Mullerian duct, the oviduct.
Mesonephros and Metanephros, the kidney, and their unseparated
ducts, the ureters.

Section 21. Hermaphrodism (i.e., cases of common sex) is
occasionally found among frogs; the testis produces ova in places,
and the Mullerian duct is retained and functional. The ciliated
nephrostomata remain open to a late stage of development in the frog,
and in many amphibia throughout life. Their connection with the renal
tubuli is, however, lost.

Section 22. The alimentary canal is, at first, a straight tube. Its
disproportionate increase in length throws it into a spiral in the tadpole
(int. Figure 11), and accounts for its coiling in the frog. The liver and
other digestive glands are first formed, like the lungs, as hollow
outgrowths, and their lining is therefore hypoblastic. The greatest
relative length of intestine is found in the tadpole, which, being a
purely vegetable feeder, must needs effect the maximum amount of
preparatory change in its food.

_The Development of the Fowl_

Section 23. The frog has an ovum with a moderate allowance of yolk,
but the quantity is only sufficient to start the little animal a part
of its way towards the adult state. The fowl, on the contrary, has an
enormous ovum, gorged excessively, with yolk, and as a
consequence the chick is almost perfected when it is hatched. The
so-called yolk, the yellow of an egg, is the ovum proper; around that is
a coating of white albumen, in a shell membrane and a shell. At either
end of the yolk (Figure 1, y.) twisted strands of albuminous matter,
the chalazae (ch.) keep the yolk in place. The animal pole is a small
grey protoplasmic area, the germinal area (g.a.), on the yolk.

Section 24. We pointed out that the presence of the yolk in the frog's
egg led to a difference in the size of the cells at the animal and
vegetable poles. The late F.M. Balfour, borrowing a mathematical
technicality, suggested that the rate of segmentation in any part of an
ovum varies inversely with the amount of yolk. In the fowl's egg,
except just at the germinal area, the active protoplasm is at a
minimum, the inert yolk at a maximum; the ratio of yolk to protoplasm
is practically infinity, and the yolk therefore does not segment at all.
The yolk has diluted the active protoplasm so much as to render its
influence inappreciable. The germinal area segments, and lies upon
the yolk which has defeated the efforts of its small m