1010 
in the cloacal cavity (s, s), these folds gradually 
subside into a few simple plications. 
In some species ( Mustelus, Cuv.) of these 
viviparous Sharks a very close attachment is 
formed between the walls of the uterine portion 
of the oviduct and the contained ovum, so much 
so indeed as to remind the anatomist very for- 
cibly of the placental connection that exists in 
the Mammifera. In these, according to J. 
Miiller, the ovum, on its arrival in the oviduct, 
is only covered with a kind of membranous in- 
vestment or chorion, which is as thin and de- 
licate as the amnion of Mammalia, and without 
apparent organization. The sac which this 
membrane forms is seven or eight times as long 
as the vitellus, and its walls being regularly 
plicated, are embraced by corresponding folds 
of the lining membrane of the oviduct, so that 
there is a very intimate adhesion between the 
two. 
In the oviparous races of the plagiostome 
cartilaginous Fishes the structure of the 
oviduct is somewhat different, in order to pro- 
vide for the formation of the egg-shell or horny 
envelope wherein the egg is contained when 
extruded from the body, the organization of 
which is not a little curious. The glandular 
portion of the oviduct is extremely thick, or 
rather is enclosed in a dense glandular mass 
(rudimental gland ), the substance of which is 
entirely made up of close-set transverse secern- 
ing tubes, which pour their secretion into the 
oviduct through innumerable orifices, which are 
aggregated together in a part where the course 
of the lining membrane of the oviducal canal 
is interrupted, and free thus left for the 
escape of the rudimental secretion, which, be- 
coming thus deposited on the surface of the 
egg, hardens into a tough horny substance, 
which constitutes its external covering or egg- 
shell. The shape of these eggs is remarkable ; 
the egg-shell when completed resembles an ob- 
long horny pillow-case, the four corners of 
which are prolonged into tendril-like processes, 
the use of which appears to be that they serve 
as anchors by becoming interlaced with the 
branches of submarine plants or ramose corals, 
and thus preserve the egg and its delicate con- 
tents from being washed away by the agitation 
of the waves. From the tough coriaceous or 
horny texture of these egg-shells, another pro- 
vision becomes necessary, in order that the 
mature embryo shall be enabled to escape from 
confinement and enter upon an independent 
existence. In the eggs of Birds this is abun- 
dantly provided for by the brittle texture of the 
calcareous substance in which they are en- 
closed, allowing the chick to break its way out 
of its fragile covering, a mode of egress which, 
in the case before us, would evidently be im- 
saocrr es This difficulty is met by a very 
utiful contrivance. The horny walls of the 
eggs of the plagiostome Fishes are continuous 
all round, except at one extremity, where, to 
use a homely illustration, the end of the pillow- 
case remains unsewn, the edges of the slit thus 
left being merely kept in apposition by the 
elasticity of the ochy envelope. By this ele- 
gant arrangement all intrusion from without is 
PISCES. 
effectually prevented, and at the same time, 
seeing that the valves will separate on the ap- 
plication of a very slight pressure from within, 
they soon yield to the efforts of the young fish 
to escape from its cradle, and afterwards close 
again so accurately that it is difficult, without 
attentive examination, to detect the existence of 
the fissure. 
As amongst mammiferous animals certain 
races are provided with a marsupium or pouch, 
in which their immature young are carried 
about for a considerable period previous to 
their birth, so do we find certain Fishes provided 
Fig. 539. 
Viscera of Syngnathus acus (male). 
a, liver; 6, communication between the swim- 
ming bladder and the alimentary canal; ¢, sto- 
mach ; d, intestine ; f, allantoid bladder; g, ae ; 
testes ; m, kidney ; m, marsupial pouch 5 of 0, 
in interior of ditto. y 
