298 The Alligator and Its Allies 



entoderm of even thickness. Dorsal to the foregut 

 are three large blood-vessels, a median, and now 

 single, dorsal aorta (ao), and a pair of cardinal veins 

 (cv). The notochord (nt) is small and is flattened 

 against the ventral side of the spinal cord (sc), 

 which latter structure needs no special mention. 

 The muscle plates (mp) are considerably elongated, 

 so that they now extend ventrally to a point slightly 

 below the upper angles of the body cavity. 



Figure 15^ is through the middle region of the 

 embryo, and, owing to the curvature of the body, 

 is not an exact dorso-ventral section; this accounts, 

 in part at least, for the unusual diameter in a 

 dorso-ventral direction of the aorta (ao), which 

 is very large in proportion to the other structures. 

 The posterior cardinal vein is shown on the left, 

 but not on the right. The relative sizes of the 

 spinal cord {sc) and notochord (nt) are very 

 different from what was seen in the preceding 

 figure. In this section the spinal cord is consider- 

 ably smaller than in the preceding, while the 

 notochord is very much larger; in fact the noto- 

 chord here seems abnormally large when compared 

 to corresponding sections of other series. It 

 is true, however, that while the spinal cord has 

 been diminishing in diameter the notochord has 

 been increasing. The spinal cord, notochord, 

 and dorsal aorta are all so large that they are 

 flattened against each other, the pushing in of the 

 ventral side of the spinal cord being even more 



