PRIMORDIAL CRANIUM OF THE CAT 357 



this notch and the tectum posterius is a broad space filled by 

 membrane continuous ^\ith that of the incisura posterior. A 

 pair of chondral centers hes in this membrane and a third center 

 stretches through the cranial roof anterior to the tectum pos- 

 terius. The latter disappears in further development; the pair 

 of centers remaining help to complete the boundary of the in- 

 cisura posterior, while the membrane between the latter and the 

 tectum posterius undergoes ossification. Voit ('09) also found 

 a space (incisura occipitalis posterior) in Lepus between the 

 dorsally open foramen magnum and the tectum posterius, and 

 since, as is explained, the latter belongs to the otic region, the 

 side boundaries of the incisura occipitahs posterior are regarded 

 as the dorsal portions of the occipital pillars. If we compare 

 now the extent of the region between the dorsal confines of the 

 foramen magnum and the tectum posterius in Hzard, rabbit, 

 cat, ape and man, it at once appears that there is a progressively 

 increasing area exhibited. This begins with Lacerta, where, as 

 Gaupp observed, the tectum is so shaped as to give to the fora- 

 men magnum an angle in the dorsal median fine, and reaches 

 the great expanse described by Bolk in the human embrj^o, in 

 which the tectum is far removed from the foramen magnum. 

 It is evident, also, that the term incisura occipitahs posterior 

 has not the same value throughout its application; in the rabbit 

 Voit apparently regards the space limited laterally by the dorsal 

 portions of the occipital pillars as the incisura occipitalis posterior, 

 while Bolk limits the term to only a small part of the region 

 encompassed by the dorsal limits of the occipital walls. 



Otic region 



Position of the otic capsules. It has been noted (p. 298) that 

 the approximately transverse position of the plane of the otic 

 capsules in the stage represented by the model forms an ex- 

 ception to the general rule in mammals of obhquity of the prin- 

 cipal otic axis toward the longitudinal axis of the skull. It 

 will be recalled that the degree of differentiation exhibited by 

 the stage of the model is one, expressed in terms of the skeleton, 

 wherein bone formation has advanced but little; of the mem- 



