George L. Streeter 151 
found until we come to embryos 20 mm. long. It will be remembered 
that His, Jr., 89, represents ampulle as forming on both ends of the 
superior and posterior canals. This was not confirmed in our models; 
the ends of these two canals where they unite to form the crus commune 
show no such enlargement. Each canal possesses but one ampulla. 
The development of the utricle and saccule is dependent on the sub- 
division of the atrium into an upper and lower compartment. The 
atrium, as has already been described is that ventral part of the vestibular 
pouch into which the endolymphatic appendage opens, and into which 
the cochlear pouch opens from below; in Fig. 7, Plate I, it is marked 
utric-sacc., and in Fig. 7 the lateral surface of it is marked sacc., and 
in Fig. & a partial median view of it is marked utric. In Figs. 7 and k, 
though the canals and ampulle are already completing their separation 
from the vestibular pouch, the atrial region has not yet begun its sub- 
division. It, however, suggests by its outer form the future saccule and 
utricle. The actual subdivision begins in embryos between 18 and 20 
mm. The initial ingrowth of the membranous partition can be seen 
in Figs. J and m, where it can be distinguished as a horizontal cleft which 
forms in front between the utricular and saccular parts of the atrium. 
Strictly speaking we cannot speak of a saccule and utricle until the inter- 
vening partition is complete. It is practically complete in Figs. a, b, c, 
Plate II; here it reaches back to the entrance of the ductus endolym- 
phaticus. It later divides the orifice of that structure, thus affording 
it separate openings into the utricle and saccule, the two openings con- 
stituting the so-called ductus utriculo-saccularis. 
In the meantime the utricle itself has developed a definite shape. As 
can be seen in the Figs. a, 6, and c, a transverse constriction divides it 
into an anterior or cephalic part and a posterior or caudal part. The 
anterior part constitutes the general utricular cavity, in the floor of 
which the nerve ends. In front, just ventro-median to the ampulla of 
the superior canal, a distinct diverticulum extends forward from it 
which is called the recessus utricularis. The posterior part consists of 
a central sinus utriculi communis, into which opens from above the 
crus commune, laterally the sinus utriculi lateralis of the lateral canal, 
from below the sinus utriculi inferioris of the posterior canal, and on 
the median side the ductus endolymphaticus. 
If one compares Figs. a, b, c, Plate II, with pictures of adult prepara- 
tions such as found in the beautiful atlas of Schédnemann, 04, it is 
apparent that the labyrinth of the 30 mm. embryo has practically com- 
pleted its gross development. In its further expansion all parts of it 
