28 GENERAL EMBRYOLOGY. 
into it, except the upper parts of the mouth and pharynx and the terminal portion 
of the rectum; thus the epithelium of the Eustachian tube and tympanic cavity, 
the trachea, the bronchi, the air-vesicles of the lungs, the gall-bladder, the urinary 
bladder, and part of the urethra is entodermal. It forms the epithelial constituents 
of the taste buds, or organs of taste, the liver and the pancreas, the epithelium 
lining the vesicles of the thyroid body and the cell nests of the thymus gland. 
From the mesoderm all the remaining structures which intervene between the 
surface ectoderm and the entodermal lining of the primitive alimentary tube are 
formed. 
The Protovertebral Somites and the Lateral Plates.—EKach protovertebral 
somite consists of numerous cells arranged radially round a central cavity—the 
myelocele ; this latter, however, quickly disappears. 
The cells of the somites are gradually grouped into three sets, two to the inner 
and lower side of 
the cavity, and 
one to its upper 
and outer side. 
The two groups 
Cutaneous 
steal on the lower and 
Proto- scle : : 
vertebral! plate inner side are an 
somite | Sclerato- 
genous 
layer 
outer, next the 
cavity, the mus- 
cle plate, and an 
inner the sclera-- 
togenous layer. 
The group on the 
upper and outer 
side of the cavity 
is the epithelial 
or cutaneous la- 
mella. 
The Sclerato- 
. genous Layer.— 
—______VD,,, ‘The cells cist 
layer proliferate 
rapidly and mi- 
Fic. 22,—FURTHER DIFFERENTIATION OF THE MESODERM. orate inwards, 
SoM 
Somatopleure 
Splanchnopleure 
Transverse section of a rat embryo, showing the transformation of the cells of the gyrrounding the 
scleratogenous layer of a protovertebral somite and their extension round the 2 
notochord and spinal cord. notochord, and 
AM. Amnion. N.  Notochord. SC. Spinal cord. passing both be- 
AMC, Amnion cavity. PA. Primitive aorta. SG. Spinal ganglion. neath the neural 
C. Ceelom, PAC, Primitive alimentary SoM. Somatic mesoderm. a 
EC, Ectoderm. canal. SpM. Splanchnic mesoderm. vue and Up 
VD. Vitello-intestinal duct. wards along 1ts 
lateral walls to 
its dorsal aspect; they intermingle above and below with the cells of the corre- 
sponding layer of the opposite side, and in front and behind with the cells of the 
scleratogenous layers of adjacent somites. In this way the neural tube and the 
notochord are gradually enveloped by a continuous sheath of mesodermal tissue, 
which forms the membranous vertebral column. This is perforated at regular 
intervals by the nerve-roots issuing from the spinal cord and brain, and by the 
vessels of supply to those structures. From its substance the vertebree and 
ligaments, the greater part of the intervertebral discs, and the investing membranes 
of the brain and cord are afterwards developed.! 
The Muscle Plates.—The cells of the muscle plate layer lose their original 
epithelial-like characters; they elongate antero-posteriorly, become spindle-shaped 
and striated, and they give rise to the striped muscles of the body. For a long 
time the fibres developed from each muscle plate remain localised and quite distinct 
from the fibres developed from neighbouring segments; the masses they form are 
1 The development of the skeleton is described in the chapter on Osteology. 
