304 AKTHUR WILLEY. 



tube lined by a columnar epithelium, the lumen of which, 

 however, becomes irregular as it proceeds backwards, where, 

 at the point marked g in the figure, its dorsal wall is seen to 

 consist of a mass of cells in place of a well-defined epithelium. 



In fact, this is the first stage in the formation of the cerebral 

 ganglion of the adult (fig. 12, g). 



Beneath the neuro- hypophysial tube lie the remains of the 

 cerebral vesicle of the larva, now filled with histolytic residue. 



Stage IV. — A sagittal section through the neuro-hypo- 

 physial tract of a young Ciona, about the time of the budding 

 off of the two intermediate stigmata from the two primary, as 

 already described by me, is shown in fig. 14. The section 

 is slightly oblique. The ganglion has attained a much greater 

 development than in the preceding stage, and, indeed, seems to 

 have been exceptionally large in this particular specimen. 



The greater part of the lumen of the primitive neural tube 

 has become obliterated, and the nervous system is now for the 

 greater part of its extent solid, with a lumen, however, still 

 persisting in front, which opens anteriorly through the media- 

 tion of a funnel-like dilatation, which has possibly arisen in 

 part by evagination from the stomodseum, into the stomodseal 

 region of the branchial sac. 



Figs. 15 — 19 are taken from an extremely instructive series 

 of transverse sections through the central nervous system 

 of a rather older individual than that from which the sagittal 

 section of fig. 14 was obtained. Starting from behind, we have 

 in fig. 15 a section through the now solid nerve-cord, which is 

 continued into the '' cordon ganglionnaire visceral" of van 

 Beneden and Julin. Advancing gradually from behind for- 

 wards, we come to fig. 16, where the transverse section has an 

 irregular contour, and in its lower portion a very distinct 

 lumen can be seen, while dorsally it consists of a solid pro- 

 liferation from the dorsal wall of the neuro-hypophysial tube, 

 the nuclei being arranged peripherally, while the central por- 

 tion of the solid mass shows the first indication of the later 

 characteristic " Punktsubstanz." In fig. 17 the transverse 

 section has" a very regular and characteristic 8-shaped outline. 



