300 The Alligator and Its Allies 



(pli), with its gill clefts and folds, is quite similar 

 to that of the corresponding structures in the chick. 

 None of the four clefts (g 1 ' 4 ) show, in the plane at 

 which the section was cut, any connection with the 

 exterior ; in fact the fourth cleft (g 4 ) 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 



FIGURES i6-i6g (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 (o) 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 

 (a/) is a rounded sac of considerable size just 



