DEVELOPMENT OF PERIOTIC TISSUE SPACES 301 



make an arbitrary rule as to how much should be included in 

 the model. The smaller spaces of the reticulum that surrounds 

 the main cavities can be seen coalescing to form larger spaces 

 and these in turn coalesce with the main cavity as it advances 

 into new territory. Thus in a given section there is a consider- 

 able range in the size and completeness of the spaces. The 

 main spaces and the larger adjacent ones that communicate with 

 them are outlined by a membrane-like border. This characteris- 

 tic was utilized as the guide for determining which spaces to 

 admit into the model; only those possessing a more or less com- 

 plete border of this kind were included. 



HISTOGENESIS OF THE PERIOTIC RETICULUM 



Although this communication is more concerned with the 

 process of conversion of the periotic reticular tissue into the 

 larger walled-off spaces, yet for the purpose of completeness a 

 brief survey will be taken of the earlier history of this tissue and 

 the nature of its histogenesis. 



The tissue in which the perioticular spaces develop is derived 

 from the condensed mesenchyme that establishes itself as an 

 encapsulating mass around the otic vesicle in embryos between 

 4 mm. and 10 mm. long. This condensed mesenchyme is sub- 

 seciuently differentiated into the cartilagenous capsule that 

 completely invests the epithehal labyrinth excepting for the 

 three openings that persist in the adult as the internal auditory 

 meatus, the aqueaductus cochleae and the aquaeductus vestibuli, 

 which openings are present in the very earliest stages. 



Originally the cartilagenous capsule abuts directh^ against 

 the epithelial wall of the labyrinth. In embryos about 14 mm. 

 long, howe^'er, the cartilage-forming tissue in the immediate 

 neighborhood of the epithelium undergoes a dedifferentiation, 

 so that an area is established all around the membranous laby- 

 rinth, and conforming to it in shape, that is less like cartilage 

 and more like embryonic connective tissue. It is this that con- 

 stitutes the foundation for the open-meshed periotic reticulum 

 which in embryos 30 mm. long everywhere bridges the space 

 existing between the membranous labyrinth and the surrounding 



