232 T'ransactions.— Zoology. 
ilie with * L.” The mid-brain (fig. 18, m.b) forms the anterior termins- 
tion of the head, and the ventral surface of the fore-brain (f.b) looks directly 
backwards. 
The mouth (fig. 14, m) is greatly reduced, and has in fact almost pre- . 
cisely the form, relative size, etc., as in Balfour’s * O.” So also have the 
branchial apertures (or. 1, br. 5), from which as well as from the spiracle 
(sp) the long external gills now emerge. The dorsal (d 1, d 2) and pectoral 
(pe) fins are beginning to assume their adult form, their line of attachment 
being no longer their greatest dimension. The caudal fin (c) shows the first 
indication of the change from diphy- to hetero-cereality: on its ventral 
edge, near the tip, is a slight emargination, evidently the commencement of 
the very marked notch in the corresponding position in the adult (fig. 15). 
In front of the anterior end of the pelvic fin (pv) the lateral ridge is now 
Lr). The length of the embryo in this stage is about 40 mm. 
Up to the present time I have been able to do very little towards the 
further examination of these embryos, and all I propose to bring forward in 
the present paper is the fact that in the third stage the lateral vein (figs. 5 
and 6, lv) is well developed, and is indeed nearly as large as the cardinal 
vein (ed), and considerably larger than the dorsal aorta (d.ao). Of the two 
sections figured, fig. 5 is taken along the line zy in fig. 18, or just in front 
of the pelvic fins, fig. 6 along s'y’ or through the pelvic fins. In both 
is seen to be a very obvious structure, a fact 
any of Balfour's figures—somewhat remarkable. Having, unfortunately, = 
ave not been able to ascertain 
8 mentioned above, however, it is present 
- A few weeks ago I received a letter from Mr. Balfour,* in which the 
following passage occurs :—“ I wag ver 
the skate's venous system. The lateral veins you describe are very peculiar, 
and I should not hesitate to consider them as confirming my view of the 
fins, were it not for the specialized character of the skate, which you your- 
self urge in your paper. One would like to find them either in the embryo 
or in some less specialized form.” 
The necessary confirmation is afforded by the facts detailed in the 
present paper. The lateral vein exists in every Selachian I have yet had the 
opportunity of examining: in all it follows the direction of Balfour's lateral 
ridge, from the anterior border of the pelvic fin forwards and upwards to 
* I little thought at the time that this letter was the last I was ever to have from the 
writer, 
y much interested in your paper on 
