146 Development of Ear and VII-VIII Cranial Nerves 
mm. long that the whole appendage wall is thinned out to a single 
layer. It seems probable that the thick median wall in embryos of 6-18 
mm. constitutes a germinating bed which furnishes the cells needed for 
the rapidly expanding appendage. It is only these cells that continue to 
multiply, and they can be imagined as moving around toward the lateral 
surface in a single layer in the order in which they are derived from 
their focus of growth. 
The diverticulum stage-—Between the primitive vesicle just described 
and the labyrinth possessing cochlea, semi-circular canals, and accessory 
recesses, there is a stage through which the ear vesicle passes which can 
be characterized as the diverticulum or pouch stage. It is represented 
by the embryos 6.6 mm. and 9 mm. long, shown in Figs. b-f, Plate I. In 
these two embryos the vesicle may be said to consist of two pouches, a 
large, bulging triangular one above, with the endolymphatic appendage, 
the vestibular pouch, and opening into it from below the more slender 
and flattened cochlear pouch. Where these two pouches meet, there is 
a portion of the vesicle which is destined to form the utricle and saccule. 
It can be distinctly seen in Fig. f, Plate I. This was observed in the 
bat by Denis, 02, who called that part of it which projects toward the 
median surface the diverticule utriculo-sacculaire. The space concerned, 
however, involves also a part of the anterior and lateral walls of the 
vesicle and perhaps it would be advantageous to include this whole region 
under the concise and descriptive name atriwm. This atrium is properly 
a subdivision of the vestibular pouch. It is in fact all that part of 
it which is left after the separation off of the semicircular canals and 
their ampulle. It is not to be confused with the cochlear pouch, which 
is phylogenetically a secondary diverticulum, which buds out from the 
atrial portion of the vestibular pouch. The embryonic relation is in- 
dicated in the following table: 
{ endolymphatic duct. 
endolymphatic appendage, Cndol aipneuiemeet 
semicircular canals. 
ampulle. 
7 utricle. 
atrium, { 
canal pockets, 4 
primary vesicle, + vestibular pouch, 
saccule. 
cochlear pouch, cochlea. 
If the words pars superior and pars inferior were substituted for the 
two pouches this conception would then be at variance with Krause, 03, 
only as regards the saccule which he describes as belonging to the pars 
inferior. This will be again referred to later. 
