of special sense. The median layer gives rise to
the voluntary muscles, bones, cartilages, &c., the nutritive systems of
the blood, the chyle, the lymph, and the muscular tube of the intestine.
Lastly, the innermost layer developes into the epithelium lining of the
intestine, with its various appendages of liver, lungs, intestinal
glands, &c.

I have just said that this three or four layered stage is shared by all
the Metazoa, except those very lowest forms--such as sponges and
jelly-fish--which do not pass on to it. But from this point the
developmental histories of all the main branches of the Metazoa
diverge--the Vermes, the Echinodermata, the Mollusca, the Articulata,
and the Vertebrata, each taking a different road in their subsequent
evolution. I will therefore confine attention to only one of these
several roads or methods, namely, that which is followed by the
Vertebrata--observing merely that, if space permitted, the same
principles of progressive though diverging histories of evolution would
equally well admit of being traced in all the other sub-kingdoms which
have just been named.

In order to trace these principles in the case of the Vertebrata, it is
desirable first of all to obtain an idea of the anatomical features
which most essentially distinguish the sub-kingdom as a whole. The
following, then, is what may be termed the ideal plan of vertebrate
organization, as given by Prof. Haeckel. First, occupying the major axis
of body we perceive the primitive vertebral column. The parts lying
above this axis are those which have been developed from the ectoderm
and mesoderm--viz. voluntary muscles, central nervous system, and organs
of special sense. The parts lying below this axis are for the most part
those which have been developed from the endoderm--namely, the
digestive tract with its glandular appendages, the circulating system
and the respiratory system. In transverse section, therefore, the ideal
vertebrate consists of a solid axis, with a small tube occupied by the
nervous system above, and a large tube, or body-cavity, below. This
body-cavity contains the viscera, breathing organs, and heart, with its
prolongations into the main blood-vessels of the organism. Lastly, on
either side of the central axis are to be found large masses of
muscle--two on the dorsal and two on the ventral. As yet, however, there
are no limbs, nor even any bony skeleton, for the primitive vertebral
column is hitherto unossi