412 DEVELOPMENT OF ELASMOBRANCH FISHES. 



Auditory Organ. With reference to the development of the 

 organ of hearing I have very little to say. Opposite the in- 

 terval between the seventh and the glosso-pharyngeal nerves 

 the external epiblast becomes thickened, and eventually in- 

 voluted as "a vesicle which remains however in communication 

 with the exterior by a narrow duct. Towards the close of stage 

 K the auditory sack presents three protuberances one pointing 

 forwards, a second backwards, and a third outwards. These are 

 respectively the rudiments of the anterior and posterior vertical 

 and external horizontal semicircular canals. These rudiments 

 are easily visible from the exterior (PI. 15, fig. 2). 



As has been already pointed out, the epiblast of Elasmo- 

 branchs during the early periods of development exhibits no 

 division into an epidermic and a nervous layer, and in accord- 

 ance with its primitive undifferentiated condition, those portions 

 of the organs of sense which are at this time directly derived 

 from the external integument are formed indiscriminately from 

 the whole, and not from an inner or so-called nervous part of it 

 only. In the Amphibians the auditory sack and lens are de- 

 rived from the nervous division of the epiblast only, while the 

 same division of the layer plays the major part in forming the 

 olfactory organ. It is also stated that in Birds and Mammals 

 the part of the epiblast corresponding to the nervous layer is 

 alone concerned in the formation of the lens, though this does 

 not appear to be the case with the olfactory or auditory organs 

 in these groups of Vertebrates. 



Mouth involution and Pituitary body. 



The development of the mouth involution and the pituitary 

 body is closely related to that of the brain, and may con- 

 veniently be dealt with here. The epiblast in the angle formed 

 by the cranial flexure becomes involuted as a hollow process 

 situated in close proximity to the base of the brain. This hollow 

 process is the mouth involution, and it is bordered on its pos- 

 terior surface by the front wall of the alimentary tract, and on 

 its anterior by the base of the fore-brain. 



