270 The Alligator and Its Allies 



is found to open ventrally and medially into the 

 anterior end of the heart. 



Figure ii^ is about a dozen sections posterior 

 to the preceding. The appearance of the over- 

 hanging yolk (3'), of the amnion (a), and of the 

 notochord {nt) is about as in the more anterior 

 section. The medullary canal {mc) is a straight, 

 vertical slit, and the depression in the floor of the 

 pharynx {ent) is much more shallow. The body 

 cavity {he) is much larger and extends across the 

 mid-ventral line beneath the heart (/?/) , which is cut 

 through its middle region. The heart may be 

 traced through about twenty sections (one tenth 

 the length of the embryo) ; its mesoblastic wall 

 (mes') is thin and irregular, and is lined by a dis- 

 tinct endothelium {en') whose exact origin has not 

 yet been worked out. 



Figure ii^ is just back of the heart, and shows 

 in its place the two vitelline veins {vv) . The depres- 

 sion in the floor of the enteron {e7tt) is entirely 

 distinct from the one that has been mentioned 

 above, and is simply the posterior limit of the 

 head-fold of the entoderm ; the fifth section poste- 

 rior to this shows where this depression opens ven- 

 trally to the yolk sac. The other structures 

 shown in the figure are not markedly different 

 from what was seen in Figure 11^. 



Figure 1 1/ is about one tenth the length of the 

 embryo posterior to Figure lie. The chief differ- 

 ences here noticed are in the enteric and coelomic 



