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GENERAL ACCOUNT OF EMBRYONIC DEVELOPMENT. 549

the embryo grows older ; the medullary folds become higher, and finally
unite over the medullary groove, which they have made by their growth.
The union of the sides begins at the anterior part. The canal thus
formed is called the neural canal, and locates the spinal cord. The
caudal end grows rapidly, pair after pair of somites being added, until
at the twelfth day the embryo presents the appearance seen in Fig. 294.
Fig. 294.
^?io

md
am

Itabbit Embryo ol" ab'Hit. the Twelfth Day (liom Balfour, after Weldou) : ce, cerebral hemisphere; fl,
to e limb; ///, liind limb; /(//, hyoid arch; u\ n, fourth ventricle; mb, mid-brain ; mx, maxillary
arch; md, mandibular arch; op, eye; l/i, thalamencephaloii ; nm, umbilical stalk.
The embryo rabbit of twelve days has reached about the stage of
the human embryo of four weeks and tlie pig 1 cm. in length.
In describing the rabbit embryo of twelve clays Foster and Balfour
say: "The latter stages in the development proceed, in the main, in the
.same manner as in the bird. The cranial flexure soon becomes very
marked, the mid-brain forming the end of the long axis of tlie embryo
(Fig. 294, mb). The sense-organs have the usual development. Under the
fore-brain appears an epiblastic involution giving rise both to the mouth
and to the pituitary body. Behind the mouth are three well-marked
pairs of visceral arches. The first of these is the mandibular arch
(Fig. 294, md), which meets its fellows in the middle line and forms
the posterior boundary of the mouth. It sends forward on each side
a superior maxillary process (nix), which partially forms the anterior
margin of the mouth.
As the embryo increases in length, the convexity of the spine be-
comes greater, and the head and tail ends approximate each other. The
chin rests hard upon the breast, and the caudal convexity comes in con-
tact with the forehead. The limbs are seen as buds springing off from
the sides of the body and hi). The umbilical stalk (nm) arises from
{fl
the concave surflice of tlie embryo and extends posteriorly.
As we have before said, a sufficient number of human embryos under
four weeks of age have not been obtained to establish any definite
description of development prior to that age. The earliest authenticated
observations were made by Allen Thomson (see Fig. 297, human embryo