The Development of the Alligator 303 



figure: one pair opens widely on either side, so 

 that there is a large area of the section that is 

 distinct from the two still larger portions and con- 

 tains a small, thick -walled cavity (g) on the right 

 side; this cavity is a gill cleft that is cut through 

 neither its outer nor its pharyngeal opening. 



No structures other than this small portion of a 

 gill cleft and a few blood-vessels are to be seen in 

 this middle region of the section. In the more 

 posterior part of the section, in which the notochord 

 (fit) is located, a pair of curved clefts may be seen, 

 opening entirely through the wall on the left, but 

 closed on the right (g). One distinct pair of aortic 

 arches is shown (ar), and also the dorsal aortse 

 (ao), which are of very unequal size. The spinal 

 cord (sc) and muscle plates need no special de- 

 scription. 



Figure i6d is in the region of the heart (/?/) and 

 lungs (111). The former is an irregular cavity whose 

 walls, especially on the ventral side {mes'), are be- 

 coming very thick and much folded. Although 

 thin, the body wall completely surrounds the heart, 

 as would be expected, since this was true of the pre- 

 ceding stage. The lung rudiments (hi) and the fore- 

 gut from which they have arisen have the same ap- 

 pearance as in the chick ; they consist of three small, 

 thick-walled tubes so arranged as to form a nearly 

 equilateral triangle. They are surrounded by a 

 swollen, rounded mass of mesoblast which almost 

 completely fills the surrounding portion of the 



