THE EAR OF MAN. 211 



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 acusticse utriculi et sacculi. This stage is likewise 

 not represented among living forms. 



Stage C is so characteristic of the Cyclosto7naia, so far as we know 

 them, that we w'U call it the Cyclostome stage. Here the cristas acusticae 

 anterior et posterior have made their appearance. 



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

 its name the Gnathostome stage. The cristae acusticae anteriores, horizon- 

 tales, posteriores and abortivae are all developed by the division of the two 

 parent ampullary organs of the Cyclostome ancestral stage, while the 

 maculae utriculi et sacculi undergo division, 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. 



Retzius finally came to the conclusion that this nerve- 

 end organ had arisen from the posterior, ampullar sense- 

 organ, and that among the higher forms, especially the 

 mammalia, it was no longer produced, or, as he expressed 

 it, the macula neglecta in these forms had disappeared 

 in the prista acustica posterior. 



After an examination of all the evidence bearing on 

 this question, both from the embryological and the 

 anatomical sides, I have solved the problem of the 

 morphological value of the parts of the internal ear 

 and their inter-relationships, by the discovery of the 

 very simple law which governs their origin and suc- 

 cession. Let me give you the whole problem in a 

 nutshell, even at the risk of some slight repetition. 



The primitive auditory sense-organ is invaginated 

 from the surface of the body, and may be said at this 

 time to be in the vesicular stage (Fig. 14, A). So far 

 as we know, this condition is not retained by any adult 



