DEVELOPMENT OF THE FACE. 



959 



FIG. 36?. 



an v 



a, 



Ao 



IHfi 



subsequently becomes segmented off, its apex or proximal portion remaining 

 as the incus and os orhiculare in Man (which represent the hyomandibular 

 and symplectic of Fish), whilst its distal portion is converted into the styloid 

 process of the temporal bone and cornu minus of the hyoid with the inte'rven- 

 ing membranous part (Fig. 361, g, m, i). The stapes proceeds from a car- 

 tilage developed in the fenestra ovalis, and is probably a periotic element. 

 In the third visceral arch, which corresponds with the first branchial arch 

 of Ichthyopsida, the cornu majus of the hyoid bone is developed on each 

 side, ossifying in the eighth month. 



796. From the foregoing brief outline of the development of the skull, it 

 will be seen that no segmentation of the cartilaginous basis takes place at all 

 analogous to that which occurs in the vertebral column at the same period 

 of its development; and therefore, as Mr. Huxley observes, it is impossible 

 to admit the existence of a 



series of cranial vertebne, 

 and still less to define their 

 several parts by means of 

 terms especially adapted to 

 the description of those bones 

 which constitute the spinal 

 column. At the same time, a 

 very superficial investigation 

 of the process of ossification 

 leads to the conclusion that 

 this takes place in such a 

 manner as to produce a series 

 of segments which are the 

 more closely analogous to 

 those of the vertebral column 

 the more posteriorly they are 

 situated, as we see in the 

 occipital and sphenoid seg- 

 ments; whilst in those that 

 are more anterior the modi- 

 fication of structure is so 

 great, that scarcely any 

 trace of the form of a ver- 

 tebra remains, as appears in 

 the case of the pre-sphenoid 

 and ethmoidal segments. 



797. Within the Cranio- 

 spiual canal thus formed, 

 the rudiment of the Cerebro- 

 spinal axis is found, at first 



Under a Very different aspect a', The outer amniotic fold or false amnion ; a, the true am- 

 froni that which it Subse- Dion ; ' tail ; CH < cerebral hemisphere ; FB, fore brain or vesi- 



QUCntlv 're*eutS eSDeciallv le " f the third ventricle ; MB,m\l-*n\n; IT*, hind brain ; 

 IS, espe ,liy ^ optjc ves . de . ^ otic vesi( . le . o/ ,, omp'ialo-mesaraic veins 



as regards the relative pro- f orm i ng the venous roots of the heart; Hl,\\-a.rt, nowcom- 



poi'tion of its different Seg- pletely twisted on itself; Ao, bulbus arteriosus, the thin aortic 



nientS. According to the arches dimly seen uniting into the aorta, still more dimly seen 



investigations of Bidder as a curved dark line running along the body; the other curved 



TT- fr> -i T^-ll'l. dark line by its side ending near the reference y is the noto- 



JVU er, ana 1VO IKCr, chord, ch; Ofa, omphalo-mesaraic arteries ; Pe, protov rtebrae 



Man and Birds, the Spinal x i S pi ace d a t the point of divergence of the splaucliopli'iire 



COl'd Consists, after the clo- folds. The blind foregut begins here and extends about up 



sure of the laminae dorsales, to y. 



