DEVELOPMENT OF PERIOTIC TISSUE SPACES 311 



It has also extended upward on tlie dorsal surface of the utricle 

 and is beginning to creep along the inner side of the posterior 

 end of the lateral canal. Ventrally it communicates freely 

 with the scala vestibuli which now extends well down along the 

 cochlear duct. 



The oldest stage studied is shown in figures 8 and 9. These 

 show two views of a wax-plate reconstruction of these structures 

 in a human fetus 130 mm. long (Carnegie Collection, No. 1018). 

 At this time the periotic cistern has spread over the vestibular 

 part of the membranous labyrinth, covering it nearly everywhere 

 excepting at the macular portions where the nerves terminate. 

 In figure 9 it can be seen that the mesial surface of the saccule 

 is not covered; this lies closely against the wall of the cartilage- 

 nous vestibule. The uppermost division of the cistern situated 

 between the crus commune and the ampulla of the posterior 

 canal does not yet open into the general cavity. It has formed 

 separately and owing to the position in which it lies its coales- 

 cence with the other parts of the cistern is retarded. Otherwise 

 free communication exists between all divisions of the cistern. 



DEVELOPMENT OF THE SCALA TYMPANI AND SCALA VESTIBULI 



The scala vestibuli may be regarded as an extension downward 

 of the cistern into the region of the cochlea and as such its growth 

 starts from a focus opposite the fenestra vestibuli (ovale). The 

 scala tympani in a similar way makes its first appearance oppo- 

 site the fenestra cochleae. From these two foci the scalae extend 

 gradually downward along the cochlear duct as two separate 

 spaces which do not communicate with each other until they 

 reach the tip of the duct, where there is finally developed a free 

 opening between them known as the helicotrema. 



In their formation they go through a series of histogenetic 

 changes in essentially the same manner that has been followed 

 in the case of the formation of the cistern. This as we shall see 

 consists of the enlargement of the spaces of the periotic reticulum 

 that originally occupies this region, the enlargement being a 

 result of the disappearance of the protoplasmic bridges of the 



