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 oesophagus 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 oesophagus are the aortas 

 (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 (;?/) 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 (111 p), 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. 



