140 THE EMBRYOLOGICAL CRITERION 



but afterwards frees itself and becomes slung to the 

 " quadrate." From the hinder edge of the hyoid arch grows 

 out the membranous operculum, in which develop later the 

 opercular bones and branchiostegal rays. The upper jaw is 

 an independent outgrowth of the serous layer. 



The serial homology of the lower jaw and quadrate with 

 the hyoid and with the true gill-arches was thus established 

 in fish, and Rathke had little difficulty in demonstrating a 

 similar origin of lower jaw and hyoid in the embryos of 

 higher Vertebrates. He could even, as we have noted before, 

 find the homologue of the operculum in a flap which grows 

 out from the hyoid arch in the embryo of birds. 



But Rathke could not altogether shake himself free from 

 the transcendental notion of the homology of jaws with ribs, 

 and this led him to draw a certain distinction between the 

 first two and the remaining gill-arches, by which the homology 

 of the former with the ribs was asserted and the homology of 

 the latter denied. He thought he could show that the 

 skeletal structures (lower jaw, " quadrate," and hyoid) of the 

 first two arches were formed in the serous layer, just like 

 true ribs, and like them in close connection with the vertebral 

 skeletal axis. The other, " true," gill-arches appeared to 

 him to be formed in the mucous layer, in the lining of the 

 alimentary canal. They had no direct connection with the 

 vertebral column, and seemed therefore to belong to what 

 Carus 1 had called the visceral or splanchno-skeleton. He 

 did not, however, let this distinction hinder him from assert- 

 ing the substantial homology of all the gill-arches inter se, 

 the first two included. 



Rathke's discoveries relative to the development of the 

 jaws, the hyoid and the operculum, enabled him to make 

 short work of the homologies proposed for them by the 

 transcendentalists. I Ie could prove from embryology that 

 the jaws were not the equivalent of limbs, as so many 

 Okcnians believed. He could reject, with a mere reference 

 to the facts of development, Geoffrey's comparison of the 

 hyoid and the branchiostegal rays in fish with sternum and 

 ribs. lie could show the emptiness of the attempts made 



1 I'tvi i/,->i Ur-Thcilen tics Knoclicn- und Schalcn-Gcrustcs, Leipzig, 

 1828. 



