DEVELOPMENT OF ELASMOBRANCH FISHES. 189 



external epiblast becomes thickened, and eventually involuted 

 as a vesicle which remains however in communication with 

 the exterior by a naiTow 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. xiv. 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 derived 

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

 division of the layer plays the major part in forming the olfac- 

 tory 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 conve- 

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

 by the cranial flexure becomes involuted as a hollow process situ- 

 ated 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. 



The uppermost end of this does not till near the close of stage 

 K become markedly constricted off from the remainder, but is 

 nevertheless the rudiment of the pituitary body. PI. xiv. 

 figs. 9 a and 12 m shew in a most conclusive manner the cor- 



