of the Fishery Board for Scotland 
305 
each side of the body of the purse, close to the origin of the lower horns, 
a large number of long silky fibres or strings are attached to the margin 
of the purse. These fibres are silky, soft, and sticky when fresh, but 
they become more consistent on exposure to sea-water. Their purpose 
is obviously to anchor the purse to a rock, weed, or other suitable object. 
Fishermen state that sometimes in summer, when the skate may deposit 
its purses after being hooked, the purses stick to the lines by these threads 
and are drawn up to the surface. At the extremities of the horns on the 
inner side the usual slit-like apertures are found. From these in old 
purses a little of the white of the egg may project. In" the purse of the 
blue skate it is somewhat difficult to distinguish which is the flatter side, 
under which, as a rule, the embryo lies. The yolk measures some 2 J inches 
in diameter, and is of a pale yellow or greenish colour — rarely rosy. 
The Thornyback Skate (R. clavata). The purses of this form occupy a 
position intermediate between those of the other species. They are of 
a deep-green colour, and become almost black in the sea-water. Including 
the horns, the purse measures rather over 6 inches in length, with a 
breadth of from 2J to 2f inches. The thickness varies from | an inch 
to f of an inch. The contrast of a flattened and a convex side is always 
as well marked as in the starry ray. The egg-case is opaque. The lower 
horns are short, usually f of an inch in length, the upper ones long, like 
those of R. circularise and rather over 2 inches long. The margins of 
the purse are broad and much flattened. The purse is built up of fine 
beaded threads. Filaments of attachment, if present, are very rudiment- 
ary. The yolk is often rosy in colour, but sometimes pale yellow, and 
has a diameter in the young egg of 1 J inches. The white, as in the other 
cases, is clear, transparent, and jelly-like, in later development it becomes 
very watery. As previously stated the egg-cases of the spotted skate (R. 
maculata) appear to differ in no respect from those of the thornyback.* 
The Development of the Blue Skate (R. batis). 
In fig. 1 a figure of one of the early stages of the segmentation is given. 
The germinal disc of the egg was prepared from a specimen freshly taken 
from the oviduct of a skate. The section figured reveals the presence of 
a limited number of rather large segmentation cells, which form a layer of 
about three cells deep on the surface of the yolk. From the upper surface 
of the latter new cells are being added to those already formed. The 
outermost layer of the cells is somewhat more regular than the inner 
layers, and forms an epithelium, — it may already be spoken of as the epi- 
blastic layer. Each cell is but roughly spherical, perhaps better wedge- 
shaped ; it contains a nucleus, and is filled with yolk particles. 
In the yolk are seen a few of the wandering cells or ' merocytes ' (me) 
of Eiickert. 
At this stage the germinal area has a diameter of about 3 mm. 
The following stage is rather more advanced (fig. 2). The section is 
from a germinal disc of a period in which the segmentation, in a strict 
sense, is almost completed. The figure is from a specimen slightly 
younger than the one depicted in fig. 9, pi. 2, of Balfour's monograph. 
The figure was drawn under the same magnification as the preceding 
one, and the much smaller size of the segmentation cells is obvious. 
In size the germinal area has not grown appreciably larger than it was 
in the stage of fig. 1. 
* From the size (which is far too small to apply to the purses of R. batis), structure, 
form of horns, colour and filaments, the purses described by Wyman appear to 
resemble very closely those of R. maculata. This species is perhaps identical with, 
or closely allied to, the R. oculata of the American seas. 
U 
