THE DEVELOPMENT OF THE HUMAN SKULL. 149 



" Sexto mense pyriforme et ovale scutiforme coaluerunt in 

 unura , tertium nonnihil auctum est magnitudine." — L. c, p. 

 224. 



The third figure of the thirty-sixth plate exhibits the condition thus described, and 



the explanation is :— " Bina in osse petroso ossicula ostendit. D, ossis petrosi 

 pars quae jam ex duobus coaluit ; c, tertium ossis petrosi ossiculum." 



" Septiino mense jam tertium illud ossiculum cluobns mense 

 superiore inter se coalitis accessit 



" Nihil ergo de mense octavo nonoque addendum, nisi quod 

 ne turn quidem foetus ullum habeat processum mamillarem, et 

 quod adhuc insigni cartilagine distet os petrosum ab occipitis 

 et syncipitis ossibus." — L. c, p. 224. 



The temporal hone of a seven months' foetus is represented in Plate xxxvii., Fig. 2, 

 with the explanation : — " Qua? primo tria, deinde bina, fuerunt in petroso ossi- 

 cula, jam in unum coaluisse, ostendit. C, ossis petrosi substantia, ex tribus 

 jam ssepe dictis in unum eoalita." 



Cassebohm (" Tractatus quatuor de Aure Humana," 1734, 

 pp. 19 and 45 ; " Tractatus Quintus," 1735, p. 15) discovered that 

 the little linear ossification mentioned in the first extract from 

 Kerckringius is developed in the immediate vicinity of the 

 fenestra rotunda, eventually surrounds it, and extends upon the 

 base of the pars jpetrosa. But the first definite light thrown 

 upon the signification of Kerckringius' " Tria ossa ' : is in the 

 following extract from Meckel's " Handbuch der Vergleichenden 

 Anatomie" (1820. Bd. iv., p. 49), though Meckel does not take 

 the trouble to refer to and explain the older observer's state- 

 ments : — 



" 4. Bony lahyrinth. — In investigating the formation of the 

 bony labyrinth, the origin of the bony substance of the petrous 

 bone is very carefully to be distinguished from that of the 

 labyrinth itself. The former begins earlier than the latter, 

 according to the ordinary type of ossification, by the develop- 

 ment of a loose, soft, reticulated tissue in the previously existing 

 homogeneous cartilage, and extends from before backwards. 



" The first part to ossify, about the end of the third month, is 

 the circumference of the fenestra rotunda, which is remarkable by 

 reason of the analogy of the fenestra rotunda to the tympanic 



