rnesoderm. 



FIG. 211. The first figure represents what might be considered a typical (lower) vertebrate skull-base. The 

 hinder portion is made by bones related with somites beside the notochord, hence is " parachordal," and 

 extends forward to the pituitary foramen. In front of this are the paired " trabeculae cranii," enclosing 

 the foramen behind, and connected by an intertrabecula in front. Special sense-capsules make up the 

 rest of the cartilaginous base : of these the nasal capsules extend forward below and in front of the 

 trabeculae, only their upper and posterior parts really coming into the floor of the proper cranial cavity : 

 the eye probably had a cartilaginous capsule originally, but this is not found now, only the fibrous sclerotic 

 possibly representing it. The auditory capsules, containing the inner ear, are partly basal, beside the 

 parachordal region, but are mainly in the lateral walls. All these structures are formed in " paraxial " 

 mesoderm, which immediately surrounds the neural tube. In the human skull (second figure) the area of 

 the base is much increased, especially in a lateral direction, as the result of brain growth. The auditory 

 capsules in consequence become definitely basal, placed on each side of the parachordal region (which 

 corresponds with the same region in the lower type, although it is developed dorsal to the notochord) 

 while the parachordal cartilage shows backward extensions meeting behind the foramen magnum. The 

 cartilage surrounding the pituitary foramen may be trabecular, but there is no definite indication of this. 

 In front of this the nasal capsules come into the base, but, as explained in the text, they only get into 

 this position secondarily. The solid cartilaginous septum between them is continuous in formation with 

 the pre-pituitary base, and may possibly be trabecular or intertrabecular. The eyes have moved forward, 

 probably on account of the development of the temporo-sphenoidal region of the brain, and a process of 

 cartilage (spheno-ethmoidal plate) is thrown over each from the nasal capsule, as indicated by the inter- 

 rupted line. A wide area of paraxial mesoderm is thus left between the eye and the auditory capsule : in 

 this a cartilaginous orbitosphenoid extends out behind the eye, and another shorter process behind this, 

 also continuous with the central basal chondrification, marks the base of the future alisphenoid. The 

 remaining paraxial tissue in this area does not ossify or chondrify, but probably forms the dura mater 

 between the orbitosphenoid and the tentorium. The greater part of the alisphenoid is formed in mem- 

 brane from (visceral) mesoderm deep to the paraxial plane, and comes up into place secondarily from 

 below ; it is indicated in the figure by darker stippling. The third figure shows the general relations of 

 the visceral mesoderm, in the floor of the pharynx, to the paraxial mesoderm round the neural tube. 

 Observe that the visceral tissue comes round the pharynx, to form a support for the paraxial. With the 

 widening of the brain a corresponding widening and thickening of the visceral mass takes place in support. 

 The relation between the two layers is only present at first behind the pituitary level, but, later, extensions 

 t (maxillary processes) from the first visceral arch grow forward to support the eyes and overhanging brain, 

 etc. The lower figures show the formation of the face from the embryonic condition ; H is the hind-brain. 

 Notice how the maxillary process from the mandibular arch (mcind.) has grown forward below the eye 

 and is applied to the side of the nasal capsule, which has been formed round the .olfactory field. 



