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 oDtic 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. 



