EXCAVATION OF THE CARTILAGINOUS OTIC CAPSULE 
a predetermined tissue but is simply precartilage that has under- 
gone dedifferentiation. In the early stages when only a few cells 
are concerned this matter cannot be so well determined, the 
histological difference between early precartilage and indifferent 
mesenchyme 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, 
and that the replacement of precartilage by a reticular connective 
tissue is accomplished by a process of dedifferentiation. By 
identifying a special area through its relation to a particular 
canal, and comparing this selected area in a series of stages, it 
is possible to observe the conversion of precartilage into re- 
ticulum, and to trace histologically step by step the manner in 
which a space occupied by precartilage in a younger stage is re- 
placed by a reticulum in an older stage. This is the same pro- 
cedure which occurs in the conversion of cartilage into pre- 
cartilage and in the latter case, on account of the more highly 
specialized structure of the tissues, the picture is even more 
striking, as will be seen in the following outline in which the 
main features of the process will be pointed out. 
DEDIFFERENTIATION OF CARTILAGE 
It has been noted that in embryos 30 mm. long the main 
capsular mass consists of true cartilage possessing encapsulated 
cartilage cells and an intervening matrix that is differentially 
stainable. A section passing transversely through the lateral 
semicircular canal of an otic capsule of this age is shown in 
figure 7. This, and figures 8 and 9, form a series showing at 
the same enlargement the same canal, i.e., lateral, cut in the 
same plane at three successive stages in its development. A 
direct comparison of these figures can thus be made and there 
is thereby seen the histological changes that occur with the growth 
of the canal. The successive figures may be superimposed 
upon each other and in this way the relative amount and position 
of the constituent tissues be determined. When this is done 
it is found that in the process of enlargement the true cartilage 
around the margin of the canal becomes replaced by precartilage 
