74 GEORGE L. STREETER 



In embryos 4 mm. long the ear vesicle consists of a simple 

 slightly elongated spherical sac tliat lies in the space between the 

 primary head vein and the lateral wall of the hind-brain. At its 

 dorsal end can be recognized a rounded pouch-like projection 

 which is quite distinctly marked off from the rest of the vesicle. 

 This is the early endolymphatic appendage. It is in relation 

 both with the brain wall and the skin, but is separated from 

 them by a scant amount of mesenchyme, in which can be seen 

 minute blood-vessels that communicate with the middle and pos- 

 terior dural plexuses. The appendage points toward the rhombic 

 lip, but does not quite reach its dorsal margin. 



In its subsequent growth the endolymphatic appendage rap- 

 idly becomes more clearly differentiated from the remainder of 

 the labyrinth. It takes on a slender tubular form, whereas the 

 vestibular part of the labyrinth expends into a voluminous tri- 

 angular pouch. The tubular character of the endolymphatic 

 appendage is pronounced in embryos from 9 mm. to 14 mm. 

 long. By its elongation it passes over the rhombic lip and in 

 14 mm. embryos we find the tip of it overlapping the ventro- 

 lateral part of the thin chorioidal roof of the fourth ventricle. 

 It, however, does not lie in direct contact with this membrane 

 as is the case in tadpole larvae, but is always separated by a 

 thin layer of the surrounding mesenchyme. 



At about the time of the closing-off of the semicircular canals 

 (embryos 15 mm. long) the, simple tubular form of the endolym- 

 phatic appendage is gradually modified by the expansion of its 

 distal half into a flattened fusiform sac, which from then on is 

 recognized as the endolymphatic sac as distinguished from the 

 remaining proximal part, the endolymphatic duct, that connects 

 it with the rest of the labyrinth. The endolymphatic sac lies 

 lateral and caudal to that part of the chorioidal membrane that 

 is to form the lateral recess of the fourth ventricle. It lies close 

 against it, but is always separated from it by the tissue that is to 

 form the arachnoid and dural membranes. 



Simultaneously with the formation of the semicircular canals 

 and the differentiation of the endolymphatic sac there occurs an 

 alteration in the large dural veins in this neighborhood that plays 



