34 THE HISTOGENESIS AND GROWTH OF THE OTIC CAPSULE AND ITS 



suspended in the reticulum, as can be seen in figures 15, 16, and 17, and from these 

 a network of small vessels branches toward the membranous duct on the one hand 

 and the cartilaginous wall on the other. 



In figure 17, which is a section through the posterior semicircular canal in a 

 fetus 52 mm. crown-rump length (Carnegie Collection, No. 96), the reticulum is 

 more mature in its appearance than any that have thus far been described. There 

 is practically no precartilage to be seen. The reticulum now only lacks the 

 membrane-like thickening of its inner and outer margins to render it complete. At 

 the inner margin the cells arrange themselves into a fibrous coat that constitutes the 

 membrana propria of the membranous duct. At the outer margin is formed the 

 perichondrium, the development of which will now be considered. 



DEVELOPMENT OF THE PERICHONDRIUM. 



In the description of the development of the periotic reticulum we have seen 

 how it begins as a small focus along the central border of the epithelial semicir- 

 cular duct and spreads at the expense of the temporary precartilage, forming as it 

 does so a crescentic-shaped area of reticulum inclosing the duct. We have also 

 seen how the invasion or spread of the reticulum into the surrounding area of pre- 

 cartilage is brought about, at least in the later stages, by a dedifferentiation of the 

 latter into the former. 



Furthermore, along with this latter process, the inner margin of cartilage sur- 

 rounding the duct is dedifferentiated into precartilage, so that a new area of pre- 

 cartilage becomes established as the old area disappears. The conversion of precarti- 

 lage into reticulum in the later stages, however, is more rapid than the conversion 

 of cartilage into precartilage, and consequently there comes a time when the pre- 

 cartilage has nearly all disappeared. In such specimens the reticulum extends 

 practically from the epithelial duct to the margin of the cartilaginous canal. The 

 qualifying term "practically" is used because the inner and outer margins of the 

 reticulum are modified in a special manner. The inner margin becomes condensed 

 into a membrane-like coat of fibrous tissue that constitutes the membrana propria 

 of the membranous canal. The outer margin at about this time undergoes changes 

 that result in the formation of the perichondrium. 



In discussing the perichondrium it is important to keep in mind the active 

 alterations in the tissue along the margin of the cartilage that accompany the 

 growth of the labyrinth. It has been seen how the enlargement of the cartilag- 

 inous canals and their alterations in form and position is obtained partly by exca- 

 vation of cartilage and partly by the laying down of new cartilage, the excavation 

 being accomplished by its dedifferentiation into precartilage and reticulum, and the 

 new cartilage being built up through a precartilage stage from the periotic retic- 

 ular tissue. Throughout the entire period of growth of the cartilaginous canals 

 the elements of this continual transformation exist along their margin. The margin 

 during this period is in a state of temporary equilibrium and is capable of advanc- 

 ing or receding as the conditions determine. 



