two spinal ganglia of one side together. These cells also gradually 
disappear, apparently being used up in nerve formation. 
g) I also found the dorsal branch, independently of Dourn, arising 
as a proliferation or wandering of cells from the undifferentiated 
ganglion. These again become arranged in chains — nearer the peri- 
phery often in plexuses — and nerve-secretion ensues. The formation 
of peripheral twigs appears, indeed, to go on for a very long period, 
and in very advanced embryos of Raja it may readily be observed. 
h) Some consideration has been given to the question of the mode 
of connection of nerve and ganglion-cell in the preceding paper, 
under circumstances in which its discussion could hardly be avoided. 
d) Histogenesis of the optic nerve. 
From the recent work of KEiBEL, His, and more especially of 
FRORIEP the optic nerve has been proved to arise in the embryonic 
retina and thence to grow centripetally to the brain'). FRORIEP ?), 
with whose work I am here more directly concerned, attained his 
results from the study of sections transverse to the developing retina 
and optic nerve, i. e. as seen in longitudinal vertical sections of em- 
bryos. Such sections cut the developing fibres transversely, and there- 
fore the appearances seen tend to lend support to the view taken by 
Hıs 3) that “die zuerst gebildeten Opticusfasern der Zellen der Retina 
entstammen und centralwärts wachsen”. 
FRORIEP judges these statements to be at most probable only, 
and regards them as lacking proof. There is in fact an error, as I 
believe, in the view taken by His, an error which the nature of Fro- 
RIEP’S sections did not enable him to detect. 
Additional facts are disclosed by sections passing horizontally 
through retina and developing nerve, and the study of such has con- 
vinced me that the mode in which the fibres of this nerve arise is in 
exact parallel with that in which the lateral nerve develops. There 
is one important difference — easily explicable — and that consists 
in the direction of growth which, as KEIBEL and FRORIEP proved, is 
centripetal in the case of the optic nerve. 
The optic nerve is, then, at first cellular, and in horizontal sec- 
tions is seen to be represented by a columnar neuroepithelium', pos- 
1) Keıser’s paper is only known to me from the HrRMANN-SCHWALBE 
Jahresbericht, and from the brief notice in the appendix of Frorrep’s 
communication cited below. 
2) Frorier, A., Ueber die Entwickelung des Sehnerven. Anat. Anz. 
1891, pp. 155—161. 
3) Archiv f. Anat. und Entwickelungsgesch., Supplement-Bd. 1890, 
p. 109. 
