6 



GEORGE L. STREETER 



morphologically from the adjacent mesenchyme, with which it 

 is directly continuous, only in its more numerous and more 

 C()nii)a('tly ari'anged nuclei and its somewhat richer network of 

 internuclear processes. This is shown in figure 3 which is 

 taken from an embryo a little larger than that in figure 2, 

 but which in its general form is apparently in about the same 

 stage of development. 



Otic copsule 



Ect o d e rm 



G.petros f/-^''^, 



GoOC?,^ 



Med. oblo na. 



Fig. 2 Section through the region of the otic vesicle in a human embryo 9 mm. 

 long (Carnegie Collection, No. 721) enlarged 66.6 diameters. The primordium 

 of the otic capsule, consisting of condensed mesenchyme, can be seen enclosing 

 the vesicle on its lateral surface. 



During the period of growth represented by embryos between 

 9 mm. and 13 mm. long, that is, up to the time when the 

 semicircular ducts begin to separate from the main labyrinth 

 through the apposition and absorption of the intervening mem- 

 branous wall, the zone of condensed mesenchyme around the 

 otic vesicle increases in extent and compactness, thereby form- 

 ing a sharply defined capsule which completely encases the 

 labyrinth. This capsule of condensed mesenchyme has the same 

 openings and corresponds closely in form to the cartilaginous 

 capsule into which it is destined soon to be converted. 



