266 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
its organs. The posterior end of the embryo, which toward the comple- 
tion of gastrulation was broader than the anterior end, becomes narrower 
and narrower, and ultimately forms the tail, which is curved ventrad 
around the trunk of the embryo within the egg membranes. 
Jefore the closure of the blastopore the chorda is a plate of cells lying 
in the dorsal wall of the archenteron, anterior and lateral to the blasto- 
pore. The portions lateral to the blastopore meet in the median plane 
when the blastopore closes. The chorda fundament then elongates, 
owing to a shoving together of its cells from each side, “Vike a pack of 
cards in shuffling” (Van Beneden et Julin), until they form, instead of 
a plate, a single median row of disk-shaped cells arranged one behind 
another like a row of coins and reaching backward underneath the nerve 
cord to the extreme posterior end of the embryo. Anteriorly it termi- 
nates not far from the middle of the trunk region. 
The muscle cells, which originally lay on each side of aud behind the 
blastopore, extend themselves a single cell deep in two bands, one on 
each side of the chorda throughout its entire length. 
The mesenchyme cells originally formed the lateral portions of the 
chorda-mesenehyme ring. As the blastopore gradually closed, they were 
thrust down to a deeper level than the muscle cells, and forward. Ulti- 
mately they come to lie wholly in the trunk region, chiefly in its pos- 
terior portion, in two pretty compact lateral masses of small deeply 
stained cells, two or more layers deep. At a still later period, these 
lateral masses are resolved into migratory cells, i. e. blood corpuscles, 
mantle cells, etc. 
Before the closure of the blastopore the endoderm forms the entire 
lining of the archenteron in its most anterior portion, where its lumen is 
almost obliterated. Farther back the chorda forms the dorsal wall of 
the archenteron, the mesenchyme cells forming its sides, the floor only 
being occupied by the endoderm cells. In the region w here the blastopore 
closes, the endoderm cells occur only as a double row ventrally situated 
along the median line. 
This double row is extended back in the larva underneath the chorda 
throughout almost the entire length of the tail, forming a “subchordal 
endoderm strand," which is ultimately resolved into wandering cells, 
or perhaps utilized as food material by the mesenchyme cells of the 
trunk region. At the posterior end of this caudal endoderm strand lie 
the small mesoderm cells which Van Beneden et Julin mistakenly 
included in the nervous fundament. These cells are to be regarded as 
the most posterior constituents of the original chorda-mesenchyme ring. 
