152 Development of Ear and VII-VIII Cranial Nerves 
become relatively more slender and the saccule draws away from the 
utricle and becomes flattened as well as biconcaved or saucer-shaped. 
The cochlea as compared with the derivatives of the vestibular part of 
the ear vesicle is less complicated in its development, presenting only 
the peculiarity of spiral growth. The cochlea has already been referred 
to as the pouch which forms the ventral tip and part of the posterior 
border of the vesicle, as seen in Figs. b-f. In Figs. g, k, it is partly 
demarcated from the saccular region by a broad fossa. At 20 mm., 
Fig. J, a sharp constriction separates it from the saccule, and this be- 
comes in the 30 mm. embryo the ductus reuniens, and in the meantime 
the cochlea has become a spiral of two turns. 
As regards the relation of cochlea to saccule we differ from the descrip- 
tion given by His, Jr., 89, who represents the saccule as budding off 
from the upper end of the cochlea, which is just the reverse of our own 
interpretation and what might be expected on the ground of the com- 
parative anatomy of these structures. We know that in certain fishes 
the ear vesicle consists of a simple utricle into which the semicircular 
canals empty. In certain other fishes pockets bud out from the utricle 
analogous to the saccule. When we come to animals that leave the 
water, the amphibians, there develops from the saccule a secondary pocket, 
which in birds and reptiles takes on the characteristics which identify it 
with the mammalian cochlea. That is to say, first utricle, then utricle 
and saccule, and finally utricle, saccule, and cochlea. The phylogenetic 
development presents here, in discrete steps, the process which we find 
in the human embryo, but in the latter case it is a matter of simultaneous 
growth of all three structures. 
A resumé of the development of the labyrinth is presented in the form 
of a diagram in the adjacent Fig. 4, which illustrates the successive steps 
by which the simple ear vesicle enlarges and becomes differentiated into 
the group of connected individual compartments which characterize the 
adult ear. 
The ear vesicle very early (6-7 mm. long, 3 weeks) assumes the form 
of two communicating pouches, the vestibular -pouch, with its endolym- 
phatic appendage, and the cochlear pouch. ‘The first gives origin to the 
semicircular canals, ampulle, utricle, and saccule. The semicircular 
canals, in consequence of the approximation and absorption of the inter- 
vening wall of the vesicle, make their appearance between the fourth and 
fifth weeks, 9-14 mm. (only one is shown in the diagrams). That 
portion of the vestibular pouch that is not involved in the formation of 
the canals and their ampulle may be called atrium, to indicate that 
a 
