THE DEVELOPMENT OF THE HUMAN SKULL. 1 T>3 



but it is equally indubitable that Kerckringius' original state- 

 ment is true, and may be readily verified in the dry skulls 

 of foetuses of the age he mentions. The beautiful series of 

 human foetuses presented by Mr. MacMurdo, in the Museum 

 of the Royal College of Surgeons, enable one easily to reconcile 

 the teachings of the older and the later observers, when taken 

 in conjunction with the study of the same parts in wet prepa- 

 rations. 



Fig. 61", A, represents the periotic capsule of a human foetus 

 live and a quarter inches long. 



One ossification in the cartilage (Op.O.) is seen surrounding 

 the fenestra rotunda (F.B.), and extending a little way upon 

 the promontory. A second, very small, quadrate ossification 

 (Pr.O.) is situated at the outer end of the superior vertical 

 semicircular canal, and apparently extends into the carti- 

 laginous tegmen tympani. There is no other ossification in 

 the cartilage than these two. As the upper part of the periotic 

 mass in man answers to the front part, and as the lower part 

 corresponds to the hind part of the same mass in the majority 

 of the Vertebrata, I term the ossification on the superior vertical 

 semicircular canal the pro-otic bone, that on the cochlea the 

 opisthotic bone. 



In some dry foetal skulls of this age the opisthotic ossifi- 

 cation only is seen, just as it is described by Kerckringius, 

 who seems not to have observed the pro-otic ossification at 

 this period. 



The pro-otic ossification rapidly extends, as Meckel states, over 

 the superior vertical semicircular canal (see Fig. 59, A, p. 144), 

 and reaching its posterior end, it includes the front and upper 

 part of the posterior vertical canal ; while, from the outer end 

 of the anterior vertical canal, or the primitive centre, a mass 

 of bone extends backwards in the periotic cartilage and, in the 

 dry skull, appears conspicuously immediately behind the edge of 

 the squamosal. (Pr.O., Fig. 61, B.) This part of it is, in 

 fact, that one of the " tria ossicula " of which Kerckringius says, 

 " pyriforma, acutiore sui parte, squamoso annectitur." 



The opisthotic ossification likewise extends backwards and, 

 its hinder extremity becoming apparent in the dry skull behind 



