The Chondrocraniiim of an Embryo Pig. 181 



The median surface of the pars vestibularis is also triangular in 

 shape. The surface is irregular, although in general character it 

 is concave, and it is perforated by three foramina. The smallest 

 of these is located in the postero-dorsal comer ; it is long and narrow 

 and opens into the posterior semi-circular canal. At this stage it 

 is filled with connective tissue ; later it disappears. On the postero- 

 ventral side is another long slit-like aperture, the foramen endo- 

 lympliaticum, or aquwdudus vestihuli (PI. I, for. endolym.). It 

 runs parallel to the sinus utriculi superior, which lies beneath it 

 within the capsule, and serves for the exit, into the cavum cranii, of 

 the ductus endolymphaticus. In its position and shape the foramen 

 is almost identical with the corresponding foramen in Talpa and 

 Semnopithecus, although it is larger. In a Lacerta embryo it is 

 oval. This oval shape is attained in older specimens of Sus and 

 is due to the thickening of the walls of the capsule and to their 

 surrounding more and more the oblique duct, thus constricting 

 the opening and bringing the internal aperture further back. On 

 the posterior face of the pars vestibularis, a short distance below and 

 back of the foramen endolymphaticum, is a small oval pit which 

 communicates internally with the fossa subarcuata. It is filled with 

 precartilaginous tissue and is probably later closed with cartilage. 



At about the middle of the median surface of the pars vestibularis 

 lies the third and largest of the three cavities, the fossa subarcuata 

 {fos. siibarc.) or floccularis. It is roughly oval in shape and extends 

 to the outer wall of the ear-capsule. Several foramina which per- 

 forate this wall (see above) place it in communication with the out- 

 side of the skull. Its posterior portion is deeper than the anterior 

 and forms a pocket which extends between the sinus utriculi and 

 the outer wall of the capsule and communicates with the small oval 

 pit above mentioned. In the embryonic Talpa the fossa is very 

 shallow, while in the adult it is deep. In the human embryo and 

 also in the Lacerta embryos, it is likewise shallow. From this 

 Pischer concluded (1901 b) that the deep pit is a secondary acquisi- 

 tion. Its presence and size seem to be dependent, to a certain 

 degree, upon the development of the flocculus, since it is occupied 

 by this; but still not entirely, since a shallow pit is present in 



