On the Perilymphatic Spaces of the Amphibian Ear. 247 



lymphatic spaces and the less restricted fluid-containing spaces lying 

 within the cranial cavity. 



We may perhaps regard the perilymphatic system as derived from 

 the irregular cavities found in the cavum perilymphaticum of fishes. 

 Two of these cavities acquired special importance in the transmission 

 of vibrations to the endolymph, and gave rise to the spaces I have 

 called the recessus partis neglectae, and the spatium sacculare. By 

 their increase in size they came into close contact with the wall 

 of the auditory capsule, and as I believe, by the slow and gradual 

 action of their pressure, in the course of many generations they x>ro- 

 duced a suppression of chondrification over the area of contact. 

 The two apertures thus formed are the foramen perilymphaticum and 

 the fenestra vestihuli. There is no doubt that the presence of soft 

 parts, such as blood vessels and lymph spaces, may exercise conside- 

 rable influence over the disposition of skeletal tissue in contact with 

 them. As a recent observation bearing on this point I may quote 

 Gaupp on the skull of R. fusca: "An der Uuterfläche der Ohrkapsel 

 findet man .... bei Larven dieses Stadiums (29 mm) hin und wieder 

 auch eine Lücke im unteren äusseren Umfang der vorderen Kuppel, ent- 

 sprechend der Stelle, wo die Vena jugularis interna der Ohrkapsel an- 

 liegt." It is not possible to study the development of the chondro- 

 cranium without finding instances in which the presence of soft struc- 

 tures, and especially fluid-containing cavities, retards the formation of 

 cartilage. From a retardation of chondrification in ontogeny, to its 

 suppression in the course of phylogeny, is a sequence we can easily 

 imagine. There is then no improbability in the view that the fenestra 

 vestibuli and the foramen perilymphaticum have been produced in con- 

 sequence of the presence of the spatium sacculare and recessus partis 

 neglectae within the auditory capsule. We have however an argument 

 from analogy having great weight. The formation of the aperture by 

 which the recessus partis basilaris leaves the auditory capsule in the 

 Anura, viz, the foramen perilymphaticum inferius, can scarcely be at- 

 tributed to any cause other than the inhibitory action of the recessus 

 over its area of contact with the cartilaginous floor of the capsule. 

 There is at least no suggestion of any other cause, and nothing passes 



