300 The Alligator and Its Allies 
(ph), with its gill clefts and folds, is quite similar 
to that of the corresponding structures in the chick. 
None of the four clefts (g’“*) show, in the plane at 
which the section was cut, any connection with the 
exterior; in fact the fourth cleft (g*) would scarcely 
be recognized as a cleft if seen in this section alone. 
One or two of the more anterior clefts are open to 
the exterior. Three pairs of aortic arches are seen, 
and each visceral fold has a central condensation of 
mesoblast. 
STAGE XIII 
FicurEs 16-16g (PLATES XXII., XXIII.) 
The embryo (Fig. 16) now lies on one side, body 
torsion being complete. The curvature of the 
body is so marked that the exact length is difficult 
to determine. The eye (e) and ear (0) have 
about the same superficial appearance as in the 
preceding stage. The nose is not shown in this 
figure. About thirty somites are present; the 
exact number cannot be determined in surface view. 
The amnion is complete, though not shown in the 
figure, and the tail (¢) is well formed. The umbili- 
cal stalk was torn in the removal of the embryo, so 
that it is not shown in the figure. The dim out- 
line of the now convoluted heart may be seen if 
the ‘‘cleared’’ embryo be viewed by transmitted 
light; it is not shown in the figure. The allantois 
(al) is a rounded sac of considerable size just 
