1894.] NATUKAIi SCIENCES OF PHILADELPHIA. 39 



see presently, from two distinct centres of ossification, these two 

 centres, in the judgment of the author are not so exactly repre- 

 sented in the skull of the lower vertebi-ata that the names pro- 

 otic and opisthotic can be given them. 



At a period about the middle of the fifth mouth of intra- uterine 

 life the temporal bone exhibits, according to the dissections of the 

 author, three distinct ossifications : the squamosal, the tympanic and 

 a ring surrouudiug the foramen rotundum ot the otherwise cartila- 

 ginous labyrinth. A little later a second centre of ossification makes 

 its appearance at the extreme end of the superior vertical canal. 

 From these two centres of ossification, which Ave will name the upper 

 and lower otic, as indicating their situation in Man, there arise, due 

 to progressive ossification extending through the labyrinth, the pars 

 petrosa and two- thirds of the pars mastoidea of the temporal, the 

 remaining third of the latter being developed from the squamosal. 

 From the lower otic ring-like ossific centre surrounding the foramen 

 rotundum arise all that part of the pars petrosa seen beneath the 

 cranium, viz.: the lower part of the cochlea, the promonotoiy, and 

 lower part of the fenestra ovalis, the fenestra rotunda, the lower 

 arm of the posterior semicircular canal, the lower part of the facial 

 canal, jugular fossa, the carotid canal and the floor of the tyrapan- 

 num. From the upper otic ossific centre, (that appearing on the 

 superior vertical canal) arise all that part of the pars petrosa seen 

 within the cranium except that entering into the formation of the 

 jugular fossa, viz.: the upper part of the cochlea including the cupola 

 and the base, the internal auditory meatus, the upper part of the 

 fenestra ovalis, the ujjper arm of the posterior and the superior and 

 vertical semicircular canals, the upper part of the facial canal, the 

 tegmeii tympani. 



The various structures just described as respectively produced from 

 the two ossific centres having coalesced about the eighth month"* or even 

 earlier, to form the osseous labyrinth, there is developed as a continuous 

 outgrowth of the latter the pyramidal and mastoid portions of the 

 petrosal, the mastoid being formed more especially by outgrowths 

 from the posterior and external semicircular canals. The outgrowth 

 from the posterior semicircular canal appears as an "elliptical islet "^" 



'"It should be mentioned in this connection that it is often impossible to de- 

 termine exactly the age of a foetus, since foetuses of the same age vary as re- 

 gards length, weight and develoiinu'iit. 



-""Epiotic," Huxley, Op. cit. p. 155. 



