38 THE HISTOGENESIS AND GROWTH OF THE OTIC CAPSULE AND ITS 



that results in the formation of the so-called perilymphatic spaces, or periotic spaces 

 as they will be referred to in this paper, the development of which will now be 

 outlined. 



Thus far attention has been directed primarily to regions included in typical 

 transverse sections through the semicircular canals. This was done for the purpose 

 of uniformity and simplicity and because of the ease with which successive stages 

 could be compared with one another. For studying the periotic spaces, however, 

 the region of the canals is not so favorable, because the spaces are late in developing 

 there, and even in their completed form they are not so well defined and highly 

 differentiated as those in the region of the vestibule and cochlea. 



The earliest evidence of a periotic space makes its appearance opposite the 

 stapes. It is developed in the reticulum that fills the interval situated between 

 the saccule, utricle, and the cartilaginous stapes. Even before the general periotic 

 reticulum becomes very extensive, in embryos between 30 and 40 mm. long, it can 

 be seen that its meshes are more irregular and more open in this region than else- 

 where. This is the rudimentary form of the periotic vestibular cistern, which is 

 the first space to become established. 



DEVELOPMENT OF THE PERIOTIC CISTERN OF THE VESTIBULE. 



Aside from the scala vestibuli and the scala tympani, the largest of the periotic 

 spaces is the large reservoir situated between the tympanic wall of the bony vesti- 

 bule with its articulated stapes and the vestibular chambers of the membranous 

 labyrinth. This is the spatium perilymphaticum vestibuli (BNA) or the cisterna 

 perilymphatica (Retzius). In order to eliminate the word lymphatic from the 

 terminology it will be designated here as the cisterna periotica vestibuli, or less 

 formally the periotic cistern. In this manner the descriptive term introduced by 

 Retzius is retained. 



Before there is any trace of the scalse the initial steps in the formation of the 

 cistern can be seen. This is well illustrated in an embryo 35 mm. long (Carnegie 

 Collection, No. 199). This particular embryo is cut in a sagittal series and the 

 sections on slides 53 and 54 show the periotic cistern in its most rudimentary form. 

 It consists of an area of reticulum bounded by the utricle, saccule, ductus reuniens, 

 the proximal end of the cochlear duct, and the ampulla of the posterior semicircular 

 duct. The greater part of the periotic reticulum at this time (35-mm embryo) is 

 characterized by a narrow and uniform mesh that is interrupted only by numerous 

 capillaries branching through it; in the area mentioned, however, the spaces are 

 larger and are more irregular both in shape and in size. They present the appear- 

 ance seen along the semicircular ducts in considerably older embryos, for instance, 

 in the 52-mm. embryo, as is shown in figure 17. From the very first the increase in 

 the size of the mesh seems to be attained by the detachment and retraction of its 

 constituent protoplasmic bridges, thereby allowing adjacent spaces to unite in the 

 formation of composite large spaces. Thus in the above section a few irregular 

 protoplasmic free-ends are seen still projecting into the newly enlarged spaces. 

 This interesting histogenetic process will be taken up again later in connection with 



