58 ASCIDIANS 



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, 

 Tt.c) 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, n.e.c). 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, c/t), 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, /), 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 (aii) 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 ciliated 

 diverticulum of the anterior end of the enteric cavity (future 

 pharynx) which enters into close relations with the front of the 

 cerebral vesicle, 1 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 



1 Possibly the diverticulum may be wholly derived from the neural tube (see 

 Willey, Quart. J. Micr. Sci. 1893). 



