CHANGES IN THE OVUM. 



95 



portion ; at the end of the second month all the vertebral bodies are car- 

 tilaginous, while the laminae are yet in a membraneous condition. In the 

 third month ossification commences, and during this process the dorsal 

 cord disappears, except between the vertebras, where it is developed to 

 form the intervertebral fibro-cartilage. 



The /^<r^ and <rrrt!«/V/?« are formed by a membrane that envelops the 

 encephalon, and which is due to the protovertebral laminae. This cranial 

 membrane become partly cartilaginous and partly fibrous, the cartilage 

 existing at the base of the cranium, and appearing to be a prolongation 

 of the bodies of the vertebrae : indeed, there is a resemblance between a 

 vertebra and the cranium, in so far as the latter can be resolved into four 

 portions, each corresponding to a vertebra. This cartilage is slowly 

 transformed into bone ; while the fibrous part, answering to the roof and 

 sides of the skull, pass directly into the osseous state. The bones of the 

 face are formed by the phary?igeal, branchial, or visceral arches : a name 

 given to four laminae which, springing from the anterior extremity of 

 the dorsal cord, curve downward to meet those of the opposite side ; 

 the spaces between them are named the "pharyngeal clefts." The 

 upper jaw, mouth, and nasal cavities — composed by the nasal, maxillary, 

 and palate bones — come from the first pharyngeal arch ; while Meckel's 

 ■cartilage, which passes from the handle of the malleus towards the lower 

 jaw, is also an appendage of it. This cartilage disappears about the 

 sixth or seventh month. At first the mouth communicates with the nasal 

 cavities ; the palate is developed in two portions, which advance towards 

 each other, but remain for some time apart \ so that during this time the 

 young animal has a cleft palate.* The second pharyngeal 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 the 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 

 protovertebral laminae which incline towards the lower face of the ver- 

 tebral 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 disappearing 

 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 vertebrae, 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 vertebrae. 



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 



* This cleft condition of the palate would sometimes appear to persist after foetal life. In April, 1876, 

 at the Middle Park Stud, in Kent, I saw a thoroughbred foal with a cleft palate. It was being suckled, 

 and a portion of the milk, instead of passing down the oesophagus, escaped from the nostrils. It was this 

 unusual course of the milk which led to the detection of the imperfection. 



