312 ARTHUR WILLEY. 



whatever of the hypophysis at this stage. The branchial or 

 stomodaeal opening of the hypophysis and its cerebral opening 

 may be conveniently referred to as its anterior and posterior 

 openings respectively. The posterior opening of the hypo- 

 physis now occurs in the region of the cerebral vesicle which 

 contains the eye — that is, still farther back than in the preced- 

 ing stage. Figs. 38 — 42 represent a series of sections through 

 the cerebral vesicle of a larva which shows the hypophysis in 

 a rather more advanced stage of development. It now projects 

 from the wall of the cerebral vesicle, and has a definitely tubular 

 appearance. Fig. 38 shows the branchial or anterior aperture 

 of the hypophysis, while fig. 42 shows its posterior communi- 

 cation with the cavity of the cerebral vesicle. It should be 

 noted that the cerebral vesicle expands in every direction, not 

 only laterally, but also in a longitudinal direction, so that its 

 anterior wall projects far beyond its previous limit, and so 

 comes to lie side by side with the anterior opening of the 

 hypophysis, where its wall, as shown in fig. 38, consists 

 almost entirely of a thin and apparently structureless mem- 

 brane. 



Stage VI. — At this stage the hypophysis no longer opens 

 into the cerebral vesicle sensu s trie to, but its posterior open- 

 ing has been carried back by progressive constriction to the 

 region of the prominent ganglionic enlargement of the ventral 

 wall of the neural tube (figs. 45 and 46) which lies between 

 the cerebral vesicle and the anterior extremity of the notochord, 

 and which has been accurately described by van Benedeu and 

 Julin. As described by the latter authors, this ganglion 

 eventually becomes absorbed and disappears entirely, leaving 

 the superjacent neural tube in the shape of a solid cordon 

 ganglionnaire visceral. After this stage the posterior 

 opening of the hypophysis becomes closed, and only the anterior 

 opening into the branchial sac persists. 



Fig. 43 represents a section taken slightly posterior to the 

 anterior opening of the hypophysis, and serves to illustrate the 

 general topographical relation of the parts. In fig. 44 a very 

 important point is illustrated — namely, the origin of the 



