VIVIPAROUS FISHES OF THE PACIFIC COAST. 
449 
between the mesoderm and ectoderm. In the anterior region the cells of the neurula 
(fig. 65) are seen to radiate from the dorsal depression. 
The ectoderm is everywhere about three cells deep. The outermost layer is dif- 
ferentiated into a flattened epithelium. 
While the relation of the head end of the embryo to the blastopore can not be 
certainly decided, it seems probable that at this period the embryo encircles consid- 
erably more than half a circumference of the yolk. The length of the embryo is now 
increased, chiefly at the expense of the caudal mass of cells. 
Anatomy of an embryo with three yrotovertebrce . — Of this stage I have two good 
series of sections. One of the embryos was cut in the sagittal plane, the other at right 
angles to it. The former was figured (figs. 5, 6) in my account of the sex cells, but 
the anterior was mistaken for the posterior. 
The entoderm in this stage forms a layer over nearly the entire yolk. The only 
region where it could not be distinguished was over the small portion of yolk between 
the caudal mass of cells and the head of the embryo. In the anterior region it grad- 
ually merges into the one-cell-deep mesoderm. Along the axial line, beneath the 
notochord, the entoderm is about two cells deep. Just to one side of this median line 
the entoderm is three or four cells deep; at the sides of the yolk it dwindles to an 
attenuated layer but one cell deep (figs 72, 73), and along the ventral line it is merged 
with the mesoderm. In longitudinal sections the entoderm is still evident, even in this 
region (fig. 67). Just in front of the caudal region, where the three layers merge into 
each other, the entoderm has a columnar arrangement and is raised some distance from 
the yolk (fig. 68 kv). This is the first indication of Kupffer’s vesicle. In cross-section 
the raised regionis seen to be quite wide, with the outer angle projecting upward and 
outward (fig. 73). The further development of Kupffer’s vesicle will be described in 
another chapter. 
The principal difference between the entoderm of Cymatogaster and other teleosts 
during this stage lies in the fact that it is composed of several layers of cells and that 
it covers the greater part if not the whole of the yolk. 
The notochord is well formed and has assumed its final outline back to the neuren- 
teric region, where its outlines merge into the general mass. In front it tapers to a 
point. The notochordal cells are as yet but little different from the cells of the other 
structures. The outlines of the nuclei and of the cell itself are a little more prominent 
and the cells are anteroposteriorly compressed. This gives their nuclei in the sag- 
gittal section (67) vertical oval outlines and gives the impression that the cells have 
a vertical columnar arrangement which they do not in reality have. 
The mesoderm consists of a single layer of cells in the anterior cephalic region 
(fig. 71). It rapidly thickens backward on either side of the notochord and neural 
ridge (fig. 70). The cephalic region, or the region in front of the first protovertebrae, 
is about the third of the entire length. There is no cavity in the protovertebrae, but 
the nuclei are arranged in an epithelial manner around a central region. The three 
proto vertebrae extend over one-fifth of the length of the embryo. In the embryo cut 
in a sagittal direction the mesoderm just behind the third protovertebra contains 
numerous sex cells; behind these the mesoderm disappears as a distinct layer. From 
the thickened masses along the notochord the mesoderm extends over the yolk in 
all directions, thinning out to two and further laterally to a single layer of cells, the 
two sheets meeting along the ventral line of the embryo. It is only in cross- sections 
F. C. B. 1892 29 
