THE DEVELOPMENT OF THE MUSCULAR SYSTEM. 



301 



is due to an abbreviation or condensation in the development of the head 

 region. Such condensations are known to occur in .the development of other 

 structures. In a human embryo 3.5 mm. long, three structures resembling 

 segments have been seen somewhat caudal to the region of the optic vesicle on 



FIG. 268.- Drawing from a reconstruction of the right side of a human embryo of 20 mm. (about 



7 weeks). Bardeen and; Lewis. 



The left body wall and viscera have been removed. Note especially the following muscles: The 

 deltoid and biceps, just to the left of the brachial plexus and below the clavicle; the internal 

 intercostals; the diaphragm, attached to the body wall; the transverse abdominal and the 

 rectus abdominis; the quadratus lumborum, just to the right of the transverse abdominal; 

 the psoas, cut just above the lumbo-sacral plexus; the levator ani, running obliquely upward 

 from the coccygeal region. 



one side. On the other side there were seven similar but smaller structures. 

 All were composed of epithelial-like cells surrounding small cavities. 

 Whether these segment-like structures bear any relation to the mesenchymal 

 condensations which appear regularly in the occipital region (p. 193), seems 

 not to have been determined. 



