910 
MR. A. ERASER OR THE DEVELOPMENT OF THE 
These fenestrse differ in their development in the following manner : in sections from 
rat embryoes 8 millims. in length, in which the wall surrounding the labyrinth can 
first be said to be roughly differentiated from the adjacent mesoblast, a gap for neither 
fenestra exists; but in sections from embryoes 13 millims. in length, while the wall is 
continuous at the position of the future fenestra ovalis (Plate 57, fig. 27), a well 
marked deficiency exists for the fenestra rotunda (Plate 57 , fig. 28). 
The primitive jugular vein and seventh. nerve divide the auditory region on an 
external view into two parts. Ventrad of these structures are the proximal extremities 
of the first two post-oral cartilages ; dorsad of them lies the semicircular canal portion 
of the labyrinth; this portion of the periotic capsule is roughly circular in outline, 
but flattened ventrally, and more or less pointed dorsally. Along the anterior and 
posterior borders run in a dorsal direction the superior and posterior semicircular 
canals; these turning ventrally unite together, and open by a common aperture into 
the vestibular portion of the labyrinth; the external semicircular canal lies between 
the ventral extremities of the last two, and has a direction parallel to the vessel and 
nerve (Plate 54, fig. 6 ; Plate 56, figs. 19, 21, and 22 ; Plate 57, figs. 33 and 34; and 
Plate 58, fig. 39). 
I shall describe the dorsal aorta and the branch passing from it in connexion with 
the stapes. 
The tympanic annulus can first be detected in Rat embryoes 2 centims. in length ■ 
it lies within the proximal extremities of the mandibular and hyoidean cartilages, 
ventrad of the external wall of the sulcus tympanicus, and internal to the bottom of 
the meatus auditorius externus ; it is deficient dorsally, the proximal extremity of the 
mandibular cartilage appears to bend round and be supported by it (Plate 55, fig. 15). 
In the oldest Rat embryoes and especially in the young of carnivorous animals at birth 
(Cat, Dog, Leopard), a membranous splint intrevenes between it and the mandibular 
cartilage (Plate 57, fig. 34) ; this, however, soon unites with the periotic, or the annulus. 
In the oldest embryoes examined there is no trace of the papery bulla or bony 
meatus auditorius externus, which are continuous with aud lie internal and external 
to the annulus in the adult (Plate 55, figs. 13, 14, and 16; Plate 57, fig. 31 ; Plate 58, 
figs. 35, 37, and 40). 
Second.—Embryonic history of the proximal extremities of the first two post-oral 
cartilages, being in effect a description of the malleus and incus, and a comparison 
between the embryonic and adult form of these ossicles. 
I have never been able to detect, notwithstanding the large number of embryo heads 
which I cut for the study of the earliest condition of the cartilages, a stage in which 
these could be described as being straight. 
There was always a slight bending of the proximal extremities of both cartilages, 
that of the mandibular depressing the dorsal wall of the meatus auditorius externus 
