THE EMBRYO. 29 
called myotomes. After a time, however, the fibres of neighbouring myotomes are 
more or less intermingled, and in the adult, except 1n certain situations, the inter- 
myotomic intervals are no longer recognisable. 
The main portions of the myotomes are converted into the muscle masses 
situated in the dorsal part of the body wall, that is, into the erectores spine and 
their main subdivisions, and the other muscles which occupy the vertebral grooves. 
In the lower vertebrates the ventral ends of the myotomes descend in the 
somatopleure almost to the mid-ventral line, and are transformed into the muscles 
of the ventro-lateral walls of the body. A similar descent of the ventral ends of 
the myotomes into.the lateral walls of the body has not been proved in the highest 
vertebrates. In mammals, including man, the ventral ends of the myotomes only 
descend for a short distance in the somatopleure, and then all trace of their char- 
acteristic structure is lost. It is presumed, however, that cells budded off from the 
myotomes descend to a lower level, 
and that they take part in the 
formation of the ventro-lateral 
muscles. 
In lower vertebrates bud-like 
projections pass from the myo- Sclemtesmons 
tomes in the thoracic and pelvic 
regions into the limb rudiments, — Muscle plate. 
and from these the muscles of the 
limbs are developed. In_ the PAAnin ig & 
highest vertebrates distinct buds lamella 
from the myotomes have not been 
observed, but it is said that out- 
growths of cells pass from the 
myotomes into the limb buds, 
where they proliferate and form 
the limb-muscles. The occurrence 
of these outgrowths into the 
limbs, like the descent of the 
lower ends of the myotomes into 
the ventral part of the body-wall, 
has not been proved in mammals ; 
possibly it occurs, but, if not, the 
gaia and, limb - muscles of Fic. 23.—CorRONAL SECTION OF A RAT EMBRYO. 
mammals must be developed Showing the relationship of the extending scleratogenous tissue 
from the somatopleural meso- s to the spinal cord and to the muscle plates. 
derm. 
The Cutaneous Lamella of the Protovertebral Somite.—The cells which form 
the outer and dorsal walls of the cavities of the protovertebral somites retain their 
epithelial-like characters for a longer period than those of other portions of the 
somites, and at the borders of the lamelle they pass by gradual transition into the 
cells of the muscle plates. After a time they undergo histological differentiation, 
and they are utilised in the formation of the subcutaneous tissues and fasciz on the 
dorsal aspect of the body, and outgrowths, which descend with the offsets of the 
muscle plates, enter into the formation of the ventro-lateral walls of the body. 
Blood-vessel 
Spinal cord 
Ectoderm 
Mesoderm 
gn OA 8 
I 
The Protovertebral Somites of the Head.—It has already been pointed out (p. 24) that 
protovertebral somites are not recognisable in mammals further forwards than the occipital 
region ; but, from the evidence obtained by examination of lower vertebrates, it is believed that 
originally nine somites were present in the cephalic region. From the first, second, and third of 
these, muscle plates form which are developed into the muscles of the eyeballs. If any muscle 
plates are formed in connection with the fourth, fifth, and sixth somites they disappear, leaving 
no traces, and the muscles developed from the remaining cephalic somites are those of the tongue 
and those connecting the head with the shoulder girdle. 
The Lateral Plates.—At an early stage, before its separation trom the paraxial 
mesoderm, each lateral plate is divided into an outer or somatic and an inner or 
splanchnic layer. The somatic layer is concerned with the formation of the 
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