l>KrKI.(il'.\(KXT or THE FiLTl'S. 103 



month. At tirst the mouth communicates with the nasal cavities ; the 

 palate is developed in two portions, which advance towards eacli other, 

 but remain for some time apart ; so that during this time the young 

 animal has a cleft palate.^ The second phai-yngeal arch forms the 

 stapes, the petrous portion of the temporal bone, the styloid arch and 

 the hyoid branch. The third originates the hyoid bone with its 

 cornua, while tiie fourth only constitutes the soft parts in this region of 

 the throat. 



The thorax, consisting of the ribs and sternum, is an appendage of 

 the proto-vertebral lamiuio, which incline towards the lower face of the 

 vertebral spine. The true ribs are developed most rapidly, and before 

 attaining the middle line they unite by their inner extremity to form a 

 moiety of the sternum. A fissure at this part separates the ribs of one 

 side from those of the other ; this gradually closing, ends by disappear- 

 ing altogether, and then the sternum is constituted. The ribs are, 

 after the petrous portion of the temporal bone, the parts of the skeleton 

 which ossify most promptly ; ossification begins in the middle bones. 

 The ribs do not belong exclusively to the dorsal vertebra}, but have a 

 tendency to be developed along the length of the spine ; it is not rare 

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

 and which is soon lost in the substance of the abdominal parietes ; in 

 Birds this body assumes large dimensions on the last cervical vertebrue. 



The shape of the thorax differs with species, being round in some 

 and oval in others, but it is always less developed in the foetus than in 

 the young or adult animal. 



The limbs do not appear until after the formation of the vertebral 

 spine, the pharyngeal arches, and the thorax. They show themselves 

 as four small prolongations from the pelvis and chest, slightly thickened 

 at their origin and contracted in the middle. Their free extremity is 

 flat, and is either simple or divided, according to the foot of the species. 

 In these prolongations the cartilaginous segments are formed, which, 

 when ossified at a later period, constitute the bones of the limbs. 



The muscles are divided into four groups, after their origin. They 

 are the vertebral juusclcs, which arise from the muscular laminae of 

 the proto-vertebrai ; the visceral 7nusclcs for the thoracic and abdominal 

 cavities, the neck and jaws, and having the same origin ; the cutancoics 

 muscles, which originate from the cutaneous lamina; of the middle layer 

 of the blastoderm ; and the muscles of the limbs, the development of 

 which is not fully understood. 



It was believed at a certain period, that the muscular fibres are 

 formed by the joining together at their ends of several elongated cells. 

 It is now known that each fibre is constituted by a single cell which 

 extends in length, and whose nuclei are multiplied and placed at the 

 surface ; while the contents are transformed into a substance that pre- 

 sents all tlie characteristics of contractile tissue. The sarcolemma is 

 formed after the fibre, by a modification of the connective tissue sur- 

 rounding it. 



The locomotory apparatus of the majority of the domesticated species 

 of animals is so developed at birth, that immediately after that event 

 the young creature can move with more or less alacrity. With the 



' This cleft condition of the palate would sometimes appear to persist after ffctal life. 

 In April, 1876, at the Middle Park Stud, in Kent, I saw a thoroughbred Foal with a 

 cleft palate. It was being s^uckled, and a portion of the milk, instead of passing down 

 the cpsophagiis, escaped from the nostrils. It was this unusual course of the milk which 

 led to the detection of the abnormality. 



