is often described as being split off from
the segmental duct, but which is, very probably, an independent
structure in the frog. A number of tubuli, at first metamerically
arranged, now appear, each opening, on the one hand, into the
coelom by a ciliated mouth, the nephrostome (n.s.), and on the other
into the segmental duct. These tubuli are the segmental tubes or
nephridia. There grows out from the aorta, towards each, a bunch, of
bloodvessels, the glomerulus (compare Section 62, Rabbit). These
tubuli ultimately become, in part, the renal tubuli, so that the primitive
kidney stretches, at first, along the length of the body cavity from the
region, of the gill-slits backward. The anterior part of the kidney, called
the pronephros, disappears in the later larval stages. Internal to the
kidney on either side there has appeared a longitudinal ridge, the
genital ridge (g.r.), which gives rise to testes or ovary, as the case
may be.

* In the discussion whether the vertebrata have arisen from some
ancestral type, like the earthworm, metamerically segmented, and of
fairly high organization, or from a much lower form, possibly even from
a coelenterate. Such a discussion is entirely outside the scope of the
book, though its mention is necessary to explain the importance given
to these organs.

Section 19. The student should now compare the figures on Sheet 17.
In the male, tubular connections are established between the testes
and the middle part of the primitive kidney (mesonephros). These
connections are the vasa efferentia (v.e.), and the mesonephros is
now equivalent to the epididymis of the rabbit. The Wolffian duct is the
urogenital duct of the adult, and the Mullerian duct is entirely
absorbed, or remains, more or less, in exceptional cases.

In the female, the Mullerian duct increases greatly in length-- so that
at sexual maturity its white coils appear thicker and longer than
the intestine-- and becomes the oviduct; the Wolffian duct is the
ureter, and the mesonephros is not perverted in function from its
primary renal duty.

Section 20. Tabulating these facts--

In the adult male:
Pronephros disappears.
The Mullerian duct (? = pronephric duct) disappears.
Mesonephros = Epididymis; its duct, the urogenital.
Metanephros and duct, not clearly marked off from
Mesonephros.
(Compare Dog-fish, Section 19.)

In the adult female:
Pronephros disappears.
The