The Development of the Frog 55 



second gill arches, and a little later a third 

 pair is formed upon the third arch. The ex- 

 ternal gills reach their greatest development 

 at about the time of the opening of the mouth, 

 and at that time each of the first two consists 

 of from five to seven main lobes, with numer- 

 ous secondary lobes along their posterior bor- 

 ders (Fig. 23, A). The gills on the third arch 

 are much smaller than those of the arches in 

 front, and are nearly covered by them. The 

 course of the circulation in the external gills 

 may be easily seen under the microscope, each 

 main lobe and each minor lobe being supplied 

 with an efferent and an afferent blood-vessel 

 (Fig. 23, FandAF). 



The opercular folds arise, before the mouth 

 opens, as two folds of skin from the hyoid 

 arches ; they unite with each other in the ven- 

 tral line, and grow backward as a sort of 

 hood over the external gills (Fig. 24). The 

 posterior border of this hood fuses with the 

 body wall behind the gills, on the right and 

 ventral sides, but remains open on the left side 

 as a sort of spout (Fig. 24, OA), through 

 which the gills of that side frequently protrude, 

 and through which the water, taken into the 

 gill chamber through the mouth, passes again to 



