ON THE SENSORY PIT OF THR OROTALIN^E. 53 



When seen in surface view (PL 4, fig. 11) tliese elevations 

 are rounded and of variable size, being from 9 to 14"5 ju in 

 diameter. The characteristic embossed appearance is due to 

 the formation of the cuticle by a lining epithelium of hemi- 

 spherical cells, each of which has a height of about 7 to 9 /,(, 

 and a flat base resting upon more numerous, smaller, sub- 

 epithelioid cells. The nuclei of these cells are large oval 

 structures, containing a very distinct nucleolus. Immediately 

 underneath the cellular stratum, «,nd forming a bed upon 

 which the latter I'ests, is a second layer consisting of dense 

 fibrous connective tissue containing blood-vessels, and be- 

 tween this layer and the hollow of the maxillary bone there 

 is a third stratum of delicate connective tissue, containing 

 numerous, relatively large, connective-tissue corpuscles. In 

 consequence of the delicate nature of this third stratum, the 

 lining epithelium of the chamber, together with the layer of 

 fibrous connective tissue upon which it rests, readily comes 

 away from the hollow of the maxillary bone, which in young 

 animals is a smooth hemispherical cavity, but in older 

 examples is generally irregularly pitted. A considerable 

 number of nerve-fibres are found in the connective tissue 

 underlying the cellular layer (cf. PI. 4, fig. 10, c. t.f.), but I 

 have not succeeded in tracing their ultimate distribution to 

 the epithelial cells. The total thickness of tissue between 

 the inner cuticle of the chamber and the maxillary bone is 

 not moi'e than about 50 fx. 



The pit-membrane forms the inner wall of the outer 

 chamber and the outer wall of the inner chamber, and, as 

 both these chambers are lined by a thin cuticle, this mem- 

 branous partition consists of a thin sheet of tissue, bounded 

 by cuticle on both its external and internal faces. It is the 

 only truly sensory part of the organ, and is a semi-transparent 

 membrane of not more than 25 fx in thickness, somewhat 

 loosely stretched between the chambers, and forming a com- 

 plete wall of separation between them. As was mentioned by 

 Leydig,' it exhibits a series of folds or ridges ; these are only 



' Leydig, 1. c. 



