The Cbondrocranium of an Embryo Pig. 207 



the middle of its passage through the skull, dips beneath the plate 

 and is connected with the dorsal wall of the pharynx in two places. 



The cartilages, which will later form the car-bones, are of the 

 type common to the mammals at this stage of development. This 

 region of the skull is very similar to the corresponding region in 

 some of the streptostylic reptiles, or those with a movable quadrate. 



A foramen nervus abducens is present. It is in the same position 

 as the similarly named foramen in the reptiles, but the two are 

 probably not homologous, that in Sus being formed by the secondary 

 union of the processus clinoideus posterior with the ear-capsule. 



The middle portion of the processus alaris is of less dense cartilage 

 than the basal cartilage or the ascending part of the ala temporalis, 

 indicating that the latter was probably formed independently in the 

 pig, and had later united with the base of the skull. 



Both roots of the ala orbitalis unite directly with the basal 

 cartilage. They are well separated, leaving between them a large 

 foramen through which passes only the optic nerve. 



The evidence supports Gaupp's conclusion that the cranial cavities 

 in the reptiles and mammals are not strictly homologous, but that 

 the cavity in the mammals is larger morphologically than that of 

 the reptiles; that it has been increased by the addition of the rep- 

 tilian cavum epiptericum. Vestiges of the primitive side wall of 

 the mammalian cranium are found in various forms. One vestige 

 that Sus offers, which has not been described in any other form, is 

 the rod of cartilage connecting the posterior clinoid process with the 

 cochlear portion of the ear-capsule. 



The commissura orbito-parietalis is very broad in Sus, resembling 

 that in Echidna. This is probably a very primitive character of 

 the mammals. 



A true interorbital septum is present, supporting Gaupp's con- 

 clusion that the mammals belong to that group of vertebrates having 

 a tropibasic or keeled type of skull. This allies them with the reptiles 

 and separates them from the amphibians. 



The nasal region is neither greatly lengthened nor shortened. 

 The nasal cavities and the turbinals are of a very simple type which 

 can be derived, without much difficulty, from that of the reptiles. 



