copied and mastered. Here we need only- [Comparing
with the rabbit, we would especially] call attention to the fact that the
vena cava inferior extends posteriorly only to the kidney, and that
there is a renal portal system. The blood from the hind limbs either
flows by the anterior abdominal vein to the portal vein and liver, or it
passes by the renal portal vein to the kidney. There the vein breaks
up, and we find in the frog's kidney, just as we find in the frog's and
rabbit's liver, a triple system of (a) nutritive arterial, (b) afferent*
venous and (c) efferent** venous vessels.

* a, ad = to;
** e, ex = out of.

{This Section missing from Second Edition.}
-Section 11. It is not very improbable that the kidney of the frog
shares, or performs, some of the functions of the rabbit's liver, or
parallel duties, in addition to the simply excretory function. Since
specialization of cells must be mainly the relatively excessive
exaggeration of some one of the general properties of the
undifferentiated cell, it is not a difficult thing to imagine a gradual
transition, as we move from one organism to another, of the functions
of glands and other cellular organs. It is probable that the mammalian
kidney is, physiologically, a much less important (though still quite
essential) organ than the structures which correspond to it in position
and development in the lower vertebrate types.-

Section 12. The lymphatic system is extensively developed in the
frog, but, in the place of a complete system of distinctly organized
vessels, there are great lymph sinuses (compare Section 1). In Figure
5, Sheet 12, the position of two lymph hearts (l.h., l.h.) which pump
lymph into the adjacent veins, is shown.

Section 13. The skull of the frog will repay a full treatment, and will
be dealt with by itself later. The vertebral column (Sheet 12)
consists of nine vertebrae, the centra of which have faces, not flat, but
hollow in front (pro-coelous), and evidently without epiphyses
(compare the Rabbit). The anterior is sometimes called the atlas, but
it is evidently not the homologue of the atlas of the rabbit, since the
first spinal nerve has a corresponding distribution to the twelfth cranial
of the mammal, and since, therefore, it is probable that the
mammalian skull = the frog's skull + one (or more) vertebrae
incorporated with it. Posteriorly the vertebral column terminates in the
urostyle, a calcified unsegmented rod. The vertebra