t6o 



A MANUAL OF ANATOMY 



Membranous Stage. — The crajiium is at first entirely membranous, and is 

 formed by the mesenchyme which surrounds the cephaHc part of the noto- 

 chord. This mesenchyme expands forwards laterally, and then backwards, 

 so as to form a membranous capsule for the cerebral vesicles. This capsule 

 forms the primitive membranous cranium, which presents a base and a vault, 

 or tegmental part. 



Cliondral or Cartilaginous Stage. — The base of the primitive membranous 

 cranium undergoes chondrification, but the vault or tegmental portion is 

 not involved in this process. Having, therefore, regard to the development 

 of the bones which construct the osseous cranium, the membranous cranium 

 is to be considered under two aspects — chondral and membranous. 



Chondrocranium. — The chondrocranium involves the basal part of the 

 primitive membranous cranium, which has become converted into cartilage. 

 Chondrification gives rise to a single plate of cartilage, extending from the 

 basi-occipital to the ethmoidal region'. This basal cartilage may be regarded 

 as consisting of two segments — chordal and prechordal. The chordal segment 

 . embraces the cephalic part of the notochord, and the prechordal segment is 

 in front of the notochord. 



Septum Nasi 

 Optic Foramen 



Presphenoid 



Basi-occipii 



-sphenoid 

 .Presphenoid 



Periotic Capsule 



— CranioPharyngea 

 Canal 



ii||_ Ex -occipital 



Foramen Magnum 



Fig. 90. — The Chondrocranium (after Huxley). 



The chordal segment represents the parachordal cartilages of certain animals. 

 The prechordal segment represents the prechordal cartilages, or trabeculae 

 cranii of Rathke, present in certain animals. In man, however, it is a con- 

 tinuous cartilage, which is interrupted at the centre by a membranous interval, 

 representing the primitive pituitary space. In this region the cranial cavity 

 communicates with the primitive mouth, or stomodasum, from which latter 

 the pouch of Rathke ascends to form the anterior lobe of the pituitary body. 



The chordal segment, as stated, embraces the cephalic end of the notochord, 

 but the chord soon disappears, and the cartilage of the chordal segment 

 extends backwards, so as to embrace the foramen magnum and involve the 

 supra-occipital region. 



The prechordal segment is continuous posteriorly with the chordal seg- 

 ment. Anteriorly, in front of the primitive pituitary space, the basal plate 

 of cartilage (prechordal segment) here spreads out laterally. Subsequently 

 the pituitary space becomes closed by the encroachment upon it, from either 

 side, of the prechordal cartilage. 



On each side of the chordal segment of the basal cartilage is the periotic 

 cartilaginous capsule. 



