Charles Eussell Bardeen and Warren Harmon Lewis 33 



occipital myotomes disappear. From the thoracic myotomes processes 

 enter the hody wall. During the fifth week the myotonies give rise to 

 a dorso-ventral muscle-mass in which the segmentation characteristic 

 of the myotomes mainly disappears. This muscle-mass becomes divided 

 longitudinally into two great divisions — a dorsal and a ventro-lateral. 

 Into the composition of the dorsal division all of the spinal myotomes 

 enter. From it is derived the dorsal musculature of the adult. The 

 ventro-lateral muscle-mass is formed from the processes which extend 

 from the thoracic myotomes into the body wall. From it are derived 

 the intrinsic muscles of the thorax and abdomen. The differentiation 

 of these muscles takes place during the fifth, sixth and seventh weeks. 



From the median surface of the myotomes near the ventral margin 

 mesenchyme springs to surround the intrinsic structures of the spinal 

 axis (Fig. 16). This mesenchyme is at first non-segmental in distribu- 

 tion. Gradually, however, it becomes denser at the posterior third of 

 each spinal segment. This condensed tissue forms the scleromeres. From 

 the scleromeres are developed the intervertebral discs, the arches and 

 transverse processes of the vertebrae, and the ribs. Between the sclero- 

 meres the bodies of the vertebras are formed. The vertebral column at 

 first surrounds only the ventral half of the spinal-cord. It is at a com- 

 paratively late period that the vertebral arches from each side meet 

 dorsally to form the vertebral spines. 



Owing to the accurate studies by His, the main stages in the develop- 

 ment of the spinal-cord and early formation of the spinal nerves are too 

 well known to demand further description. We find, however, that the 

 dorsal divisions of the spinal nerves are given off after the division of 

 the spinal nerve into somatic and sympathetic branches, a period later 

 than that described by His. When the dorsal musculature begins to be 

 formed from the tissues derived from the myotomes, the dorsal divisions 

 appear and pass into the differentiating musculature, where they give 

 rise to the characteristic median and lateral trunks from which muscular 

 and cutaneous branches spring. The ventral trunks of the spinal 

 nerves in the cervical and lumbo-sacral regions unite to form plexuses 

 from which in turn the nerves of the neck and limbs arise. In the 

 thoracic region the ventral trunks pass inio the body wall as inter- 

 costal nerves. Sympathetic branches are given off at the end of the 

 fourth week, at the period when the thoracic nerves reach the dorsal 

 margin of the coelom. The lateral and ventral cutaneous branches 

 are given off during the fifth, the main muscular branches during the 

 sixth week. 



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