DEVELOPMENT OF PERIOTIC TISSUE SPACES 307 



otic cistern. In tliis manner the descriptive term introduced by 

 Retzius is retained. 



Before there is any trace of the scalae 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 into a sagittal series and the sections 

 on slides 53 and 54 show the periotic cistern in its most rudi- 

 mentary 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 canal. The re- 

 ticulum here is of the type seen along the semicircular canals in 

 considerably older embryos. Whereas the reticulum elsewhere 

 in this 35 mm. embryo presents a uniformly narrow mesh that 

 is interrupted only by the numerous capillaries branching 

 through it, this particular field gives the appearance of spaces 

 which are more open and which are irregular both in shape and 

 in size. 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 larger spaces. Thus 

 in the above section a few irregular protoplasmic free-ends are 

 seen still projecting into the newly enlarged spaces. The area 

 of this rudimentary periotic cistern is as yet very small and 

 merges indefinitely into the adjoining reticulum. It is not 

 until we come to fetuses about 40 mm. long that it develops 

 spaces of any considerable size, and it is not until we come to 

 fetuses about 50 mm. long that we find a single large space with 

 walls that are definitely outlined so that it can be satisfactorily 

 modelled. 



In a fetus 43 mm. long (Carnegie Collection, No. 886), the 

 spaces forming the rudimentary cistern stand out much more 

 definitely than is the case in the 35 mm. embryo that has just 

 been referred to. There is now just opposite the stapes one space 

 which is much larger than the adjoining spaces. On part of its 

 margin the protoplasmic bridges are stretched along so as to 

 form a smoothly curved continuous boundary. This boundary 

 is defective in some portions and at such places the space merges 



