THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 651 



'27. The membranous hyoid arch furnishes, [beginning with] its 

 uppermost part, 



(a) The bow of the stapes, whereas its plate is derived from 

 the cranial capsule and is, as it were, cut out to form the 

 fenestra ovalis, 

 (6) The processus styloideus, 



(c) The ligamentum stylohyoideum, and 



(d) The lesser horn and body of the hyoid bone. 



28. The third membranous visceral arch is chondrified only in 

 its lowest [ventral] part, to form the greater horn of the hyoid 

 bone. 



29. At no stage of its development does the primordial cranium 

 exhibit evidence that, like the vertebral column, it is composed of 

 separate segments. 



30. The original segmentation of the head is expressed in only 

 three ways in the appearance of several primitive segments (myo- 

 tomes), in the arrangement of the cranial nerves, and in the funda- 

 ment of the visceral skeleton. 



31. The primordial cranium is therefore an unsegmented skeletal 

 fundament in a region of the body that is segmented in another 

 manner. 



32. The ossification of the head-skeleton is a much more com- 

 plicated process than that of the vertebral column. 



33. Whereas in the vertebral column there are developed bones of 

 only one kind, through substitution for cartilage, there are to be 

 distinguished in the ossification of the head-skeleton, according to 

 their formation and source, two different kinds of bone primary 

 and secondary. 



34. The primary bones of the head arise in the cartilaginous 

 primordial cranium and visceral skeleton, like the separate bone- 

 nuclei in the cartilaginous vertebral column. 



35. The secondary bones, covering or membrane-bones, arise 

 outside the primordial skeleton of the head in the connective-tissue 

 foundation of the skin and mucous membrane ; they are therefore 

 dermal and mucous-membrane ossifications, and constitute in lower- 

 Vertebrates a portion of a dermal skeleton that covers the surface 

 of the whole body. 



36. The covering bones are developed in some instances, which 

 can be regarded as reproductions of the original method, by fusion of 

 the bony bases of numerous denticles which arise in the skin and 

 mucous membrane. 



