder and the oral tract a wide 
flattened sheet of hypoblast, the pharynx being a separate and later formation than 
the mesenteron proper. They consider Kupffer’s vesicle as the first indication of the 
alimentary tube, and it is only in this region that the alimentary tract is ever open to 
the yolk below. The hypoblast reaches as far as the cardiac region, where it thins out. 
With the thickening of the hypoblast the embryo is raised from the yolk. Beneath 
the eyes the hypoblast becomes thickened as two longitudinal ridges. The ventral 
wall is formed by hypoblast cells pushed in from the side, or of periblast cells. They 
seem to think that the periblast contributes in some degree to budd up the entire 
mesenteron. The mid and fore portions are said to form a dense cord, in which a 
lumen appears later by a forward extension of the posterior enteric chamber. In other 
words, the lumen is formed from behind forward, the mouth and anus being the last 
parts formed. The oesophageal portion they found ciliated in later stages. On the 
thirteenth day the alimentary tract of the gurnard is differentiated into the following- 
regions: (1) Oral chamber large, but depressed. (2) A wide (esophagus, the lumen of 
which is a horizontal fissure; from this part the pneumatic duct is given off. (3) An 
enlarged stomach with the hepatic mass below. (4) A pyloric portion into which the 
ductus choledochus passes. (5) The intestine. 
Wilson, 1890, found that the alimentary canal was formed u by a process of folding, 
essentially akin to that- found in amniota. After the formation of the notochord the 
entoderm is one cell thick, the cells being uniform. In the trunk region the cells 
become thicker and a fold rises up at this point, the two sides being separated by a slight 
slit. In the anterior region there is a fold on each side which grows up and forms the 
embryonic gill-slits. In the posterior part of the trunk the entoderm becomes thick- 
ened along the median line” but is not raised up in a fold ; this region is transformed 
into the postanal gut. 
Kupffer’s vesicle. — Kupffer’s vesicle, as has been hinted in the preceding pages, 
has a remarkable history in Gymatogaster. It arises as an up-pushing of the hypo- 
blast which has previously become columnar, in the posterior part of the embryo 
when about three protovertebrae have been formed (figs. 67, 68, 70, 73). It later devel 
ops a floor of hypoblast as usual among fishes. But here comparison must stop. While 
in teleosts in general it is quite a minute structure* and entirely disappears behind the 
region of the anus in some teleosts, in Gymatogaster it becomes frequently half the length 
of the yolk into which it" pushes or eats a large pit and finally is divided into three dis- 
tinct vesicles, each one of which is larger than the ordinary Kupffer’s vesicle, and each 
of which has a different history and fate. The middle one remains as an enlargement 
*In Ctenolabrus it measures 0 - 03 mm. during its largest stage. In Cymatogaster it reaches a length 
of 0T3 mm., which is about half the length of the yolk. Its maximum height, exclusive of the depres- 
sion formed in the yolk, is about 0-05 mm. 
