somatopleur, and from it will be formed the dermis, the muscles of the
body wall, almost all the cartilage and bone of the skeleton, the
substance of the limbs, the kidneys, genital organs, heart and
bloodvessels, and, in short, everything between the dermis and the
coelom, except the nervous system and nerves, and the notochord.
The inner mesoblast, the mass of the splanchnopleur, will form the
muscle and connective tissue of the wall of the alimentary canal, and
the binding substance of the liver and other glands that open into the
canal.

Section 12. Figure 8 is one which we reproduce, with the necessary
changes in each plate of embryological figures given in this book, so
that the student will find it a convenient, one for the purpose of
comparison. The lines of dashes, in all cases, signify -epiblast-
[hypoblast] , the unbroken black line is -hypoblast-, [epiblast] dotting
shows mesoblast, and the shaded rod (n.c.) is the notochord. c.s. is
the spinal cord; br.1, br.2, br.3 are the three primary vesicles which
constitute the brain, and which form fore, mid, and hind brain
respectively. I. is the intestine and Y. the yolk cells that at this early
stage constitute its ventral wall.

Section 13. Figure 9 gives a similar diagram of a later stage, but here
the blastopore is closed. An epiblastic tucking-in at st., the
stomodaeum pre-figures the mouth; pr., the proctodaeum, is a
similar posterior invagination which will become the anus. Y., the
yolk, is evidently much absorbed. Figure 10 is a young tadpole, seen
from the side. The still unabsorbed yolk in the ventral wall of the
mesentery gives the creature a big belly. Its mouth is suctorial at this
stage, and behind it is a sucker (s.) by which the larvae attach
themselves to floating reeds and wood, as shown in the three black
figures below.

Section 14. We may now consider the development of the different
organs slightly more in detail, though much of this has already been
approached. The nervous system, before the closure of the neural
groove, has three anterior dilatations, the fore-, mid-, and hind-brains,
the first of which gives rise by hollow outgrowths to two pairs of lateral
structures, the hemispheres and the optic vesicles. The latter give rise
to the retina and optic nerve as described in {Development} Section
40.

Section 15. The hypoblastic notochord is early embraced by a
mesoblastic sheath derived from the protovertebrae. This becomes
tru