TEE FCETUS. 



913 



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, styloid arch, and 



branch of the hyoid. The Fig. 439. 



third develops the hyoid 



with its cornua ; while the 



fourth arch only constitutes 



the soft parts of the neck. 



C. DEVELOPMENT OF THE 

 THORAX. The ribs are de- 

 pendencies of the proto- 

 vertebral laminae, which 

 curve towards the lower 

 face of the vertebral column. 

 The true ribs are most 

 rapidly developed, and 

 before attaining the middle 

 line are united by their 

 internal extremity, and 

 form a moiety of the ster- 

 num. A fissure separates 

 the costal arches of the 

 right side from those of 

 the left; this gradually 

 contracts, and finally dis- 

 appears, and the sternum 

 is. then formed. The ribs 



are, after the petrous bone, the parts of the skeleton which are most 

 promptly ossified, ossification commencing in the middle ribs. 



The costal arches do not belong exclusively to the dorsal vertebrae, but 

 have a tendency to form along the whole length of the spine ; and it is not 

 rare to see, attached to the lumbar vertebrae, a small cartilaginous nucleus 

 which is soon lost in the texture of the abdominal walls. This nucleus 

 assumes large dimensions on the last cervical vertebrae of birds. 



The form of the thorax varies with the species ; in some it is circular, 

 in others it is flattened laterally ; and in all cases it is less developed in the 

 foatus and young animal than in the adult. It is in the latter that the 

 thoracic cavity presents, proportionately, its greatest dimensions. 



D. DEVELOPMENT OF THE LIMBS. The limbs do not show themselves 

 until after the formation of the spinal column, the pharyngeal arches, and 

 the thoracic parietes. They appear as four little prolongations from the 

 thorax and pelvis, and are slightly enlarged at their origin and constricted 

 in the middle. Their free extremity is flattened, and either divides or 

 remains single, as the animal has one or more apparent digits. It is in 

 these prolongations that the cartilaginous rays are developed, which, at a 

 later period, become the bones of the limbs. For the manner in which ossi- 

 fication is carried on in each bone, reference must be made to Articles IV. 

 and V., pages 71 and 91. 



2. MUSCLES. The muscles are developed around the bones when these 

 have become perfectly distinct. They may be divided into four groups : the 

 vertebral muscles, which come from the muscular laminae of the protovertebrae ; 

 the visceral muscles thoracic and abdominal cavities, neck and jaw having 



THE HEAD OF A FCETAL LAMB DISSECTED TO SHOW 



MECKEL'S CARTILAGE. 



Mm, The malleus; i, Incus; zy, The tympanic; H, The 

 hyoid ; sg, The squamosal ; p, Pterygoid ; pi, Palatine ; 

 L, Lachrymal; pmx, Premaxilla; N, Nasal sac; Ew, 

 Eustachian tube. 



