256 H. S. Harrison, 



9. The observations made in the course of this investigation point 

 to the conchision that the origin of the fenestra vestibuU in the 

 Vertebrata may, ivitli some degree of prohahility, he attributed to the 

 influence of the x)erilymphatic sjKUium saccidare on the process of 

 chondrification over that area of the wall of the capsule with which 

 it was originally in contact. That is to say. the continued pressure 

 of a fluid-containing space has acted first of all by a retardation of 

 chondrification iu ontogeny, which in the course of phylogeny has re- 

 sulted in suppression: 



The chief arguments in favour of this theory are: — 

 a) In the Urodela, two apertures, the foramen perilymphaticum and 

 the fenestra vestibuli, are associated with perilymphatic spaces, 

 to the exclusion of other structures. 

 h) In the Anura, where the recessus partis basilaris first becomes 

 of large size, it also possesses the monopoly of an aperture in 

 the wall of the capsule. The study of development supports the 

 view that the perilymphatic space has actually caused the for- 

 mation of the aperture. 



c) In tire Anuia the recessus partis basilaris passes out through the 

 above aperture to become closely applied to the outer perichon- 

 drium of the chondrocranium. The spatium sacculare in the same 

 way is applied to the membrane of the fenestra vestibuli, and 

 this membrane represents a part of the outer perichondrium of 

 the auditory capsule. 



d) The perilymphatic spaces have been shown to exercise conside- 

 rable influence over the disposition of skeletal tissue in contact 

 with them, as is seen e. g. in the change of coui'se undergone by 

 the ductus reuniens. In Pelobates larvae at one stage this part 

 can be seen "eating its way" through a bar of cartilage. 



e) Tlie presence of structures, such as bloodvessels, is known to 

 retard chondrification over areas in contact with them. Assu- 

 ming this to have been the first effect of the pressure of the 

 spatium sacculare over the area which is now the fenestra vesti- 

 buli, then it is easily conceivable that the more direct relations 



