THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 



189 



ing the internal ear to form the periotic capsule which subsequently unites with 

 the occipital and sphenoidal cartilages. The pieces of cartilage thus formed con- 

 stitute the chondrocranium. 



In connection with the development of the caudal part of the occipital cartilage there is 

 an interesting feature which is at least indicative of a segmental character. In some of the 

 lower Mammals there are four fairly distinct condensations of embryonic connective tissue 

 just cranial to the first cervical vertebra, corresponding to the first cervical nerve and the 

 three roots of the hypoglossal. These condensations bear a general resemblance to the 

 primitive segments and indicate the existence of four vertebrae which are later taken up into 

 the chondrocranium. In the human embryo the condensations are less distinct, but the 

 existence of a first cervical and a three-rooted hypoglossal nerve in this region suggests an 

 original segmental character. If this is true, then the base of the human skull is formed 

 from the unsegmented chondrocranium plus four vertebrae which become incorporated in 

 the occipital region. 



Optic foramen 



Ala magna (sphenoid) 



Ala parva (sphenoid) 



Vomer 

 Palate bone 



Mandible 



Meckel's cartilage 



Cricoid cartilage 



\ Styloid process 

 Cochlear fenestra 



Foramen (XII Nerve) 



Thyreoid cartilage 



FIG. 174. Lateral view of primordial cranium of human embryo of 80 mm. 



(3rd month). Gaupp, Hert-wig. 



The membrane bones of the roof of the skull have been removed. Compare with FIG. 173. 

 maxilla, vomer, palate, and mandible are membrane bones. 



The 



In addition to the chondrocranium, other cartilaginous elements enter into 

 the formation of the skull, all of which are derived from the visceral arches. 

 Not all the arches, however, produce cartilage; for in the maxillary process of 

 the first arch, which forms the upper boundary of the mouth, cartilage does not 

 appear, and the bones which later develop in it are of the membranous type. 

 The mandibular process of the first arch produces a rod of cartilage Meckel's 

 cartilage. This gives rise, at its proximal end, to a part of the auditory ossicles, 

 but the cartilage in the jaw proper soon wholly or almost wholly disappears. 

 The cartilage of the second arch becomes connected with the skull in the region 



