The Development of the Alligator 325 



its walls form a solid, bow-shaped mass of cells. 

 Since there is a narrow space between this mass of 

 cells and the surrounding mesoblast, it might be 

 thought that the lumen of the oesophagus had been 

 closed by the simple shrinkage of its walls; higher 

 magnification, however, fails to show any sign of 

 a collapsed lumen. It is doubtless the problematic 

 and temporary closure of the oesophagus that is 

 noticed in other forms. On each side of the 

 oesophagus, in close relation with the anterior 

 cardinal vein {ac), is noticed a nerve (ai) cut 

 through a ganglionic enlargement. When traced 

 forward these nerves are seen to arise from the re- 

 gion of the medulla, and when followed caudad they 

 are found to be distributed chiefly to the tissues 

 surrounding the newly formed bronchi; they are 

 doubtless the tenth cranial nerves. On the right 

 side of the figure the close connection of this nerve 

 with the near-by gill cleft is seen. Above the 

 paired aortas {ao) the sympathetic nerves (sy) 

 will be noticed. The mesoblast surrounding the 

 spinal cord (sc) and notochord (nt) is distinctly 

 condensed (more so than the figure shows) to 

 form what may be called the centrum (c) and neu- 

 ral arch {no) of the vertebrce. The arch, owing to 

 the slight obliquity of the section, shows here only 

 on one side. The spinal cord is not yet completely 

 enclosed by the neural arches. The muscle plates 

 (mp) are in close connection with the rudiments 

 of the vertebrae just mentioned. The spinal cord 



