nd, bone is formed only in the muscle-layer (i., p. 155, ii.,
pp. 92-3).

This relation of tissue to germ-layer was more fully discussed and
brought into greater prominence by Remak, from the standpoint of the
cell-theory, and it will occupy us in a later chapter (Chap. XII.).

The fourth Scholion elaborates the analysis of developmental processes
still further, and discusses in particular the scheme of development
which is shown by the Vertebrata. The characteristic structure of the
vertebrate body is brought about by a "double symmetrical" rolling
together of the germ-layers, whereby two main tubes are formed, one
above and one below the axis of the body, which is the chorda. The
dorsal tube is formed by the two animal layers, the ventral tube by all
the layers combined (see Fig. 7).

The process is indicated with sufficient clearness in the diagram. It
will be seen that the real foundation and framework of the arrangement
is the muscle-layer, with its two tubes, one surrounding the central
nervous system and forming the "dorsal plates," the other surrounding
the body cavity and forming the "ventral plates." In the dorsal plates,
which early show metameric segmentation, the investing skeleton of the
neural axis develops; in the ventral plates are formed the ribs, the
ventral arches of the vertebrae, the hyoid, the lower jaw and other
skeletal structures.

The alimentary or "mucous" tube and the part of the vessel layer which
invests it become so closely bound up with one another as to form a
single primitive organ--the alimentary canal. The muscles of the
alimentary canal are accordingly in all probability developed in the
investing part of the vessel layer. From the "Gekroese," or remaining
part of the vessel layer develop the Wolffian bodies (_Urnieren_,
Pronephros), the kidneys, the sex glands, and the series of
"blood-glands"--suprarenals, thyroid, thymus and spleen. Baer did not
attach any special morphological significance to the peritoneal lining
of the body cavity, as is done in more modern forms of the germ-layer
theory. The gill-slits were largely formed by outgrowths from the
alimentary canal.

_a._ Chorda.
_b._ Dorsal plates.
_c._ Ventral plates.
_d._ Spinal cord.
_e._ Vessel-layer.
_f._ Alimentary tube.
_g._ Pronephros.
_h._ Skin.
_i._ Amnion.
_k._ Serous membrane.
_l._ Yolk-sac.

In his germ-layer theory von Baer was influenced a good deal by
Pander, to whom the actual discovery of the pro