650 EMBRYOLOGY. 



19. In the Selachians the cartilaginous primordial cranium is a 

 permanent structure, and possesses rather thick uniform walls ; in 

 Mammals and Man, on the contrary, it is of only short duration, 

 serving as foundation for the bony cranial capsule that takes its place; 

 it is therefore less completely developed than in Selachians, for only 

 the base and lateral parts are in all cases cartilaginous, whereas the 

 roof presents large openings closed by dermal membranes. 



20. From its relation to the chorda dorsalis, there are dis- 

 tinguishable in the cartilaginous primordial cranium two chief 

 portions, a vertebral (chordal) and a non-vertebral (prechordal), 

 or, according to its relations to the sensory organs, it may be 

 divided into four regions ethmoidal, orbital, labyrinthal, and 

 occipital. 



21. As the ribs are associated with the vertebral column in the 

 form of ventral arched structures, so also the visceral skeleton is 

 united to the primordial cranium in the head-region. 



22. The visceral skeleton is composed of segmented cartilaginous 

 rods, which have arisen by a process of chondrification in the tissue 

 of the membranous visceral arches between the successive visceral 

 clefts. 



23. The cartilaginous throat- or visceral arches are well developed 

 only in the lower Vertebrates (permanently in the Selachians), and are 

 distinguished, according to differences of position and form, as jaw- 

 arch, hyoid arch, and branchial arches, the last being variable in 

 number. 



24. The jaw-arch is divided into the cartilaginous upper jaw 

 (palato-quadratum) and the cartilaginous lower jaw (mandibulare) ; 

 the hyoid arch into the hyomandibulare, the hyoides, and the unpaired 

 copula. 



25. In Mammals and Man the cartilaginous visceral skeleton 

 attains only a very rudimentary condition, and is converted into the 

 cartilaginous fundaments of the three auditory ossicles and the hyoid 

 bone. 



26. In the membranous jaw-arch arise 



(a) The incus, which corresponds to the palato-quadratum of 

 lower Vertebrates ; 



(6) The malleus, which is the representative of the articular 

 part of the cartilaginous mandibulare ; and 



(c) The cartilage of MECKEL, which corresponds to the remain- 

 ing portion of the mandibulare, but which afterwards 

 completely degenerates. 



