ther side of the cloaca in
either sex. They can always be readily demonstrated by probing out
from the body cavity, in the direction indicated by the arrow (a.p.) in
Figure 1, Sheet 15. They probably serve to equalize the internal and
external pressure of the fish as it changes its depth in the water, just
as the Eustachian tubes equalize the pressure on either side of the
mammal's tympanic membrane.

Section 17. The musculature of the dog-fish body is cut into
V-shaped segments, the point of the V being directed forward. The
segments alternate with the vertebrae, and are called myomeres.
Such a segmentation is evident, though less marked, in the body wall
muscles of the frog, and in the abdominal musculature of the rabbit
and other mammals it is still to be traced.

Section 18. The uro-genital organs of the female dog-fish (Figure 1,
Sheet 17) consist of an unpaired ovary (ov.), paired oviducts (o.d.),
enlarged at one point to form an oviducal gland (o.d.g.), kidneys (k.),
with ureters (ur.) uniting to form a urinary sinus (u.s.) opening into the
cloaca by a median urinary papilla separate from the oviducal
openings. The eggs contain much yolk, and, like those of the fowl, are
very large; like the fowl, too, one of the ovaries is suppressed, and it
is the right ovary that alone remains. The two oviducts meet in front of
the liver ventral to the oesophagus, and have there a common opening
by which the ova are received after being shed into the body cavity.
The eggs receive an oblong horny case in the oviduct; in the figure
such a case is figured as distending the duct at e. The testes of the
male (T. in Figure 2) are partially confluent in the middle line. They
communicate through vasa efferentia (v.e.) with the modified anterior
part of the kidney, the epididymis (ep.), from which the vas deferens
(v.) runs to the median uro-genital sinus (u.g.s.), into which the
ureters (ur.) also open. The silvery peritoneum (lining of the body
cavity) covers over the reddish kidneys, and hides them in
dissection.

Section 19. Figure 3, Sheet 17, is a generalized diagram of the
uro-genital organs in the vertebrata; M.L. is the middle line of the
body, G. is the genital organ, Pr. is the pronephros, or fore kidney,
a structure which is never developed in the dog-fish, but which has
functional importance in the tadpole and cod, and appears as a
transitory rudiment in the chick. A duct, which is often spoken of as
the pronephr