CONTAINED PERIOTIC TISSUE-SPACES IN THE HUMAN EMBRYO. 27 



this region, and although it does not directly concern the development of the con- 

 tained perioticular spaces, yet it may not be out of place to point out some of the 

 elements of this process as they are seen in our material. In fact, the cartilaginous 

 capsule of the ear is an especially favorable place for studying the general question 

 of growth of cartilage, for two reasons: (1) there are, on account of the intricacy of 

 form of the labyrinth, many kinds of cartilaginous changes found there that are 

 necessary to accommodate its growth, including the deposit of new tissue and the 

 removal of old tissues; (2) the topography is so well marked by known landmarks 

 that all of these changes as well as the location and direction of growth can be 

 easily followed. 



Growth of cartilage is usually considered to be of two kinds, which are dis- 

 tinguished from each other by being either interstitial or perichondrial. Inter- 

 stitial growth is described as consisting of an increase in the amount of hyaline 

 matrix and the growth and proliferation of the encapsulated cartilage cells. The 

 new cells form new capsules to a certain extent, but a point is finally reached beyond 

 which the newly proliferated cells continue to occupy their parent capsule . From 

 this variety of growth there results a uniform intumescence of the tissue without 

 producing any marked change in its form. This manner of growth forms a large 

 element in the increase in size of some parts of the capsule of the ear. In those 

 parts, however, where a change in form is involved the growth is more like that 

 described under perichondrial growth and consists of a new deposit of cartilage 

 along the borders of the older cartilage, the consituent cells passing through a pre- 

 cartilage stage. In the otic capsule this latter type of growth is actively going on 

 even before a definite perichondrium is established. The deposit of new cartilage 

 along the margin of older cartilage and the removal of old cartilage by dedifferen- 

 tiation are indeed the main factors in the process through which the form of the 

 ear-capsule is modeled. 



The excavation of established cartilage can be studied by comparing sections 

 through the semicircular canals at different stages, such as appear in figures 11, 12, 

 14, and 15. These are all sections through the same canal (lateral), taken in about 

 the same position, and are enlarged the same number of diameters. It is, of course, 

 possible that they were shrunken in different degrees in the process of embedding; 

 this discrepancy, however, is probably not enough to interfere with their showing 

 the approximate increase in size of the cartilaginous canal at the respective ages. 

 A crude measurement of the perimeters of the canals as seen in the original photo- 

 graphs (100 diameters) yields the following circumferences: 30-mm. embryo, 115 

 mm. circumference; 37-mm. embryo, 132 mm. circumference; 43-mm. embryo, 152 

 mm. circumference; 50-mm. embryo, 192 mm. circumference. It is evident that we 

 are dealing with an enlarging space and that a study of its receding edge must give 

 the histological picture of the replacement of true cartilage by other tissue, either 

 by dedifferentiation or by direct metaplasia. 



If, with this process in mind, one makes an examination of the specimen shown 

 in figure 11 (Carnegie Collection, No. 86) it will be seen under higher magnification 

 that a rather definite border can be made out separating the general mass of true 



