452 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
cells have become very different from the hypoblast beneath and have become re- 
arranged. In the caudal swelling- the two structures are not separated by any distinct 
line, and the difference between the cells of the future chorda and the gut are not so 
well differentiated. In the head the hypoblast underlying the chorda is quite thin; 
caudad it becomes gradually thicker and finally columnar in arrangement. This 
columnar region stains darker than the surrounding cells. In the posterior part of 
this region the columnar hypoblast is raised from the yolk to form a distinct arch, or 
rather, as is seen in transverse section, a transverse fold raised from the yolk (fig. 73). 
The outer edges of this fold are raised a little higher than the median portion, 
and it extends and gradually disappears backward. The floor of this arch is very 
largely formed by the yolk, its posterior extension entirely so. Below the arch there 
are a few cells which are very probably derived from its own roof, from which several 
cells project (figs. 68-73). This space is the first observed condition of Kupffer’s 
vesicle. Gross-sections show that at this time the hypoblast extends over the greater 
part of the yolk, if not entirely over it (figs. 71-74; hypoblast cells have nucleoli 
indicated). Near the middle it is several (5) cells deep over nearly half tbe circum- 
ference of the yolk. It is columnar only below and a short distance on either side of 
the chorda; latterly it becomes thicker, but thins out again to a thickness of one cell 
(fig. 72). The section extending through Kupffer’s vesicle shows that at its outer 
margins it extends as short pockets toward the ectoderm. In this fact it resembles 
very closely the conditions found later in the region of the forming gills, which arise 
as similar outward extensions from the alimentary tract. 
In an embryo just freed from the zona radiata, and which still greatly resembles 
the stage just described, nine protovertebrse are formed, and the development of the 
intestine has made great progress. The anterior part of the chorda is well formed, 
and the hypoblast below it is but one cell deep. Caudad the thickness of the hypo- 
blast increases till it is about five cells deep (fig. 75). In the anterior part, where the 
hypoblast is but one cell deep, evaginations extend upward and outward toward a 
point below the fundaments of the ear. These folds are of considerable cephalo- 
caudad extension, reaching in one embryo through nineteen cross-sections (figs. 109, 
115). These outpushings are for the most part solid at this stage. The peripheral 
cells of this structure (the gills) are columnar. The outer layer of cells extends some 
distance over the yolk. Immediately behind the gill region the method of the forma- 
tion of the floor of the intestine becomes evident (fig. 110). The upper cells of the 
hypoblast have a dorsoventral columnar arrangement. The lower cells, on the other 
hand, have the longer axes of their nuclei in a horizontal position, the edges of the 
hypoblast having become turned in. No space is at first evident between this 
lower and the upper layers, the infolding resulting in a solid bilateral mass of cells. 
The lateral extent of the hypoblast is no longer as great as it was in the preceding 
stage (figs. 110-112). Farther back, where the ingrowth to form the potential floor 
of the intestine does not take place until some time later, and where the intestine, as 
will be seen later, is very wide from the beginning, the hypoblast still extends over a 
considerable portion of the yolk (figs. 112, 113). The same process of forming the 
intestine takes place in this region later, i. e., the hypoblast becomes columnar and 
cells grow from the margin inward to form a layer between the yolk and the main 
mass of hypoblast cells without at once forming a lumen. 
