912 



EMBBYOLOGY. 



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 Ijeen 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 OP 

 THE CHORDA DORSALIS IN THE MIDDLE OF 

 THE BODIES OP THE VERTEBRA, WHILST 

 PRESERVING ITS ORIGINAL DIAMETER IN 

 THE INTERVERTEBRAL SPACES. 



ch, Chorda dorsalis ; a, Body of vertebra ; 

 li, Intervertebral spaces. 



DIAGRAM SHOWING THE POSITION OP THE 

 CHORDA DORSALIS IN THE BODY OP THE 

 VERTEBRA, AND THE FORMATION OP THE 

 NEURAL ARCHES. 



ch, Chorda dorsalis ; cv, Body of the verte- 

 bra ; (/, Neural arch, or neurapophysis ; 

 c, Eib ; pr, Transverse process. 



as peculiar to animals whose anterior dorsal vertebrae 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 op the Cranium and Face. — The encephalon is en- 

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

 This cranial membrane becomes partly cartilaginous, partly fibrous ; the 

 cartilage exists at the base of the ci'anium, 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 

 arches. This designation is given to four laminae (or lamellce) 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 



