20 GEORGE L. STREETER 



an important factor in the growth and changes in the cartilage. 

 In later periods its influence on cartilaginous changes cannot be 

 easily determined, but fortunately for the solution of this point 

 it happens that the perichondrium is late in making its appearance 

 and therefore cannot take any part either in the deposit of new 

 cartilage or in the excavation of the old until after a considerable 

 part of this transformation is already completed. 



The zone of precartilage surrounding the margins of the 

 canals in embryos about 50 mm. long might be mistaken for 

 perichondrium, such for instance as is shown in figure 9. If 

 this area, however, is followed to a slightly older stage it will 

 be found to be converted almost entirely into reticulum. The 

 section shown in figure 10 is through the posterior semicircular 

 canal of an embryo of the same length, 50 nnn., but a little 

 older in development. It is just at this age that precartilage 

 very rapidly reverts to reticulum, much more rapidly than the 

 surrounding cartilage in reverting to precartilage; and therefore 

 in sections at this period we find only a thin rim of precartilage 

 around the margins of the canals. The real perichondrium 

 makes its first appearance when the fetus has reached a length 

 of about 70 mm. A photograph of a section of the posterior 

 semicircular canal of a fetus 73 mm. long (Carnegie Collection, 

 No. 1373) is shown in figure 11. Examination of this section 

 reveals along the outer margin of the periotic reticulum a conden- 

 sation of its trabeculae resulting in the formation of a thin fibrous 

 lamina or membrane near the margin of the cartilage. 

 This is the perichondrium in its early form. It does not abut 

 directly against the cartilage but is separated from it by a zone of 

 transition tissue which consists partly of precartilage and partly 

 of reticulum. This transitional precartilage-reticular zone, be- 

 comes narrower and more abrupt in later stages. In all of the 

 specimens studied, however, it was found intervening between the 

 perichondrium and the surrounding cartilage. It will thus be seen 

 that the perichondrium is a derivative of the periotic reticulum. 

 It forms an outer limiting membrane along the cartilaginous 

 margin of the latter in a manner somewhat similar to that in 

 which the membrana propria forms an inner one along its epithe- 



