EXCAVATION OF THE CARTILAGINOUS OTIC CAPSULE 17 



in the latter but it has moved outward into the area that was 

 previously true cartilage. At this period the outer perimeter 

 of the precartilage is 192 mm. as compared with 115 mm. in 

 figure 7. As the old area of precartilage disappeared, a new and 

 more peripheral area became established. Thus it may be seen 



Ductus semicirc. lat. 



Cartilaqe 



Precarlilaqe" 



,^: - " . ''''';'''!vv?.i' ./'/'•'; ' T ?eticulum 



Fig. 9 Section through the lateral semicircular canal in human fetus 50 

 mm. long (Carnegie Collection, No. 95). Enlarged 100 diameters. This sec- 

 tion is taken at the same relative position and at the same enlargement as those 

 in figures 7 and 8, so that they may be directly compared. It will be seen that 

 the area of precartilage in figure 7 is now entirely replaced by reticulum, where- 

 as a new and more peripheral area of precartilage has formed at the expense of 

 surrounding cartilage. This more peripheral precartilage likewise in the end 

 becomes reticulum. 



that true cartilage has been dedifferentiated into precartilage 

 and this in turn into the periotic reticulum. It is in this way 

 that the enlargement of the canals is accomplished, a process of 

 excavation based on the dedifferentiation of a specialized tissue 

 into a more embryonic type, followed by a readjustment of re- 

 differentiation of this simpler form into a tissue adapted to the 

 new conditions. 



THE AMERICAN JOUR.N'AL OF ANATO.MY, VOL. 22, NO. 1 



