The Chondrocranium of an Embryo Pig. 179 



vein traverses the foramen petroso-oceipitale. There is no indica- 

 tion of the presence of a processus operciilaris, such as Fischer de- 

 scribed in Talpa. 



Posteriorly the auditory capsules are united with the lateral parts 

 of the regie occipitalis by two small bridges of cartilage, between 

 which lies a long narrow slit filled with precartilaginous tissue 

 {V orhnorpel) . This slit is probably the homolog of the dorsal part 

 of the reptilian fissura metotica. Above the upper bridge lies the 

 foramen petroso-oceipitale, formed, as Fischer says (1901 b), by 

 a bridge of cartilage dividing the original foramen jugulare spurium 

 into two parts. In the reconstruction of a younger Sus embryo neither 

 of the bridges forming the foramen petroso-oceipitale are present, 

 nor is the one which closes anteriorly the foramen jugulare spurium, 

 so that the auditory capsules are entirely free from the side-walls of 

 the cranium. 



The foramen jugulare (Pis. I and III) is a large irregular open- 

 ing posterior to the cochlear portion of the auditory capsule and 

 ventral to the pars vestibularis. Externally it looks forward, down- 

 ward and outward; internally, toward the foramen magnum. It 

 serves for the exit of the glossopharyngeus-vagus group of nerves 

 and the internal jugular vein. The paroccipital process extends 

 forward and under the foramen, so that it is not seen when the skull 

 is viewed directly from beneath. Laterally it has a large opening 

 into the fenestra cochlearis (rotunda). Postero-dorsally the foramen 

 is continued as a narrow slit, which is later closed as the vestibular 

 portions of the ear-capsules fuse more firmly with the exoccipital 

 elements. The thin lamina of cartilage lying between the cochlear 

 part o'f the auditory capsule and the basal plate has already been 

 described (p. 172). The auditory capsules are free anteriorly, with 

 the exception of two narrow bars of cartilage, one connecting the 

 upper part of the pars vestibularis with the lamina parietalis and 

 the other going from the pars conchlearis to the outer end of the 

 processus clinoideus posterior and forming the foramen above men- 

 tioned through which passes the nervus abducens. In front of the 

 capsules lie the large fenestra spheno-parietales. No trabecula ali- 

 cochlearis, such as occurs in Lepus and Talpa, is present in Sus, nor 



