The Development of the Alligator 315 



knob contains a distinct cavity (not shown in 

 the figure), the myocoel. 



In Figure 17^, owing to the curvature of the body, 

 the plane of the section passes through the body 

 at three places : through the region of the heart and 

 lungs (Fig. I yd) , through the region of the posterior 

 appendages, and through the tail. In fact, the 

 plane of the section represented by each of the 

 preceding figures cut the embryo in more than one 

 region, but for the sake of simplicity only one 

 region was represented in each figure. In the figure 

 under discussion only the leg and tail regions have 

 been drawn, though the latter region (/), being cut 

 through one of its curves, is seen as an elon- 

 gated body with a section of the spinal cord, noto- 

 chord, etc., at each end. Both regions shown in 

 the figure are enclosed in the same fold (a) of the 

 amnion. Of the structures in the dorsal side of the 

 larger or more anterior part of this figure nothing 

 need be said. The most striking feature of the 

 section is the presence of the large posterior leg 

 rudiments {pa). As was noted in the preceding 

 figure, they are, as usual, merely local enlarge- 

 ments or projections of the mesoblast (covered, 

 of course, with ectoblast) of the Wolffian ridge. 

 They are, as shown in this section and in the sur- 

 face view of this stage (Fig. 17), bluntly pointed pro- 

 jections from the sides of the body. The anterior 

 appendage seems to be slightly more developed 

 than the posterior, as was noted in describing the 



