of the Fishery Board for Scotland. 
309 
gill-cleft, as I showed some years ago. It would extend this paper too 
much to say more about this question now. In front of the facial nerve 
lies the fifth nerve (v.) and behind the auditory organ the glossopharyngeal 
nerve (ix.) and vagus nerve (x.) are seen. I now pass over many stages, 
and proceed to describe the oldest embryo I have till now obtained. 
The embryo is shown life-size in figs. 16 and 17. 
It is fully 9 centimetres in length. The purse from which the embryo 
was taken had lain in sea-water for at least six winter months, and none 
the less the embryo still had a large yolk-sac attached to it, and must have 
been some two or three months distant from the time at which, in the 
natural course of events, it would have 'hatched out.' The yolk-sac 
weighed (in spirit preparation) 22 grms., so that a good supply of food 
material was still present in it. 
The undescribed stages between this embryo and the preceding one 
would have shown us, among other things, the development and degenera- 
tion of the anal fins originally described by Wyman. In this embryo 
there is no trace of them. 
The younger embryos described and figured were shark-like in appear- 
ance. Any zoologist would recognise the present one as that of a skate. 
The broadened anterior paired fins and the long, striking tail are very 
characteristic. At this stage the proportionate size of tail to body is much 
greater than in the young skate. 
Fig. 16 is a dorsal view. Attention may be drawn more particularly to 
the greatly developed eyes (o.) the spiracular cleft (s.p.) just behind the eyes, 
some greatly developed canals of the future mucous or sensory tubes, 
(s.t.) and the unpaired fins (u.f.). In the figure of the ventral aspect, 
the mouth (m.) and nostrils (m.o.l.o.), the numerous and long external gill- 
filaments, the stump of the cut umbilical cord, the anus, and the posterior 
paired fins must be noted. What appear to be the rudiments of the 
claspers show that the embryo would probably have been a male skate. 
When the purse containing the living embryo was opened, the beautiful 
bright red appearance of the gill-filaments, and of the numerous blood- 
vessels in the anterior fins, attracted the eye at once. A considerable 
amount of what appeared to be sea-water was also contained in the purse, 
and in this fluid the gill-filaments floated loosely. 
The External Gilk Filaments of the Skate-Embryo. 
The bright red external gills which are such striking objects in advanced 
stages of development, are structures over whose meaning many zoologists 
have puzzled themselves. Home, Cuvier, Davy and Johannes Muller were 
perhaps the first to pay much attention to them. Davy assumed that one 
of their purposes was to absorb the yolk from the yolk-sac, an explana- 
tion Muller was inclined to accept. 
Others (Home and Wyman) have seen in these structures respira- 
tory organs, and in order to support their views have assumed that sea- 
water finds its way through the slits, previously described in the horns of 
the purse, into the body of the egg-case. Wyman (p. 32) even speaks of 
'an inward and outward flow of water which passes through the egg 
during incubation.' It is needless to say that such an inward and out- 
ward current is non-existent, indeed, there is no mechanism present in the 
egg which would cause such a current to flow. 
As Hartog has recently pointed out (with reference to another animal in 
which sea-water is present within the animal), it is unnecessary to assume an 
outward and inward flow in the case of an animal living in sea-water. 
What holds for an animal in sea-water applies also to the skate-purse. 
