No. 387.] THE OSSICULA AUDITUS. 223 
meatus. The tissue between these two tubes is to form the 
tympanic membrane, its plane being now nearly horizontal 
instead of oblique, as in the adult. In this membrane is formed 
a rod of procartilage entirely unconnected with any other skel- 
etal structure, the Anlage of the manubrium mallei. In front 
this procartilage gradually shades off into the looser mesen- 
chyme between it and the mandibular arch, while behind it is 
very sharply delimited from the undifferentiated tissue lying 
between it and the hyoid. 
In front of the Eustachian cleft and external meatus is 
another mass of procartilage, the anlage of the mandibular arch. 
At first it consists of a continuous stroma, which extends proxi- 
mally and dorsally to a point just in front of the stapes, distally 
into the lower jaw. This procartilage rod is not equally dense 
throughout, but at the level of the future tympanic membrane 
the nuclei are less crowded than they are above and below. 
This indicates a division which later becomes more marked, 
separating a proximal element, the incus, from a more distal 
portion which will give rise later to a proximal body of the mal- 
leus and its processus longus and a more distal rod of cartilage 
which extends into the lower jaw. So far, with the exception 
of the separate origin of the manubrium of the malleus, all who 
have approached the problem of the mammalian ear-bones from 
the developmental standpoint are in agreement. The differences 
of opinion are regarding the nature of the incudal element. 
In the pig embryo, twenty millimeters long, several changes 
have been introduced. Chondrification of the otic capsule is well 
advanced, the foramen ovalis being formed around the base of 
the stapes in such a way that it lies within the opening. In 
the stapes itself the process of chondrification has set in, while 
outside the junction of its crura arises a short process which 
articulates with a stapedial process of the incus. The body of 
the incus is now an elongate plate with rounded extremities, 
the whole occupying a vertical position. The dorsal end extends 
up to, and may be said to articulate with a depression in the 
outer wall of the otic capsule, just above and outside the fora- 
men ovale. Below, the stapes also articulates with the body of 
the malleus. The stapedial process of the incus is a slender 
