292 The Alligator and Its Allies 



becomes still thinner as it is followed posteriorly 

 over the hindbrain (hb). Between the floors of 

 the fore- and hindbrains, in the acute angle caused 

 by the cranial flexure, is the anterior end of 

 the notochord (;?/), the only part of that struc- 

 ture that lies in the plane of the section. Ventral 

 and posterior to the notochord is a large cavity, 

 the pharynx (/>//), whose entoblastic lining can 

 scarcely be distinguished under this magnification 

 from the surrounding tissues. The stomodeal 

 opening being as yet unformed, the pharynx is 

 closed anteriorly; posteriorly also, owing to the 

 plane of the section, the pharynx appears to be 

 closed, since its connection with the yolk stalk is 

 not shown. In the floor of the pharynx, almost 

 under the reference line ph, a slight depression 

 marks the position of the first gill cleft. In the 

 mesoblast ventral to the pharynx and near the gill 

 cleft just mentioned, a couple of irregular openings 

 represent the anterior end of the bulbus arteriosus, 

 posterior and ventral to which is the heart (///), a 

 large, irregular cavity. The dorsal aorta (ao) may 

 be seen as an elongated opening in the mesoblast, 

 extending in this section from the middle region of 

 the pharynx to the posterior end of the figure where 

 it is somewhat torn. Two of the eighteen or 

 twenty pairs of mesoblastic somites possessed by 

 this embryo are shown at the posterior end of the 

 figure (s), where the plane of the section was far 

 enough from the median line to cut them. 



