" 
68 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
extending forward (fig. 4, ch.). This band separates off 
from the hypoblast, which closes in beneath it, and thus 
the notochord is formed (fig. 5, ch.). Thesame cells further 
laterally and posteriorly become mesoblast, and separate 
off as lateral plates which show no trace of metameric 
seomentation (fig. 7a, m.b.). The remainder of the archen- 
teron will become the branchial sac, and by further growth 
bud off the rest of the alimentary canal. 
The medullary groove now becomes converted into the 
closed neural canal by the growing up and arching inwards 
(fig. 7a, n.c.) of the laminee dorsales, which unite with one 
another from behind forwards in such a way that the 
blastopore now opens from the enteron into the floor of 
the neural canal, forming the neurenteric passage (fig. 5, 
m.e.c.). Hor a time the anterior end of the neural canal 
remains open as a neuropore. The posterior end of the 
body is now elongating to form a tail, and the embryo is 
rapidly acquiring the tadpole shape (fig. 6) characteristic 
of the free larva. 
The tail grows rapidly, curves round the body, and also 
undergoes torsion so that its dorsal surface comes to lie 
on the left side. It contains ectoderm cells on its surface, 
notochordal cells (in single file) up the centre (see fig. 7, 
n.ch.), a neural canal dorsally, and a row of endo- 
derm cells (hy.) representing the enteron ventrally to the 
notochord. Later on the mesoblast also is prolonged into 
the tail where it forms a band of striated muscle-cells at 
each side of the notochord. When the ectoderm cells 
begin to secrete the cuticular test 1t forms two delicate 
transparent longitudinal (dorsal and ventral) fins in the 
tail (fig. 7), and especially at its extremity where radial 
thickenings form striz resembling fin rays. The ecto- 
derm on the anterior end of the body grows out into three 
adhering papillee (figs. 8 and 9), 
