150 Development of Ear and VII-VIII Cranial Nerves 
endolymphaticus, all of which stand out prominently. The intervening 
vestibular epithelium, which is doomed to absorption, consists of a single 
layer of cuboidal cells, as shown in B, in contrast to the thick outer edge 
which is to become canal. 
This process of absorption may be described histologically as a con- 
version of the definite epithelial membrane into a line of cells which 
seem to fuse with and cannot easily be distinguished from the adjacent 
mesodermal cells, the line finally becoming broken and irregular. The 
transition from one step in this procedure to the next is quite abrupt; 
thus in B the thin membrane is sharply cut off from the absorption focus. 
Several specimens were examined of about this age, and in one case, 
embryo No. 175, 13 mm. long, it was found that absorption of the 
epithelium was going on before the lateral and median walls of the 
vesicle had actually come together. So it is possible that during this 
process the vesicle cavity is in some cases left temporarily in open com- 
munication with the spaces of the adjacent mesoderm. The final curling 
in of the edges and closure of the canal tube repeats in a way the pro- 
cedure which we have already seen in case of the auditory cup during 
its conversion into the auditory vesicle. It is probably likewise mechani- 
cally brought about by the arrangement of the epithelial cells. Section 
C shows the canal after the formation of the closure seam, the so-called 
raphe of Hasse. The thickness of the epithelium of the outer edge and 
presence of division figures indicate that the activity of growth still 
continues. Section D shows a canal in an embryo 30 mm. long, the 
same stage as that shown in Figs. a, b, c, Plate II. Here the epithelium 
is reduced to a single layer and division figures have disappeared. It 
can be seen, however, that traces still exist of the thickened outer edge 
and the raphe of Hasse. This stage differs from the adult canal prac- 
tically only in its diameter, which there is 3-4 times greater. Doubtless 
this growth is in large part accomplished simply by the flattening out and 
expansion of the individual cells. 
The formation of the ampulle can be seen by comparing the figures 
on Plates I and II. It will be noticed that their development proceeds 
simultaneously with that of the canals. In their histogenesis they re- 
semble the canals, in having a thin single layer of epithelium on the 
inner rim and the thick 2-3 layered epithelium on the outer surface. It 
is out of the latter primitive epithelium that the macule are developed, 
and they make their appearance before ampulle and canals are com- 
pletely separated from the remainder of the vestibular sac; they can 
be seen in the 11 mm. stage, but a high degree of differentiation is not 
