The Cbondrocraniiiin of an Embryo Pig. 175 



Regio Occipitalis. 



The regio occipitalis forms a complete cartilaginous ring, the 

 lateral parts being connected by a dorsal band of cartilage, the 

 tectum iiosterius or synoticum (Pis. I-III, tectum post.). In the 

 remainder of the skull a roofing over is present only in the anterior 

 half of the regio ethmoidalis. In the central portion of the skull 

 the roof of the brain-box is formed only by the membrane bones, and 

 at no time is it covered over by the chondrocranium. 



The basal portion of the regio occipitalis has already been de- 

 scribed. It is broad and rectangular in shape and passes without 

 demarcation into the pars lateralis. Near its lateral border and 

 postero-internal to the foramen jugulare, it is perforated by the 

 foramen nervi hypoglossi, already referred to. At this stage the 

 foramen is filled mostly with connective tissue. The three ventral 

 roots of the nervus hypoglossus, corresponding to the roots passing 

 through the three foramina in Lacerta embryos, unite inside the 

 cavum. cranii and so pass through this foramen as a single nerve. 

 N^o indication of the dorsal roots of this nerve could be found. These 

 three roots are the nerves which were taken into the skull during the 

 phylogenetic history when the three cervical vertebra3 were added 

 to the amphibian skull to complete the occipital segment of the am- 

 niotic skull. Before entering the brain each of these three roots 

 splits up into a number of rootlets (4-6), as in man, which enter 

 the brain singly. 



The paroccipital processes, just lateral to the foramina jugulares 

 and hypoglossi, are remarkably large even at this early stage, an 

 indication of the great length they attain in the adult pig. Each is 

 rounded and blunt and extends forward so as to hide the greater 

 part of the foramen jugulare. 



The occipital condyles are large, but as yet they are only slightly 

 convex and scarcely project out from the general surface of the skull. 

 They face downward, backward and outward and bridge over 

 the boundary between the pars basalis and the pars lateralis. They 

 are weU separated, forming two distinct atlanto-occipital articular 

 surfaces, although the capsule surrounding them is continuous from 

 side to side in both the embryo and the adult. 



