THE SKULL. 433 



With regard to the cartilaginous bars developed in the visceral 

 arches, the maxillary or palatopterygoid bar forms the basis of 

 the upper jaw, but it is not clear whether it arises independently, 

 or as an outgrowth from the maiidibular bar. 



The mandibular bar is a rod of cartilage, which along the 

 greater part of its length is known as Meckel's cartilage (Figs. 

 151 and 156, MC) ; it forms the basis of the lower jaw, the 

 bones of the mandible being formed around it, though not in 

 direct connection with it, except at the chin. 



The hyoid bar is at first cartilaginous along its whole length, 

 but subsequently disappears in great part. Its lower or ventral 

 end forms the basis of the anterior, or lesser cornu of the hyoid 

 bone of the adult. 



Of the first branchial bar, the only part that persists is the 

 ventral end, which forms the basis of the posterior, or greater 

 cornu of the hyoid bone. The body of the hyoid bone is 

 formed from the median elements of the hyoid and first 

 branchial arches. 



The development of the auditory ossicles, and their relations 

 to the mandibular and hyoid bars have already been considered 

 in the section dealing with the development of the ear (p. 398). 



Concerning the appendicular skeleton there is nothing special 

 to note, except that Kolliker has shown that the clavicle in 

 rabbit embryos of about the seventeenth day is cartilaginous ; 

 and that the clavicle, though presenting some peculiarities in 

 the details of its mode of ossification, ought to be viewed as a 

 cartilage bone, and not, as is commonly stated, as a membrane 

 bone. 



DEVELOPMENT OF THE SKIN. 



1 . The Hairs. 



Hairs are epidermal structures, and are as characteristic of 

 Mammals as are feathers of Birds. The first stage in the for- 

 mation of a hair consists in the growth of a small solid process 

 from the deeper or mucous layer of the epidermis, into the 

 underlying connective tissue (Fig. 156, DE). A small papilla 

 of vascular connective tissue grows into the deeper end of the 

 epidermal process, and serves for its nutrition. The hair itself 



F F 



