58 ASCIDIANS CHAP. 
becomes the branchial sac, and by further growth buds 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. 25, G, 
nc) of the medullary folds, 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. 25, F, nec). For a time the anterior 
end of the neural canal remains open as a neuropore. By this 
_time the posterior end is elongating to form a tail, and the 
embryo is acquiring the tadpole-shape (Fig. 25, H) 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, noto- 
chordal cells (in single file) up the centre (see Fig. 25, H, ch), a 
neural canal dorsally, and a row of endoderm cells 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 this forms two delicate 
transparent longitudinal (dorsal and ventral) fins in the tail (Fig. 
25, K, f), and especially at its extremity where radial thickenings 
form striae resembling fin-rays. The ectoderm on the anterior end 
of the body grows out into three adhering papillae (Fig. 26, A). 
The neural canal now differentiates into a tubular dorsal 
nervous system. The anterior end dilates to form the thin- 
walled cerebral vesicle (see Figs. 25, I, and 26, A), containing later 
the intra-cerebral, dorsal, pigmented eye (oc), and the ventral 
otolith (aw) of the larva. The next part of the canal thickens 
-to form the trunk-ganglion, and behind that is the more slender 
“spinal cord,” which runs to the extremity of the tail. A cilated 
diverticulum of the anterior end of the enteric cavity (future 
pharynx) which enters into close relations with the front of the 
cerebral vesicle,’ and later opens into the ectodermic invagination 
which forms the mouth at that spot, is evidently the rudiment of 
the neural duct or hypophysial canal. The future branchial sac 
(pharynx), with a ventral median thickening which will be the 
endostyle, is by this time clearly distinguishable by its large size 
' Possibly the diverticulum may be wholly derived from the neural tube (see 
Willey, Quart. J. Mier. Sci. 1893). 
