PRIMORDIAL CRANIUM OF THE CAT 323 



the confines of the brain case of the 23.1 mm. embryo. The ex- 

 clusion of the space from the cavmn cranii is brought about by a 

 strong membrane, conspicuous in the sections, which arises from 

 the base of the cranium and is apphed to the medial surface of 

 the semilunar ganglion, excluding this structure, as it rests on 

 the ala temporalis, from the cranial space occupied by the brain. 

 Less dense mesenchyma surrounds the ganglion and extends lat- 

 erally as far as the temporal muscle ; forward through the spheno- 

 orbital fissure and about the ala temporalis; backward, into the 

 cavum supracochleare. The medial limiting membrane calls for 

 special description. We may tentatively refer to it as the mem- 

 brana limitans (figs. 19 to 23). It fills the foramen spheno- 

 parietale attaching itself to its margins, i.e., to the commissura 

 orbito-parietalis, conm^iissura suprafaciahs, where it is perfor- 

 ated by the roots of the trigeminus, and to the metoptic root of 

 the ala orbitalis; then upon the base of the cranium, from the 

 anterior pole of the cochlear capsule along the commissura ah- 

 cochlearis, crista transversa and dorsum sellae, the lateral ele- 

 vated margins of the hypophyseal fossa, and on to the tubercu- 

 lum ephippii. In the bottom of the sella turcica and upon the 

 dorsum sellae the membrane is continuous from side to side. 

 At the saddle-back it is continuous with a stout meningeal mem- 

 brane which ascends into the narrow space in the flexure of the 

 mid-brain and diencephalon; this may be refered to as the sep- 

 tum transversum (see p. 327). The thickness of the limiting 

 membrane is not the same throughout. It is densest at its 

 broad attachment to the cranial base where it covers the carotid 

 foramen. Also a definite band of condensed tissue extends 

 through the septum transversmn from the crista transversa, 

 crossing the N. abducens and the roots of the N. trigeminus run- 

 ning parallel with the commissura suprafaciahs and reaching 

 laterally the parietal plate. It is along this stretch that a small 

 cartilaginous bar is later developed (embryo of 7 cm.) over the 

 dorso-medial surface of the semilunar ganghon and roots of the 

 trigeminus where the latter cross the suprafacial conmaissure. 



The floor of the epipteric cave is formed by the lamina ascen- 

 dens of the ala temporahs. The following structures are found 



