308 GEORGE L. STREETER 



with the adjoining seconchiry spaces. Within the space are 

 some faintly refractive branching threads of coagulated plasma. 

 The scala A^estibuli is not 3^et laid down and the scala tympani is 

 only represented by a moderate widening of the meshes of the 

 reticulmn in the neighborhood of the fenestra cochleae (rotun- 

 dum), 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 4 and 5. They represent a median and a lateral view of 

 a waxplate reconstruction of this region in a human fetus 50 mm. 

 long (Carnegie Collection, No. 84). Onh^ 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 bj' the main cavity as it advances into the 

 new territory. These smaller incomplete spaces w^ere omitted 

 in constructing the plates of the model. 



It will be seen then from figure 4 and 5 that the periotic 

 cistern in 50 mm. embryos consists of a flattened rounded bursa- 

 like cavity that intervenes 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 crus 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 mem- 

 branous labyrinth on the one side and the tympanic wall of the 

 cartilagenous vestibule on the other, being separated from them 

 only b}^ a thin layer of the original reticulum. Along the dorsal 

 margin of the cistern, however, there is room for expansion and 

 the reticulum in this region shows enlarging spaces in the proc- 

 ess of uniting with the main cavity. On its ventral margin 

 near the cochlea and extending along the apical surface of the 



