262 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
to form the medullary plate. Those cells of this neuro-museular ring 
which lie on each side of and posterior to the blastopore are for the 
most part invaginated, and form the entire longitudinal musculaturo of 
the tail. Some of them, however, form the most posterior portion of 
the nerve cord. 
3. Lying just within the margin of the blastopore, and encircled by 
the neuro-museular ring, is another ring of cells, interrupted at the pos- 
terior end of the embryo only. Its anterior portion gives riso to tho 
greater part of tho chorda ; its remaining (lateral) portions produce the 
mesenchyme or trunk mesoderm, besides contributing to the chorda a 
single cell at each lateral margin of the blastopore. The descendants 
of these two chorda cells meet in the median plane at the closure of the 
blastopore. They form the most posterior portion of the chorda. 
We may regard the chorda-mesenchyme ring as being completed mor- 
phologieally by the two small sub-chordal mesoderm cells 078, [75 which 
have been wedeed in between the most posterior cells of the neuro- 
muscular ring. Like the other cells of the chorda-mesenchyme ring, 
they lie in contact with the endoderm cells ou one side, and with cells of 
the neuromuscular ring on the other. Ultimately they probably form 
mesenchyme in the tail region. Possibly by a eoenogenetis reduction 
in size to their present minute dimensions, a gap has been left on each 
side of the embryo between them and the lateral portions of the chorda- 
mesenchyme ring. This change may have attended a camogenotio 
lengthening of the posterior end of the organism to subserve locomotion. 
There is evidence from other sources that the trunk of Ascidians formerly 
extended farther back into what is now the tail region of the larva. At 
that time the mesenchyme also probably extended farther back, and the 
chorda-tmesenchyme fundament was in ontogeny, as we suppose it to 
have been in phylogeny, an uninterrupted ring. 
1, The blastopore, at first widely open, closes more rapidly from the 
anterior margin and from the sides than from behind. Consequently it 
comes to lie in the posterior portion of the dorsal surface of the embryo, 
and is triangular in form. The right and left sides of the triangular blas- 
topore, however, fuse from behind forward, beginning in the region of the 
pair of small, flattencd mesoderm cells, 076, D75, Alone the line of union 
of the lateral lips of tho blastopore lies superficially on cach side of the 
median plane a row of nervo cells. These are subsequently covered in 
by ectodorm from the sides and from behind, and form the posterior por- 
tion of the nerve cord. 
Underneath them, and at first not distinguishable 
from them in histological characters 
5, are other cells, likewise derived from 
