ly cartilaginous, and at regular intervals is alternately thicker and
thinner, compressing the notochord at the thicker parts. Hence the
notochord has a beaded form within this, at first, continuous
cartilaginous sheath. This sheath is soon cut into a series of
vertebral bodies by jointings appearing through the points where the
cartilage is thickest and the notochord most constricted. Hence what
remains of the notochord lies within the vertebral bodies in the frog;
while in a cartilaginous fish, such as the dog-fish, or in the embryonic
rabbit, the lines of separation appear where the notochord is thickest,
and it comes to lie between hollow-faced vertebrae. Cartilaginous
neural arches and spines, formed outside the notochordal sheath,
enclose the spinal cord in an arcade. The final phase is ossification.
As the tadpole approaches the frog stage the vertebral column in the
tail is rapidly absorbed, and its vestiges appear in the adult as the
urostyle.

Section 16. The development of the skull is entirely dissimilar to that
of the vertebral column. It is shown on Figures 1 and 8, Sheet 14; and
in the section devoted to the frog's skull a very complete account of
the process is given. The process of ossification is described under
the histology of the Rabbit.

Section 17. The origin of the circulatory and respiratory organs is
of especial interest in the frog. In the tadpole we have essentially the
necessities and organization of the fish; in the adult frog we have a
clear exposition of the structure of pigeon and rabbit. The tadpole has,
at first, a straight tubular heart, burrowed out in somatic mesoblast,
and produced forward into a truncus arteriosus. From this arise four
afferent branchial arteries, running up along the sides of the four
branchial arches, and supplying gills. They unite above on either side
in paired hyper-branchial arteries, which meet behind dorsal to the
liver, to form a median dorsal aorta. Internal and external carotid
arteries supply the head. These four afferent branchial arches are
equivalent to the first four of the five vessels of the dog-fish. At first,
the paired gills are three in number, external, and tree-like, covered by
epiblast (Figures 10 and 11, e.g.), and not to be compared to fish gills
in structure, or in fact -with- [to] any other gills within the limits of
the vertebrata. Subsequently (hypoblastic) internal gills (int.g., Figure
12), strictly homologous with the gills o