MORPHOLOGY OF THE SKELETAL MUSCLES. 413 
against the medullary tube and notochord on either side. The cleft between its two layers 
represents the remains of the ccelomic cavity. In young embryos it is to be seen in an 
active condition of growth. On its inner side masses of cells arise, which grow inwards 
and surround the medullary tube and notochord to form the foundation of the vertebral 
column. On its outer side cells appear to be given off which participate in the 
formation of the cutis vera. At the same time the dorsal and ventral borders of 
the myotome continue to extend, and present extremities (growing points) with an 
epithelial structure for a considerable period. On the dorsal side it overlies the medullary 
tube, and gives rise to the muscles of the back; while by its ventral extension, which 
traverses the somatopleuric mesoblast in the body wall, it produces the lateral and 
ventral muscles of the trunk. By an inward extension it probably gives rise also to the 
hypaxial muscles of the neck and loin. The cells of the inner layer of the myotome are 
responsible for the formation of the muscle fibres. Elongating in a direction parallel to 
the long axis of the embryo, they give rise by fusion “fl the cells of neighbouring myo- 
tomes to the columns and sheets of muscles of the back and trunk. For the most part 
(e.g. back and abdomen) the originally segmental character of the muscular elements is 
lost by the more or less complete fusion of adjacent myotomes. The intercostal muscles, 
however, are the direct derivatives of individual myotomes. 
Muscles of the Limbs.—In fishes (elasmobranchs) and reptiles there is evidence 
that the myotomes are concerned in the formation of the limb-muscles by the extension 
of the myotomes into the limb-bud in a manner similar to that described for the trunk. 
In birds and mammals, however, in which the limb-bud arises as an undifferentiated, 
unsegmented mass of mesoblastic tissue, partly from the mesoblast surrounding the noto- 
chord, and partly from the somatopleuric mesoblast, the myotomes stop short at the root 
of each limb, and do not penetrate into its substance. Instead, the muscular elements of 
the limb take origin independently as double dorsal and ventral strata of fusiform cells on 
the dorsal and ventral surfaces of the axial cartilages of the limb. These strata are 
unsegmented ; they are grouped around the skeletal elements of the limb, and they 
gradually become “differentiated into the muscle masses and individual muscles of the 
limb. 
Muscles of the Head.—Notwithstanding the obscurity and complexity of this 
subject, it appears certain that at least two series of elementary structures are concerned 
in the formation of the muscles of the head and face—the cephalic myotomes and the 
muscular structure of the branchial arches. 
The number of myotomes originally existing in the region of the head is not known 
although it is stated with some authority that nine were primitively present. The jist 
three are described as persisting in the form of the ocular muscles, the dast three in relation 
to the muscles of the tongue, while the three intervening myotomes disappear. 
The following table shows the probable fate of the cephalic myotomes :— 
First, Superior, internal and inferior recti, obliquus inferior, levator palpebré superioris. 
Second, Obliquus superior. 
Third, Rectus externus. 
Fourth, Fifth and Sixth, Alsent. 
Seventh, 
Highth, | Muscles of the tongue. 
Ninth, i? Muscles connecting cranium and shoulder-girdle. 
Tenth (or first cervical) 
The mesoblastic tissue of the branchial arches is probably concerned in the production of 
the following muscles of the face and neck :— 
First (mandibular) arch : ‘ Muscles of mastication. 
Platysma myoides and facial muscles. 
Second (hyovd). arch 5 , ; Muscles of soft palate. 
Stapedius, stylo-hyoid, and digastric. 
(Stylo- pharyngeus. 
\ Superior constrictor. 
yi dd ; ° 2 fea se. j Middle and inferior constrictors. 
Fourth and Fifth (branchial) arches 1 hunealeecin leneaere 
Third (thyro-hyoid) arch 
