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 
(nt) 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 aorte 
(ao), which are of very unequal size. The spinal 
cord (sc) and muscle plates need no special de- 
scription. 
Figure 16d is in the region of the heart (ht) and 
lungs (Ju). 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. Thelung rudiments (Jz) 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 
