PRIMORDIAL CRANIUM OF THE CAT 355 



and the occipital condyles referred to on p. 292, which is in 

 contrast with their caudal position in the adult. The immediate 

 cause of this basal position is not difficult to find. Since the 

 lateral occipital arches are quite narrow antero-posteriorly and 

 are joined directly with the otic capsules, it is e\ddent that their 

 basal inclination cannot be attributed either to growth, as in 

 man, or to the presence of wide fissures between them and the 

 ear capsules. The explanation is to be found in the flexures 

 to which the whole chondrocranium is subjected. One result of 

 these flexures upon the longitudinal axis of the cranium is to 

 put its anterior and posterior halves nearly at right angles 

 with each other. It follows that the plane of the foramen mag- 

 num stands parallel with that of the floor of the nose. It is of 

 interest to find that, with the formation of the bony cranium, a 

 nearly straight longitudinal axis is substituted for the primary 

 angular axis and, with it, the plane of the foramen magnum 

 becomes less obhque. 



A primitive condition, and an exceptional one in the primor- 

 dial skull of mammals, it seen at the stage represented by the 

 model in the abrupt ascent of the lateral occipital walls from the 

 basal plate. The mamjnahan chondrocranium, as contrasted 

 with that of lower animals, shows a tendency toward lateral ex- 

 trusion of these walls; as Gaupp ('06) says: "Die Seitenteile der 

 Occipitahegion sind bei Saugern nicht mehr steil aufgerichtet, 

 sondern nach hinten hin basalwiirts niedergelegt am starksten 

 und vollkommensten beim Menschen." 



Basal fissures. The fissura basicochlearis posterior of cat is 

 apparently comparable with the opening of the same name in 

 the chondrocranium of Talpa, first described and named by 

 Noordenbos ('05). Mead has noted a probable homologue of 

 the posterior basicochlear fissure in Sus. In Lepus this opening 

 is not present in the stages of development studied by Voit, 

 but an anterior basicochlear fissure was observed by that in- 

 vestigator. These fissures are filled with mesenchyma, which, 

 in the cat embryo, passes into a zone of precartilage at the edges 

 of the opening. The posterior basicochlear fissure, present in 

 Talpa embryos of 14 mm., disappears in embryos of 19 to 20 



