12 GEORGE L. STREETER 



reticulum which eventually intervenes between the capsule and 

 the epithelial labyrinth. The relation existing between this 

 reticulum and the three stages of cartilage that have just been 

 defined must therefore now be referred to. The formation of 

 the periotic reticulum is first indicated by a cluster of deeply 

 stained nuclei that can be seen along the central edge of the 

 semicircular ducts in embryos soon after the ducts are formed, 

 and at about the time the otic capsule begins to change from 

 condensed mesenchyme into precartilage. These nuclei con- 

 stitute a focus at which the development of the reticulum and 

 its blood vessels takes origin. Here the tissue of the capsule 

 gradually takes on an appearance less like a cartilage-forming 

 tissue and more like embryonic connective tissue. Spreading 

 from this focus a narrow area is established which soon encircles 

 the semicircular ducts and becomes the open-meshed vascular 

 reticulum which in embryos 30 mm. long everywhere bridges 

 the space existing between the epithelial labyrinth and the sur- 

 rounding cartilage. 



While in the stage of condensed mesenchyme and in the earlier 

 part of its precartilage period the tissue of the otic capsule to 

 all appearances abuts directly against the epithelial wall of the 

 labyrinth as shown in figures 2, 3 and 4. It is possible, how- 

 ever, that some of the cells directly adjacent to the epithelium 

 do not properly belong to the tissue of the otic capsule. It is 

 conceivable that such cells may represent indifferent mesenchyme 

 and perhaps angioblasts which were originally enclosed, along 

 with the otic vesicle, by the condensed tissue of the capsule 

 where they remain in contact with the epithelial wall in a resting 

 condition until the embryo attains a length of 20 mm. We 

 might regard as an indication of their resumed activity the forma- 

 tion of the deeply stained foci along the central margins of the 

 canals which have been described above. It might thus be 

 maintained that the periotic reticulum is derived from a few 

 predestined mesenchyme cells which after a latent period undergo 

 proliferation and occupy the space vacated by the receding pre- 

 cartilage. On the other hand one may also maintain that the 

 reticulum is derived from cartilage-forming tissue; that it is not 



