232 H. S. Harrison, 



tions of the perilymph must be transmitted to the tympanal areas, 

 each of which is in close proximity to a macula acustica. The walls 

 of the ductus are in the adult so thickened that delicate vibrations 

 could not pass into the perilymphatic tissue (figs. 3, 4, 5). The only 

 course would therefore be as follows: — Perilymph of spatium saccu- 

 lare is affected by vibrations of the membrane of the fenestra vestibuli, 

 tympanal area of sacculus wall receives vibrations and passes them 

 on to endolymph, by which the cells of the macula acustica are more 

 or less directly affected; vibrations also pass along ductus perilym- 

 phaticus and set in motion the tympanal areas of the wall of the pars 

 neglecta and pars basilaris respectively. Eventually the vibrations are 

 lost in the spatium meningeale, which has probably a function compa- 

 rable with that of the membrane of the foramen rotundum of higher 

 forms. It is interesting to note that the pars superior uf the laby- 

 rinth is not in a position to receive vibrations directly from the peri- 

 lymph. These must be transmitted chiefly from the endolymph of the 

 pars inferior. This fact is not without importance in view of the 

 serious doubt that exists as to whether the pars superior is concerned 

 in audition. 



In the development of the perilymphatic system in Triton taeniatus, 

 the first portion to appear is the recessus partis neglectae, which is 

 present in a larva of 12 mm. The foramen perilymphaticum is also 

 present as an independent aperture, though at this stage it is filled up 

 by connective tissue. The first portion of the spatium sacculare to 

 develop is that lying above the fenestra vestibuli. The ductus perilym- 

 phaticus grows ^) out from the recessus partis neglectae and joins the 

 spatium sacculare. The spatium meningeale is also formed by the in- 

 growth of the recessus partis neglectae through the foramen perilym- 

 phaticum into the cavum cranii. When the larva has reached a length 

 of 30 mm the essential conditions are as in the adult, the chief diffe- 

 rences lying in the smaller extent of the spatium sacculare, in the 

 straighter course of the ductus, and in the inconspicuous size of the 

 spatium meningeale. The recessus fenestrae vestibuli is being formed 



^) The connective tissue cells in the region concerned no donl)t take part in 

 foniiing the walls of the advancing spaces. 



