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 cesophagus had been 
closed by the simple shrinkage of its walls; higher 
magnification, however, fails to show any sign of 
acollapsedlumen. It is doubtless the problematic 
and temporary closure of the cesophagus that is 
noticed in other forms. On each side of the 
cesophagus, in close relation with the anterior 
cardinal vein (ac), is noticed a nerve (cn) 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 aortz (ao) the sympathetic nerves (sy) 
will be noticed. The mesoblast surrounding the 
spinal cord (sc) and notochord (né) is distinctly 
condensed (more so than the figure shows) to 
form what may be called the centrum (c) and neu- 
ral arch (za) of the vertebree. The arch, owing to 
the slight obliquity of the section, shows here only 
onone 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 vertebre just mentioned. The spinal cord 
