STUDIES ON THE PEOTOOHORDATA. 333 



Fig. 33. — Stage IV. Shows opening of neuro-bypopliysial canal into still 

 larger cerebral vesicle, between the region of the otocyst and of the eye. 

 3, D. 



Figs. 34 — 37. — Stage V. Series showing relation of neuro-hypophysial 

 canal to cerebral vesicle at this stage. Fig. 37 passes through the vesicle 

 behind the posterior opening of the canal. For the anterior opening into 

 the branchial sac in this larva see I, Plate XXXI, fig. 28. 2, D. 



Figs. 38 — 42. — Similar series through somewhat older larva, showing the 

 neuro-hypophysial canal from its anterior opening into the branchial sac to its 

 posterior opening into the cerebral vesicle. 2, C. 



Figs. 43 — 46. — Stage VI. Fig. 43 shows an entire section slightly posterior 

 to the anterior opening of the neuro-hypophysial canal, and in front of the 

 atrial cavities (cf. I, Plate XXXI, fig. 29) ; 2, D. Fig. 44 is a most im- 

 portant section, and shows the origin of the adult ganglion in company with 

 the neuro-hypophysial canal from the left (right of the figure) latero-dorsal 

 wall of the cerebral vesicle. Figs. 45 and 46 pass through the region of the 

 visceral ganglion, which later becomes absorbed. 2, J. 



PLATE 20. 



Letters for Plate 20. 



p. I. Prseoral lobe. p.p. Praeoral pit or proboscis pore. n. p. Neuropore. 

 m. Mouth, end. Endostyle. n. c. Neural canal. 7ich. Notochord. gl. and 

 h. Proboscis-gland and heart of Balanoglossus. 



Fig. 1. — Diagram of anterior portion of an Ascidian larva (e. g. Ciona) 

 about the time of fixation. The features possessed by the larva at the stages 

 immediately prior to aud after fixation are thrown into one diagram. 



Fig. 2. — Diagram of anterior region of larva of Amphioxus. 



Fig. 3. — Similar diagram of Balanoglossus (compiled from Bateson). 



