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II. On the Development of the Spinal Nerves in Flasmobranch Fishes. Dy F. M. 
Balfour, P.A., Fellow of Trinity College , Cambridge. Communicated by Dr. 
Michael Foster, F.B.S., Fraelector in Physiology and Fellow of Trinity College , 
Cambridge. 
Received October 5, — Read December 16, 1875. 
Ix the course of an inquiry into the development of Elasmobranch Fishes, my attention 
has recently been specially directed to the first appearance and early stages of the spinal 
nerves, and I have been led to results which differ so materially from those of former 
investigators that I venture at once to lay them before the Society. I have employed 
in my investigations embryos of Scyllium canicula, Scyllium stellare, Pristiurus, and 
Torpedo. The embryos of the latter animal, especially those hardened in osmic acid, 
have proved by far the most favourable for my purpose, though, as will be seen from 
the sequel, I have been able to confirm the majority of my conclusions on embryos of 
all the above-mentioned genera. 
A great part of my work was done at the Zoological Station founded by Dr. Doiirn 
at Naples; and I have to thank both Dr. Dohrx and Dr. Eisig for the uniformly 
obliging manner in which they have met my requirements for investigation. I have 
more recently been able to fill up a number of lacunae in my observations by the study 
of embryos bred in the Brighton Aquarium ; for these I am indebted to the liberality 
of Mr. Lee and the Directors of that institution. 
The first appearance of the Spinal Nerves in Pristiurus. 
In a Pristiurus-e mbryo, at the time when two visceral clefts become visible from the 
exterior (though there are as yet no opening^ from without into the throat), a transverse 
section through the dorsal region exhibits the following features (Plate 16. fig. A): — 
The external epiblast is formed of a single row of flattened elongated cells. Verti- 
cally above the neural canal the cells of this layer are more columnar and form the 
rudiment of the primitively continuous dorsal fin. 
The neural canal ( nc ) is elliptical in section, and its Avails are composed of oval 
cells two or three deep. The wall at the two sides is slightly thicker than at the 
ventral and dorsal ends, and the cells at the two ends are also smaller than elsewhere. 
A typical cell from the side Avails of the canal is about xskTu inch in its longest 
diameter. The outlines of the cells are for the most part distinctly marked in the 
specimens hardened in either chromic or picric acid, but more difficult to see in those 
prepared with osmic acid; their protoplasm is clear, and in the interior of each is 
an oval nucleus very large in proportion to the size of its cell. The long diameter of 
a typical nucleus is about -^foo inch, or about two thirds of that of the cell. 
2 B 
MDCCCLXXVI. 
