48 THE HISTOGENESIS AND GROWTH OF THE OTIC CAPSULE AND ITS 



impression on its basal surface. This fundament of the aquseductus cochleae is 

 present in fetuses S5 mm. crown-rump length, but is longer in the 130 mm. fetus, 

 where it nearly reaches the scala tympani. The communication must be estab- 

 lished soon after this. 



SUMMARY. 



The changes in size and form which the cartilaginous capsule of the ear under- 

 goes during its development in the human embryo are accomplished in part by a 

 progressive and in part by a retrogressive 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 tendencies 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 it is in this way that the progressive alterations in 

 the size, shape, and position of the cavities are produced, due to which 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 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 con- 

 cerned follow a definite and 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 dedifferentiation. 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 syn- 

 cytium. In the latter form it redifferentiates into a more specialized tissue, in 

 this case for the most part into a vascular reticulum. 



The formation of the periotic reticulum is first indicated by a cluster of deeply 

 staining nuclei that can be seen along the central edge of the semicircular ducts in 

 embryos soon after the ducts are formed, and at about the time the otic capsule 

 begins to change from condensed mesenchyme into precartilage. These nuclei 

 constitute a focus at which the development of the reticulum and its blood-vessels 

 takes origin. Here the tissue of the otic capsule takes on an appearance that is 

 less like that of a cartilage-forming tissue and more like that of an embryonic con- 

 nective tissue. Spreading from this focus, a narrow area is established which soon 

 encircles the semicircular ducts and becomes the open-meshed vascular reticulum 

 which, in embryos 30 mm. long, everywhere bridges the space existing between the 

 epithelial labyrinth and the surrounding cartilage. In the earlier stages it could 

 not be definitely shown that the primordium of the periotic reticular tissue is not 

 derived from a few predestined mesenchyme cells which become inclosed, along with 

 the otic vesicle, by the condensed tissue of the capsule and after a certain latent 



