THE FCETUS. 



1039 



The bodies of the vertebras are developed more rapidly than their spinal 

 portion : thus, towards the end of the second month, all the vertebral bodies 

 are already cartilaginous, while the vertebral laminae are yet membranous. 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 p. 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 as peculiar to animals the anterior dorsal 

 vertebrae of which 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 brain is enveloped 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 vertebrse 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 — comprising 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 



Fig. 574. 



arches, and the spaces between 

 them are called the pharyngeal 

 clefts. The upper jaw, mouth, 

 nasal cavities : i.e. the nasal, 

 maxillary, and palatine 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 disap- 

 pears towards the sixth or 

 seventh month. It is to be 

 remarked that, at the com- 

 mencement, the mouth com- 

 municates with the nasal cavi- 

 ties ; the palate is developed in 

 two moieties which advance 

 towards each other, though they 

 remain a long time apart ; so 

 that at this time the young 

 animal really has a hare-lip. The second pharyngeal arch forms the stapes, the 

 pyramidal process of the temporal bone, the styloid arch, and the branch of 

 the hyoid. The third develops the hyoid bone, with its cornua ; while the 

 fourth arch only constitutes the soft parts of the neck. 



THE HEAD OF A FCETAL LAMB DISSECTED TO SHOW 

 MECKEL'S CARTILAGE. 



M, Meckel's cartilage; wi, the malleus; t, incus ; Zy, th* 

 tympanic; H, the hyoid; Sg, the squamosal; Vt, 

 pterygoid ; pi, palatine ; L, lachrymal ; pmx, pre- 

 maxilla ; N, nasal sac, Em, Eustachian tube. 



