The ClioiKlrocraiiium of an Embryo Pig. 173 



Looking now at the inside of the skull, it is seen that the base 

 rises gradually and in an almost straight line from the foramen 

 magnum to the dorsum ephippii. From side to side the curve on 

 the inner wall of the brain-caso is nearly uniform, thus closely 

 simulating the half of a cylinder. 



On each side of the anterior end of the basal plate there is a lai-ge 

 triangular ojiening (PI. T, for. uhduc). This is bounded anteriorly 

 by the widely projecting jn'ocessus clinoideus posterior, and postero- 

 laterally by the cochlear portion of the ear-capsule. It is filled mostly 

 with indifferent tissue, but serves also for the passage of the nervus 

 abduc^ns. This is especially noteworthy since such a foramen has 

 not been described in any mannual. In the chondrocranium of 

 Semnopithecus Fischer found the nervus abducens passing through 

 a groove, which was closed by a band of connective tissue, but no 

 distinct foramen was present. A similar groove has also been de- 

 scribed in some human embryos. At a later stage in Sus the lateral 

 boundary of the foramen, i. e., the rod of cartilage, connecting the 

 lateral end of the processus clinoideus posterior with the front end 

 of the ear-capsule, is absorbed, leaving a condition similar to that 

 in the Semnopithecus embryo. At this stage the processus clinoideus 

 posterior is relatively much larger than in the adult. This foramen I 

 do not hold to be the exact homolog of the foramen for the nervus 

 abducens in the reptiles, although it is similarly located, but rather 

 I am inclined to look upon it as a secondary formation which in- 

 cludes more than the original or true reptilian foramen abducens, 

 and is formed by a secondary fusion of the processus clinoideus 

 posterior with the cochlear capsule. The nervus abducens (Figs. 1 

 and 8) leaves the cranial cavity through the large foramen just 

 mentioned, and, continuing forward, passes dorsal to the ala tem- 

 poralis and through the fissura or'hitalis superior, in company with 

 the oculomotor and trochlear nerves and the first two branches of the 

 trigeminal nerve. 



The course of the notochord through the skull in Sus embryos has 

 been described and figured by Parker and Kolliker. In his stage 

 three (length 1 1/3 inch) Parker (1874, page 310) says that "the 

 notochord has retired from the posterior clinoid wall and has been 



