CONTAINED PERIOTIC TISSUE-SPACES IN THE HUMAN EMBRYO. 39 



the development of the two scalse. 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 con- 

 siderable 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 satis- 

 factorily modeled. 



In a fetus 43 mm. long (Carnegie Collection, No. 886), which is cut in a coronal 

 series, 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, which is defective in some portions, 

 and at such places the space merges with the adjoining secondary spaces. Within 

 the space are some faintly refractive branching threads of coagulated plasma. The 

 scala vestibuli is not yet laid down and the scala tympani is only represented by 

 a moderate widening of the meshes of the reticulum in the neighborhood of the 

 fenestra cochleae (rotunda), along the basal border of the first turn of the cochlear 

 duct. 



In fetuses 50 mm. long the outlines of the cistern become very distinct, due to 

 the marked increase in the size of its main cavity and to the more definite membrane 

 at its junction with the rest of the reticulum. Its form and relations are shown in 

 figures 26 and 27. They represent a median and a lateral view of a wax-plate 

 reconstruction of this region in a human fetus 50 mm. long (Carnegie Collection, 

 No. 84). Only the main cavity is shown in the model. At certain places around 

 its borders the meshes of the reticulum are uniting into larger spaces and these in 

 turn are taken up by the main cavity as it advances into the new territory. These 

 smaller incomplete spaces were omitted in constructing the plates of the model. 

 The rule was adopted that only the spaces that were outlined by a membrane-like 

 border should be traced on the plates and included in the model. This rule was 

 adhered to in all the models of this series. 



Figures 26 and 27 show that the periotic cistern in 50-mm. embryos consists 

 of a flattened, rounded, bursa-like cavity intervening between the stapes and the 

 lateral surface of the saccule and adjoining utricle. It extends forward to the 

 ampulla of the lateral canal and upward to the beginning of the cms commune. 

 Posteriorly it crowds backward against the ductus reuniens, filling in the space 

 between the utricle, saccule, and the proximal end of the cochlear duct. Both on 

 its median and lateral surfaces there is no further opportunity for expansion except 

 as the vestibule itself enlarges. The delicate membrane-like wall of the cistern 

 hugs closely against the parts of the membranous labyrinth on the one side and the 

 tympanic wall of the cartilaginous vestibule on the other, being separated from them 

 only by a thin layer of the original reticulum. Along the dorsal margin of the cis- 

 tern, however, there is room for expansion, and the reticulum in this region shows 

 enlarging spaces in the process of uniting with the main cavity. On its ventral 

 margin, near the cochlea and extending along the apical surface of the latter, there 



