244 ON THE STRUCTURE OF THE SKULL. 



other point.* The middle, larger part of the auditory capsule, 

 therefore, for the present, remains cartilaginous. 



In the beginning of the fourth period, a third ossific centre f 

 arises in the upper angle of the capsule, whereupon all three 

 grow towards one another. But the mode of enlargement and 

 coalescence of these bony nuclei is very remarkable. They do 

 not unite with one another in such a manner as to form a con- 

 tinuous bony capsule for the membranous part of the labyrinth, 

 but are permanently separated by cartilagino-membranous 

 and very narrow symphyses. On the other hand, one [the 

 epiotic] coalesces, in the most intimate manner, with that edge 

 of the supra-occipital which is nearest to it ; so that, even in the 

 more advanced embryos, this bone and it form a moderately 

 long oblong plate, each end of which constitutes a small, tole- 

 rably deep, and irregularly-formed shell, containing a part of 

 the anterior, or upper, semicircular canal. The second bony 

 centre [the opisthotic] becomes anchylosed with the anterior edge 

 of the lateral part of the occipital bone, and also forms a small, 

 irregularly-shaped, but longish scale, which contains the deeper, 

 or lower part, of the posterior crus of that semicircular canal, 

 and besides this, the lower sac, or representative of the cochlea. 

 The remaining bony mass developed in the auditory cartilage 

 [the pro-otic], however, includes the greater part of the mem- 

 branous portion of the labyrinth, and is the largest. The same 

 phenomenon, viz. that the petrosal bone breaks up, as it were, 

 into three pieces, of which two coalesce with the occipital 

 bone, occurs also in Lacerta agiJis, and probably takes place in 

 like manner, if we mav conclude from the later condition of the 

 petrous bone to the earlier, in Crocodilia and Chelonia. 



It has been seen that subsequent observers have fully justi- 

 fied the conviction here expressed by Rathke. 



* The opisthotic ossification. f The epiotic ossification. 



