STUDIES ON THE PEOTOOHORDATA, 331 



EXPLANATION OF PLATES 18, 19, and 20, 



Illustrating Mr. Arthur Willey's paper, " Studies on the 

 Protochordata." 



Letters for Plates 18 and 19. 

 ant. p. Anterior opening of neuro-hypophysial canal into branchial sac. 

 at. Atrial involution, br. s. Branchial sac. cer. ves. Cerebral vesicle. 

 cer. ves. res. Histolytic residua of cerebral vesicle, e. Eye, or eye-tract. 

 eet. Ectoderm. end. Endostyle. ent. Entoderm, ent. c. Enteric cavity. 

 ep. r. Epibranchial ridge, g. Cerebral ganglion, gl. Subneural gland, hyp.f. 

 Funnel of hypophysis, int. Intestine, m. Mouth, mes. Mesoderm, n. c. 

 Neural canal, nch. Notochord. n. hyp. Neuro-hypophysial canal, n. p. 

 Neuropore. ot. Otocyst, or otolith, post. p. Posterior opening of neuro- 

 hypophysial canal into cerebral vesicle, st. Stomodseum. t. Remains of 

 tail. vac. Vacuolar spaces in cells of optic region, vise. g. Visceral ganglion 

 (Rumpfganglion of Kowalevsky). 



PLATE 18. 



Figures 1 — 5 relate to Ascidia mentula, and were drawn from living 

 object with camera lucida. 



Fig, 1, — Optical section of young embryo with closed nervous system, 

 showing first appearance of pigment-granules of eye in dorsal wall of brain, 

 Zeiss, 3, C. 



Fig. 2, — Older embryo, to show depth of stomodseal invagination. Behind 

 cerebral vesicle is seen the right atrial involution, Zeiss, 3, C. 



Fig, 3, — Cerebral vesicle of young embryo (a trifle older than that shown 

 in Fig. 1), to show primary relation of otocyst to eye-tract. Zeiss, 4, D. 



Figs. 4 and 5, — Cerebral vesicles of somewhat older embryos in optical 

 section, to show stages in the migration of the otocyst. Zeiss, 3, D. 



Figures 6 — 24 relate to Ciona intestinalis. 



Figs. 6 — 8, — Transverse sections through cerebral vesicle of newly hatched 

 larva. Fig. 6 shows the neuro-hypophysial canal in left wall (right of figure) 

 of vesicle. Pig, 7 shows its communication with cavity of vesicle. Fig. 8 

 shows cerebral vesicle posterior to neuro-hypophysial region, Zeiss, 3, E, 



Fig, 9, — Transverse section through an older larva of the age of that 

 figured in I, Plate XXX, fig, 1 (this Journal, Jan., 1893), 



Fig. 10 shows posterior communication of neuro-hypophysial canal with 

 cerebral vesicle — farther back than in Fig, 7. Zeiss, 3, E. 



