192 Vertebrate Embryology 



structures is to be explained only by as- 

 suming that the birds are descended from 

 aquatic and gill-breathing animals. The frog, 

 being less distantly removed from the fish, or 

 fish-like ancestors, still retains the gills in a 

 functional condition, during the early part of 

 its development. 



Although the changes involved in the 



FIG. 63. TRANSVERSE SECTION THROUGH THE ANTERIOR PART 

 OF AN EMBRYO OF 68 HOURS. Owing to the curvature of the 

 embryo the section passes through it twice. (After Duval.) 



ABi-^ first to fifth branchial arches. AAo^ aortic arch of the fifth branchial 

 arch. A0, aorta. Ci, bulbus arteriosus. Ch, notochord. CM, medullary canal. 

 CR, lens, fb 1-4, first to fourth visceral clefts. GS^ spinal ganglion. NO, optic 

 nerve. Ph., pharynx. 7!&, thyroid gland, ^i, fore-brain. VCA , anterior cardinal 

 vein. VO) optic cup. 



ultimate fate of the visceral 

 do not take place until a 

 will be convenient briefly to 

 changes at this point, and 

 merely recall them to mind 

 times. 



As has been said, all four 

 clefts become closed, after a 



clefts and folds 



later period, it 



describe those 



then, perhaps, 



at the proper 



of the visceral 

 short time, so 



