332 ARTHUR WILLET. 



Fig. 11. — Transverse section through cerebral vesicle of newly fixed larva. 

 Shows disintegration of the vesicle, and complete separation of neuro- 

 hypophysial canal. Zeiss, 3, E. 



Fig. 12. — Sagittal section through neuro-hypophysial system of same stage 

 as preceding, showing first appearance of adult ganglion as a proliferation in 

 the dorsal wall of the tube. Zeiss, 3, J, water immersion. 



Fig. 13. — Entire section, of which Fig. 12 was a part, to show topographi- 

 cal relations. Zeiss, 3, C. 



Fig. 14. — Sagittal section through more advanced neuro-hypophysial sys- 

 tem. Posteriorly in the region of the ganglion the section is tangential. 

 The nervous system proper has become solid. Zeiss, 3, J, water immersion. 



Figs. 15 — 19. — Transverse sections through neuro-hypophysial system, 

 slightly older than preceding. Fig. 15 is the most posterior section, and 

 passes through the hinder portion of the ganglion or the anterior extremity 

 of the " cordon ganglionnaire visceral," which is now solid. Figs. 16 — 18 

 show the ganglion developing from the dorsal wall of the neuro-hypophysial 

 canal. In Fig. 18 the remains of the eye are involved in the section. Fig. 19 

 passes through the anterior extremity of the ganglion, which overlaps the 

 funnel of the hypophysis. Zeiss, 4, J, water immersion. 



Figs. 20 — 24. — Transverse sections through the hypophysis and ganglion 

 of young immature adult. Fig. 20 is the most anterior section, passing 

 through funnel of hypophysis and two cerebral nerves. Fig. 21 is taken 

 slightly further back, aud Fig. 22 passes through the point at which the lumen 

 of the hypophysis is temporarily obliterated by mutual approximation of the 

 cells forming its wall. Fig. 23 shows the lumen widening out again posterior 

 to this point, and finally Fig. 24 shows the origin of the glandular portion of 

 the hypophysis by cell-proliferation from its ventral wall in its posterior 

 portion. Zeiss, 2, J, water immersion. 



PLATE 19. 



All the figures on this plate relate to Clavelina lepadiformis, and all 

 represent transverse sections. 



j'lg, 25. — Stage I. Through cerebral region of very young embryo, of an 

 age corresponding to that shown in Plate 18, fig. 1. Shows transversely 

 elongated lumen of neural tube in this region. 3, D. 



Figs. 26— 29.— Stage II. Fig. 26, through ueuropore ; 3, J. Fig. 27, 

 just behind neuropore. Fig. 28, through neuro-hypophysial region. Fig. 29, 

 posterior to this. 2, D. 



Fig. 30. — Intermediate between Stages I and II, showing otocyst in right 

 (left of the figure) wall of cerebral vesicle. 3, D. 



Figs. 31 and 32. — Stage III. Fig. 31, through region of otocyst, shows 

 increase in size of cerebral vesicle. Fig. 32 shows the communication between 

 the hypophysial and cerebral portions of the nervous system. 3, D. 



