The Development of the Alligator 311 
wall is torn, probably in handling, so that it appears 
to be incomplete around the ventral side of the 
heart. Dorsal to the heart two small circular holes 
(ent) with thick walls are the cesophagus and 
trachea, cut anterior to the point of bifurcation of 
the latter into the bronchial or lung rudiments. 
On either side of these structures is an elongated 
blood-vessel (ac), the anterior cardinal vein, its 
elongation being due to the fact that it is cut at the 
place where it turns downward to empty into the 
heart. Dorsal to the cesophagus are the aorte 
(ao), which are here cut just at the point where 
the two vessels unite to form one; the next section, 
posterior to the one under discussion, shows an 
unpaired aorta. The notochord (nt) and spinal 
cord (sc) need no description, except to note that 
the latter shows active histological differentiation, 
numerous mitotic figures being seen under higher 
magnification, especially in the cells that line the 
spinal canal. On the right of the cord the edge 
of a spinal ganglion (sg) is seen, in connection with 
which in other sections are seen the clearly de- 
fined nerve roots. The condensation of mesoblast 
around the notochord is quite evident, and in close 
contact with this medial condensation are two very 
characteristic, S-shaped muscle plates (mp), which 
extend from the level of the dorsal side of the spinal 
cord to the upper limits of the cardinal veins. In 
some sections the muscle plates even yet show 
slight remains of the myoccel at the dorsal end. 
