912 



EMBRYOLOGY. 



It is only in the third month that ossification begins in the vertebral 

 column. The number of osseous nuclei, primary and complementary, is 

 not the same in all species ; they have been enumerated at page 20. In 

 a large number, the spinous process is regarded as the result of the joining 

 together of the two moieties of the vertebral arch ; in the Sheep, on the 

 contrary, the spinous process forms a nucleus altogether independent of the 

 vertebral arches. Thomas has noted this disposition, and he considers it 



Fig. 437. 



Fig. 438. 



DIAGRAM SHOWING THE ATTENUATION OF 

 THE CHORDA DORSALIS IN THE MIDDLE OF 

 THE BODIES OF THE VERTEBRA, WHILST 

 PRESERVING ITS ORIGINAL DIAMETER IN 

 THE INTERVERTEBRAL SPACES. 



ch, Chorda dorsalis ; u, Body of vertebra ; 

 li, Intervertebral spaces. 



DIAGRAM SHOWING THE POSITION OF THE 

 CHORDA DORSALIS IN THE BODY OF THE 

 VERTEBRA, AND THE FORMATION OF THE 

 NEURAL ARCHES. 



eh. Chorda dorsalis; cu, Body of the verte- 

 bra ; a, Neural arch, or neurapophysis j 

 c, Rib; pr, Transverse process. 



as peculiar to animals whose anterior dorsal vertebras are furnished with a 

 long spinous process. 



During ossification, the chorda dorsalis disappears, except between the 

 vertebrae, where it is developed to form the intervertebral substance or 

 discs. 



B. DEVELOPMENT OF THE CRANIUM AND FACE. The encephalon is en- 

 veloped by a membrane formed at the expense of the protovertebral laminae. 

 This cranial membrane becomes partly cartilaginous, partly fibrous; the 

 cartilage exists at the base of the cranium, and seems to prolong the bodies 

 of the vertebrae into this region ; indeed it is known that the skull may be 

 resolved into four portions, each corresponding to a vertebra. The cartilage 

 is insensibly transformed into bone ; while the fibrous part, composing the 

 roof of the skull and its lateral walls, passes directly into an osseous state. 



The bones of the face are developed at the expense of the pharyngeal 

 arclies. This designation is given to four laminae (or lamellae) which spring 

 from the anterior extremity of the chorda dorsalis, and double downward and 

 inward to join those of the opposite side. They are also named the branchial 

 and visceral arches, and the spaces between them are called the pharyngeal 

 clefts. The upper jaw, mouth, nasal cavities : i.e. the nasal, maxillary, and 

 palate bones, are furnished by the first arch. Meckel's cartilage, which arises 

 from the handle of the malleus to pass towards the inferior maxilla, is also a 

 dependency of this branchial arch ; it disappears towards the sixth or seventh 

 month. It is to be remarked that, at the commencement, the mouth com- 

 municates with the nasal cavities ; the palate is developed in two moieties 



