40 THE HISTOGENESIS AND GROWTH OF THE OTIC CAPSULE AND ITS 



is a definite row of reticular spaces actively coalescing and constituting the begin- 

 ning of the scala vestibuli. These are shown in figure 21, which is a section of a 

 fetus of about the same age. The spaces of the scala vestibuli lie between the 

 cochlear duct and the cistern. This section also shows very well the relation of 

 the stapes to the cistern. The scala tympani is already well started at this time, 

 but its development is quite independent of the cistern. Within the cistern can 

 be seen scattered clumps of faintly refractive granular threads of what seems to be 

 a coagulated constituent of the plasma. 



The subsequent growth of the cistern is shown in figures 28 to 31. Figures 28 

 and 29 show respectively a median and lateral view of a wax-plate reconstruction 

 of the membranous labyrinth and its periotic spaces in a human fetus 85 mm. 

 long (Carnegie Collection, No. 1400-30). The growth of the cistern here has kept 

 pace with the increase in size of the labyrinth and maintains the same general 

 relations as regards the stapes and the parts of the membranous labyrinth. The 

 view of the cistern in figure 28 is an oblique one which would tend to mislead 

 one as to its width. In reality it is relatively a little wider. It has also extended 

 upward on the dorsal surface of the utricle and is beginning to creep along the 

 inner side of the posterior end of the lateral semicircular duct. Ventrally it com- 

 municates freely with the scala vestibuli, which now extends well down along the 

 cochlear duct. 



The oldest stage studied is shown in figures 30 and 31. 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 31 it can 

 be seen that the mesial surface of the saccule is not covered; this lies close against 

 the wall of the cartilaginous vestibule. The uppermost division of the cistern, 

 situated between the crus commune and the ampulla of the posterior semicircular 

 duct, does not yet open into the general cavity. It has formed separately and owing 

 to the position in which it lies its coalescence with the other parts of the cistern is 

 retarded; otherwise, free communication exists between all divisions of the cistern. 



DEVELOPMENT OF THE PERIOTIC SPACES OF THE SEMICIRCULAR DUCTS. 



From the descriptions given of the adult the reticulum along the ducts never 

 develops a single continuous wide periotic space like that of the cistern and the 

 two scalae. There always remain a few trabecula?, such as are seen in the cistern 

 and scalae in their earlier stages, and these constitute partitions which traverse the 

 .--pace and give it in sections the appearance of a series of separate spaces extending 

 along the inner margins of the semicircular ducts. Although these spaces along 

 the ducts arc incomplete as compared with the cistern and scalse, they are, however, 

 entirely analogous with them in their formation. 



The space along the lateral semicircular duct is the largest. Its posterior end 

 exists as a continuation of the cistern. This can be seen in the lateral view of the 

 model shown in figure '.>(), where the cistern extends for a considerable distance 



