THE ROLE OF THE AUDITORY SENSORY EPITHE- 

 LIUM IN THE FORMATION OF THE 

 STAPEDIAL PLATE 



FRANKLIN PEARCE REAGAN 



Department of Comparative Anatomy, Princeton University 



TEN FIGURES 



Certain structures which seem to have undergone transforma- 

 tion in the process of transition in vertebrate life from aquatic 

 to terrestrial conditions have always been of great morpho- 

 logical interest. Structures of this sort are abundant in the 

 pharyngeal region, whieh, especially in earlier times, furnished 

 a most productive field of study. Such study was often concerned 

 with the homologies among the nervous, muscular, vascular, 

 skeletal and epithelial derivatives of this pharyngeal region. 



Between the intermittent communications of the pharynx 

 with the exterior there are developed mesenchymatous visceral 

 arches in which chondrification takes place, forming the so-called 

 'visceral ribs;' these may ossify and retain their original position 

 as components of the pharyngeal skeleton, or they may become 

 greatly modified in the higher forms, acquiring a special function 

 very unlike that of their more primitive homologues. In the 

 lower vertebrates the pharyngeal ribs or components of the 

 visceral skeleton tend to preserve their original resemblances 

 to each other. Particularly striking here, also, are the great 

 independence and the wide separation of the visceral skeleton 

 from the skeleton of the central nervous system — the cranial 

 skeleton. In some of the primitive elasmobranchs, for in- 

 stance, the mere cutting of three ligaments may be sufficient for 

 the complete separation of these two skeletal complexes (i.e., 

 visceral and cranial). But even in forms as low as the holo- 

 cephali, the visceral skeleton has become immovably articu- 



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