182 
MR. F. M. BALFOUR ON THE DEVELOPMENT OF 
It is there effected (Plate 18. fig. L) in each case by a protoplasmic commissure with 
imbedded nuclei*. Near its dorsal extremity each posterior root dilates, and from the 
dilated portion is given off on each side the commissure uniting it with the adjoining 
roots. 
Considering the clearness of this formation in this embryo, as well as in the embryo 
belonging to the stage under description, there cannot be much doubt that at the first 
formation of the posterior rudiments a continuous outgrowth arises from the spinal cord, 
and that only at a later period do the junctions of the roots with the cord become, 
separated and distinct for each nerve. 
I now return to the more complete series of Pristiurus-e mbryos, the development of 
whose spinal nerves I have been able to observe. 
Second Stage of the Spinal Nerves in Pristiurus. 
In the youngest of these (Plate 17. fig. E) the notochord has undergone but very slight 
changes, but the segmental duct has made its appearance, and is as much developed 
as in the Torpedo-e mbryo from which fig. D b was taken. 
(The embryo from which fig. E a was derived had three visceral clefts.) 
There have not as yet appeared any connective-tissue cells dorsal to the top of the 
muscle -plates, so that the posterior nerve-rudiments are still quite free and distinct. 
The cells composing them are smaller than the cells of the neural canal ; they are 
round and nucleated ; and, indeed, in their histological constitution the nerve-rudi- 
ments exhibit no important deviations from the previous stage, and they have hardly 
increased in size. In their mode of attachment to the neural tube an important change 
has, however, already commenced to be visible. 
In the previous stage the two nerve-rudiments met above the summit of the spinal 
cord and were broadly attached to it there ; now their points of attachment have glided 
a short distance down the sides of the spinal cordf. 
The two nerve-rudiments have therefore ceased to meet above the summit of the 
canal ; and in addition to this they appear in section to narrow very much before 
becoming united with its walls, so that their junctions with these appear in a transverse 
section to be effected by at most one or two cells, and are, comparatively speaking, very 
difficult to observe. 
In an embryo but slightly older than that represented in fig. E a the first rudiment of 
the anterior root becomes visible. This appears, precisely as in Torpedo, in the form of 
a small projection from the ventral corner of the spinal cord (fig. E b, ar). 
The second step in this stage (Plate 17. fig. F) is comparable, as far as the connective- 
tissue is concerned, with the section of Torpedo (Plate 16. fig. D d). The notochord (the 
* This commissure is not satisfactorily represented in the figure. Vide Explanation of Plate 18. 
t [May 18, 1876. — Observations I have recently made upon the development of the cranial nerves incline 
me to adopt an explanation of the change which takes place in point of attachment of the spinal nerves to the 
cord differing from that enunciated in the text. I look upon this change as being apparent rather than real, 
and as due to a growth of the roof of the neural canal in the median dorsal line, which tends to separate the 
roots of the two sides more and more, and cause them to assume a more ventral position.] 
