254 
of the infundibulum, but the cells of the dorsal and lateral walls are 
closely crowded together and the lumen they formerly bounded has 
entirely disappeared. At either side of the line inf, fig. 5, the neural 
wall comes into close contact with the external ectoderm, and as 
development continues, the region of contact increases towards the 
median line, until ultimately the entire fundus of the infundibulum 
comes into contact with the ectoderm, and the anterior portion of the 
primitive alimentary canal is thereby severed from the posterior and 
permanent canal. 
It may be here noticed that the downgrowth in the floor of the 
brain which gives rise to the infundibulum is primarily bilateral and 
not median. 
The fate of the cells that formed the anterior portion of the 
alimentary canal will be considered with the mesodermic somites. 
2. The Notochord. 
In regard to the differentiation of the notochord from the endo- 
derm, I have little to add to what is already well known. The ap- 
pearance of a longitudinal groove in the dorsal wall of the alimentary 
canal, the concentric arrangement of the cells about this groove, its 
final closure, the extrusion of the chorda from the endoderm, and the 
reuniting of the walls of the alimentary canal below, have been des- 
cribed, and do not differ essentially in Acanthias from similar events 
described by Kurrrrer for Petromyzon !), or by HarscueK for Am- 
phioxus ?). The first stage in the differentiation of the chorda, namely, 
the deepening of the cells in the dorsal wall of the alimentary canal, 
and the appearance there of a longitudinal groove, extends in Acanthias 
to the anterior extremity of the primitive canal (fig. 4 g). The 
chorda-Anlage, however, undergoes here a retrogressive development 
corresponding to that which affects the alimentary canal, and the defi- 
nitive chorda does not extend beyond the cells which, in the dorsal 
wall of the alimentary canal unite primitively the walls of the mandi- 
bular cavities. The peculiar contortions to which the anterior end of 
the notochord is subject after its complete separation from the endo- 
derm, show that the retrogressive development begun at the anterior 
extremity of the embryo with the reduction of the chorda-Anlage, 
still continues in the earliest stages of the fully formed chorda. 
1) C. Kuprrer, „Die Entwickelung von Petromyzon Planeri“. Archiv 
f. mikroskop. Anat., 1890. 
2) B. Hartson, „Studien über Entwickelung des ianaphioxus Arbeiten 
a. d. Zool. Institut d. Univ. Wien, 1881. 
