238 BEATRICE WHITESIDE 
morphological. At the start the whole structure is lined with a 
one-layered epithelium of cylindrical cells. 
For comparison, it can be noticed that Norris (92) found the 
first indication of thesaccus endolymphaticus in alarva of Amblys- 
toma of 9 mm. length, whose auditory organ, similar to the 
above-described frog larva, already possessed the rudiments of 
the semicircular canals and the lagena. Fleissig (08) mentions 
the first appearance of the saccus endolymphaticus of Phyllo- 
dactylus in an embryo 4 mm. long, at a time when the develop- 
ment of the labyrinth proper is far advanced. In reference to the 
origin of the saccus in man, Streeter (16) writes that this organ 
appears at about the time of the closing off of the semicircular 
canals. 
Stage II (figs. 3 and 11) 
Stage II is based on a larva of 10 mm. length. No indications 
of extremities are visible. 
The development of the labyrinth is far advanced and all its 
morphological parts have appeared (fig. 38). <A cartilaginous 
ear capsule is completely formed, in whose median surface the 
foramen endolymphaticum (f.e.) is situated. In this stage as 
well as in the two following ones, this opening lies lateral to the 
anterior end of the plexus choroideus of the fourth ventricle. 
The ductus endolymphaticus has now attained its definitive 
form. Its long axis has changed its dorsoventral direction and 
inclines somewhat in a craniocaudal direction, so that its entry 
into the cranial cavity lies a little more caudad than its exit from 
the sacculus. In consequence of this rotation, only its upper 
part is to be seen in figure 3. The ductus at this stage begins 
with a slightly broadened piece at the upper median part of the 
sacculus, and runs dorsally, median to the utriculus as far as the 
foramen endolymphaticum. Here it turns inward and leads 
through this aperture into the cranial cavity, where it immediately 
expands into the saccus endolymphaticus. As a comparison of 
figures 2 and 3 shows, the ductus endolymphaticus now appears 
smaller in proportion to the rest of the labyrinth, which has 
grown considerably more than the ductus. 
