14 



GEOKGE L. STREETER 



and the precartilage in its turn becomes converted along its 

 inner margin into the reticular mesenchyme which finally be- 

 comes the periotic reticulum. In other words, cartilage of the 

 third stage as above described, reverts or is dedifferentiated 

 into cartilage of the second stage and this in turn is dedifferen- 



."Ductus semicirc. lai". 



•/Carrilaqe 



Precarhilaqe 



'. ' •' ' ■ .• ' ■' -Reticulum 



Fig. 7 Section passing transversely through the lateral semicircular canal in 

 a human embryo 30 mm. long (Carnegie Collection, No. 86), enlarged 100 di- 

 ameters. The canal at this time is only slightly larger than the contained epi- 

 thelial duct, but the zone of temporary precartilage marks out an area that is 

 soon to be excavated by the process of dedifferentiation through which it be- 

 comes converted into a reticular connective tissue. 



tiated into a tissue approximating the first stage. It is this ret- 

 rogressive adaptability of its tissues combined with their pro- 

 gressive development which render possible the enlargement of 

 the otic capsule and the alteration in form and position of its 

 contained cavities. 



In the 30 mm. embryo shown in figure 7, the first of these 

 three figures, it will be seen that the epithelial duct is sep- 



