24 GEORGE L. STREETER 



the reiuiuints of tlu^ capsiihir margins. The appearance of activ- 

 ity in this zone corresponds to the unstable condition of the 

 margin of the cartilage which is still undergoing gradual ex- 

 cavation. 



SUMMARY 



From a study of the development of the cartilaginous capsule 

 of the ear in human embryos it is found that the changes in 

 size and form which it undergoes during its development are 

 accomplished in part by a progressive and in part by a retro- 

 gres.sive differentiation of its constituent tissues. Throughout 

 the entire period of growth, as far as material was available for 

 study, it was found that the margins of the cartilaginous cavities 

 "undergo a process of continual transformation. They exhibit 

 a state of unstable equilibrium, in respect to the opposing tend- 

 encies toward a deposit of new cartilage on the one hand and 

 toward the excavation of the old on the other. The margins 

 thereby are always either advancing or receding and in this 

 way are produced the progressive alterations in their size, shape 

 and position. In this manner a suitable suite of chambers is 

 always provided for the enlarging membranous labyrinth. 



The general tissue mass of the otic capsule during the period 

 represented by embryos from 4 mm. to 30 mm. long passes 

 through three consecutive histogenetic periods, namely, the stage 

 of mesenchymal syncytium, the stage of precartilage and the 

 stage of true cartilage. In the subsequent growth of the capsule 

 it is found that in areas where new cartilage is being deposited 

 the tissues of the areas concerned follow the same progressive 

 order of development. In areas, however, where excavation 

 occurs, where cartilage previously laid down is being removed, 

 it is found that the process is reversed. The tissue in such areas 

 returns to an earlier embryonic state, that is it undergoes de- 

 differentiation. Tissue that has acquired all the histological 

 characteristics of true cartilage can thus be traced in its reversion 

 to precartilage and from precartilage in turn to a mesenchymal 

 syncytium. In the latter form it redifferentiates into some 



