CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 265 
in the region shown in Figure 105, it fills the entire interior of the 
section. 
In Figure 106 (Plate XIII.) is shown a section through an embryo 
in which the tail is already recognizable as a distinot portion of the 
embryo, though it has not yet reached anything like its maximum length. 
It is curved ventrad under the trunk, so that the section passes 
transversely through both trunk and tail. The section passes through 
the trunk in the brain region, but intersects only one of the mesenchyme 
bands, the other one not extending so far forward in the embryo. The 
endoderm cells are seen to have arranged themselves round a potential 
lumen in the form of an epithelium. However, they still lie two deep 
in places. Their shape is clearly becoming columnar. 
In the tail region appears the chorda, now transformed into a single 
row of flattened, disk-shaped cells, rapidly becoming vacuolated. They 
form an axial rod extending through the entire tail region and the pos- 
terior portion of tho trunk, Dorsal (right in the Figure) to the chorda 
lies the nerve cord of the tail, composed in cross section of about four 
small cells. 
Ventral to the chorda is the sub-chordal endoderm strand consisting 
of a double row of cells (en'drm.). On each side of the chorda are seen 
in the section about three muscle cells. 
SUMMARY ON FORMATION OF THE LARVA, 
l. The nerve cord in the limited region of concrescence of the lips of 
the blastopore is covered over by the ectoderm first at its posterior end 
and then successively in its more anterior regions, following the course 
of concrescence. The nerve cells in that portion of the embryo never 
form a real canal, but only a potentíal one. "They are arranged in a 
solid strand, which usually shows in cross section four cells placed round 
à common centre, the potential canal, 
The medullary plate arises wholly anterior to the blastopore. At the 
time when the blastopore is about to close, the medullary plate has come 
to extend over a great part of the length of the embryo, and has sunk 
down in the form of a shallow grove deepest at its posterior end, the 
anterior margin of the blastopore. When the blastopore closes, it be- 
gins to form a canal. This process, like the fusion of the lateral mar- 
gins of the blastopore, advances from behind forward. 
2. Beginning shortly before the closure of the blastopore, a rapid 
elongation of the embryo takes place, accompanied by a considerable 
change m its form and a rearrangement, of the cells composing some of 
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