18 GEORGE L. STREETER 



In addition to the excavation of cartilage there occurs, in 

 connection with the growth and alteration of form of the otic 

 capsule, the deposit of new cartilage. As the lateral cartilaginous 

 canal, for instance, enlarges it also moves laterally, so that the 

 distance between it and the cartilaginous vestibule increases, 

 thereby producing a lateral migration of the space as a whole. 

 Such a migration must involve an excavation of the established 

 cartilage on its lateral margin and the formation of new cartilage 

 on its median margin. Therefore on the lateral margin we find 

 true cartilage being dedifferentiated into precartilage and on 

 the median margin precartilage being differentiated into true 

 cartilage. The margins of the cartilaginous canals throughout 

 the whole embryonic period are in an unstable condition and are 

 constantly undergoing changes. These are either in the nature 

 of a uniform excavation throughout their whole contour, re- 

 sulting in a simple enlargement of the canal, or of an excavation 

 in certain parts combined with a deposit of additional cartilage 

 in others resulting in a change of form and position of the canal. 

 On account of the well defined landmarks that characterize 

 the labyrinth, it is possible to orient points at which excavation 

 and new deposit respectively are occurring. Thus one can follow 

 the histological phenomena of these two processes with great ac- 

 curacy. Where new cartilage is being deposited, the tissue shows 

 all the stages of development from an embryonic connective tissue 

 on its central margin through an aj^ea of precartilage to a true 

 cartilage on its more peripheral margin. These different grades 

 merging into one another repeat stages which characterized the 

 whole capsule in embryos between 14 and 30 mm. Where 

 the cartilage is undergoing excavation the same transitions 

 exist, but the changes are more abrupt and there is a sharper 

 line of transition between the different zones. The \\ddth, how- 

 ever, and the sharpness of the zones vary somewhat, being 

 relatively ^\'ider and less abrupt in youngef stages and becoming 

 narrower and more abrupt in their transition in older fetuses. 

 It is quite possible that these changes occur in waves and when 

 the zones are wider and less abrupt it is due to the greater acti\aty 

 of this process of dedifferentiation and when the zones are nar- 



