EXCAVATION OF THE CARTILAGINOUS OTIC CAPSULE 6 



are maintained but there are alterations in the detailed form. 

 As the canals become larger and longer they describe arcs of 

 lesser cm'vature. If one compares the superior canal of an 80 

 mm. fetus with that of a 30 mm. fetus it will be found that in 

 the former it has doubled its diameter and trebled its length. 

 There is, moreover, a constant change in the relative position 

 of the cartilaginous canals. The lateral canal, for instance, 

 progressively recedes from the lateral wall of the vestibule. In 

 studying this canal, therefore, one may know that it is steadily 

 becoming larger by means of a process of excavation, but this 

 is so managed that the canal as a whole moves in a lateral direc- 

 tion through the substance of the cartilaginous capsule. The 

 topography of the cartilaginous labyrinth is so well provided with 

 known landmarks that these changes in its size and form can be 

 accurately followed. It is possible to determine deductively 

 at what points new cartilage is being laid down and at what 

 points it is being removed. On this account the cartilaginous 

 capsule of the ear is a particularly favorable place for determin- 

 ing the histological features of the growth of cartilage. 



As has been noted above the growth of the cartilaginous otic 

 capsule resolves itself into an increase in its external dimensions 

 with a simultaneous hollowing out and reshaping of its contained 

 chambers. It at once becomes evident that this cannot be 

 accounted for on the basis of a simple interstitial increase in the 

 mass of cartilage together with its passive rearrangement to 

 allow for the enlarging cavities, due for instance to a mechanical 

 expansive pressure from the growing membranous labyrinth 

 with its surrounding tissue and fluid. Such a passive rearrange- 

 ment could only occur in a tissue that is very plastic, whereas 

 cartilage is one of the least plastic of the embryonic tissues. 

 Moreover the histological picture is not that of mechanical pres- 

 sure. The cartilaginous chambers are always excavated slightly 

 in advance of the space actually required by the membranous 

 labyrinth, and there is no evidence of the labyrinth being 

 cramped or of the creation of pressure grooves in the margins of 

 the cartilage. Nor is the situation improved by the introduc- 

 tion of the conjectured activity of the perichondrium, either in 



