32 THE HISTOGENESIS AND GROWTH OF THE OTIC CAPSULE AND ITS 



the otic vesicle by the condensed tissue of the otic capsule, where they remain in 

 contact with the epithelial labyrinth in a resting condition until the embryo ap- 

 proaches 20 mm. in length. They then show activity and by the time the embryo 

 is 30 mm. long we find them converted into a vascularized reticulum which forms a 

 definite area surrounding each semicircular duct and completely separating it from 

 the receding precartilage. The area of reticulum advances as the precartilagc 

 becomes hollowed out. This can be seen by comparing figures 11, 12, 14, 15, and 

 16, all of which are reproduced on the same scale of enlargement. 



From the histological appearance one could maintain that the reticulum is 

 derived from a few predestined mcsenchymatous cells which, after a latent period, 

 undergo proliferation and occupy the space that is vacated by the receding pre- 

 cartilage in the manner described above, the growth of the reticulum perhaps being 

 the cause of the recession of the cartilage. But one could equally well maintain 

 that the reticulum. is derived entirely from the precartilage; that it is not a pre- 

 determined tissue, but simply precartilage that has undergone dedifferentiation. 

 It is entirely possible that the isolated cells included with the epithelial labyrinth 

 are angioblasts only, everything else being indifferent mesenchyme. In the early 

 stages, where only a few cells are concerned, this matter can not be determined, 

 the histological difference between early precartilage and other embryonic cells not 

 being sufficiently great for their certain recognition. In the later stages, however, 

 it is quite evident that precartilage tissue is actually converted into a reticulum; 

 that the replacement of the temporary precartilage by a reticular connective tissue is 

 accomplished by a process of dedifferentiation, or direct metaplasia, just as we have 

 previously seen in the case of the dedifferentiation of cartilage into precartilage. 



In this connection it is instructive to compare again figures 11 and 15, and also 

 figures 2 and 3, which are details of the same under higher magnification. They 

 show under the same enlargement a section through the lateral canal made in about 

 the same position and cut at the same thickness. It will be noticed that the space 

 occupied by precartilage in the younger stage is entirely filled in by reticulum in 

 the older stage. There is in the older stage, however, more precartilage than before, 

 but it now occupies a more peripheral position. With the change in the position 

 of the precartilage area there is a corresponding enlargement of the lumen of the 

 true cartilage, i. e., the cartilaginous canal. It is clear that we are dealing here 

 with a dedifferentiation of true cartilage into precartilage on the one hand and a 

 dedifferentiation of precartilage into reticulum on the other. These factors, as we 

 already have seen, are of great importance in the alteration in form and size of the 

 cartilaginous canals. 



In younger stages, as in figure 10, the epithelial semicircular duct lies near the 

 center of the area of temporary precartilage. When the reticulum develops it 

 makes its first appearance, and its growth continues more marked along the concave 

 side of the duct than on the convex side that is, on the side toward the utricle 

 rather than toward the periphery of the capsule. On this account the epithelial 

 duct loses its central position and gradually comes to lie along the peripheral border 

 of the cartilaginous canal, where it eventually becomes attached to the periosteum. 



