DEVELOPMENT OF THE EAR IN THE COMMON FROG. 169 



by their thickened epithelium before they can be distinguished by 

 their shape. 



The last division which takes place in the sensory tract separates 

 it into two parts, one of which lies in the utriculus and one in the 

 sacculus. It does not take place for some time after the others, 

 and even in the adult these two parts are not far removed from 

 each other. 



e. The Becessus Labyrinthi. — The existence of this peculiar organ, 

 the anatomical relations of which have been described by Hasse,* 

 has been overlooked in the Anura. The animals that I have 

 examined (fiana temporaria, Bufo vulgaris, and Dadylethra larva) all 

 possess very similar structures. Taking the adult frog as a type, the 

 relations of this part of the auditory sac may be briefly noted here. 

 A narrow duct leads from the sacculus near its opening into the 

 utriculus, and, running up the inner surface of the vestibule, per- 

 forates the skull wall. Within the skull it is dilated to form a large, 

 thin-walled, vascular sac, which extends some distance both in front 

 of and behind the duct. The sacs of the two sides are connected by 

 bands of spongy tissue above the cerebellum, and below the brain 

 just behind the pituitary body. 



The walls are vascular, and Hasse has assigned to this part of the 

 ear the function of supplying the endolymph. It cannot be sensory, 

 for at no period are its walls supplied with nerves in sufficient 

 quantity, and its histological structure will not admit of such an 

 interpretation. It would seem to have some function of importance 

 in the adult, as it steadily increases in size during the growth of the 

 tadpole, and it is after the tadpole stage is passed that this increase 

 in size becomes most rapid and the blood-supply most copious. 



The relations of this organ at different stages are shown in Figs. 

 4, 5, 7, 9, 17, and 18. Its mode of origin and early stages have 

 already been described, and until the semicircular canals are formed 

 little is to be noted except a general growth in size, accompanied by 

 a movement towards the brain, so that it comes to lie in close contact 

 with this organ. As the distal part comes close to the brain, it 

 begins to expand and its duct narrows ; at the same time the upper 

 lip of the duct elongates so as to carry the vestibular opening down- 



* " Anatomische Studien," lid. iv., "Die Lymphbahnen des inneren < Mires der 

 fVirbelthiere." 



