Amongst these, notice
a. The changes of the mesoblastic somites: the com-
mencing formation of the musele-plates.
b. The position of the Wolffian duct and the forma-
tion of the germinal epithelium.
c. The aorte and the cardinal veins.
d. The great increase in depth and relative diminu-
tion in breadth of the section.
V. Examination of an Embryo of the Fourth Day.
A. Opening the egg,as in II. A.
Great care will be required not to injure the
embryo, which now lies close to the shell-membrane,


B. Examination in situ. Observe:—


1. The now conspicuous amnion.
2. The allantois, a small, and as yet hardly vascular
vesicle, beginning to project from the embryo into
the space between the true and the false amnion.
3. The rapidly narrowing somatic stalk.
29—2


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PRACTICAL DIRECTIONS. (APP.
Removal of the embryo, as in I. C. and IV. C.
The remarks made in the latter place apply with
still greater force to an embryo of the fourth and
succeeding days.
Surface view of the transparent embryo, For
manipulation, vide IV. D.
The points to be observed are :—
The formation of the fifth, seventh, and ninth
cranial nerves.
To observe these, a small amount of pressure
is advantageous.
The formation of the fourth visceral cleft, and the
increase in size of the superior maxillary process.
The formation of the nasal pits and grooves.
The great relative growth of the cerebral lobes and
the formation of the pineal gland from the roof of
the vesicle of the third ventricle.
The great increase in the investing mass.
The formation and growth of the muscle-plates,
which can now be easily seen from the exterior.
The allantois. Make out its position and mode of
opening into the alimentary canal.
