212 



AVERS. 



[Vol. VI. 



B 



A Auditory vesicle. 



B Utriculo-sacculus. 



C Utricular sacculus + two ampullary canals. 



D Utriculo-sacculus + two ampullary canals and lagenar canal. 



Cu( 2^. — Stage A represents the undivided superficial sense organ of the vertebrate 

 ancestor, as it is invaginated from the surface and inclosed within the auditory 

 vesicle to function as the macula acustica vesiculi; the only auditory sense organ 

 of this stage. It is but little removed from the canal organs as they exist in, e.g. 

 Amia, and differs from it mainly in size. This condition is not represented in the 

 adult of living Vertebrates. 



Stage B represents the first division of the macula vesiculi, into its two offspring, the 

 maculae acusticas utriculi et sacculi. This stage is likewise not represented among 

 living forms. 



Stage C is so characteristic of the Cyclostomata, so far as we know them, that we will 

 call it the Cyclostome stage. Here the cristse acusticae anterioris et posterioris have 

 made their appearance. 



Stage D shows the condition of the organs in the Gnathostomata, hence its name, the 

 Gnathostome stage. The cristse acusticse anteriores, horizontales, posteriores, and 

 abortivas are all developed by the division of the two parent ampullary organs of 

 the Cyclostome ancestral stage, while the maculpe utriculi et sacculi undergo divis- 

 ion, giving rise to the parents of the utricular and saccular complexes of sense 

 organs. These latter reach their highest differentiation in some Rodentia and 

 porcine species. 



