STUDIES ON THR PROTOCHORDATA. 311 



a secondary infolding of relatively inconsiderable extent 

 takes place from the wall of the branchial sac — i. e. from the 

 base of the stomodseum, — and carries the original branchial 

 opening of the neuro-hypophysial tube farther inwards in the 

 same way as I have suggested above for Ciona. 



Van Beneden and Julin, on the contrary, whose account of 

 the origin of the hypophysis diflFers essentially from that which 

 I am giving, speak of the entire hypophysis, including the 

 glandular portion of it, as arising from an endodermic diver- 

 ticulum of the branchial sac, and Professor Kupffer (10) has 

 recently seized on this statement to confirm him in his opinion 

 that the so-called hypophysis of the Ascidians is really nothing 

 of the kind, but merely a " Kiemendarmdriise." The whole 

 development of the Ascidian hypophysis, however, obviously 

 opposes itself to such a view. 



To return then to fig. 33, we see here a further progress in 

 the distension of the cerebral vesicle, while the section also 

 shows the posterior opening of the hypophysis into the vesicle. 

 The division of the primitive neural tube into two does not 

 extend to its anterior extremity, but the whole of that portion 

 of the neural tube which in the previous stages lay between the 

 point at which the neuro-hypophysial constriction commenced 

 and the neuropore becomes bodily taken up in the service of 

 the hypophysis, and at its front end comes to open into the 

 base of the stomodaeum as described above. 



Stage V. — At this stage the cerebral portion of the medul- 

 lary tube has assumed its definite vesicular character with the 

 accompanying local thinning out of its wall, which has been 

 already referred to in the case of Ciona. The contrast between 

 the cerebral vesicle and the hypophysial tube in point of size is 

 now very great (figs. 34-, 35). The latter here appears in the 

 form of a minute lumen in the thickness of the cerebral wall 

 just as we found it in Ciona. 



Figs. 34 — 37 are taken from a series of transverse sections 

 which show very clearly the way in which the lumen of the 

 hypophysis opens posteriorly into the cerebral vesicle. In fig. 

 37, the most posterior of the sections drawn, there is no trace 



