C1I. L1X.] 



FORMATION OF FACE AND SKULL 



847 



A. 



pits, and the sides and the lower boundary of the orifice are formed by 

 the first or mandibular arches. From the upper ends of the mandibular 

 arches the maxillary processes grow forwards immediately beneath the 

 eyeballs (which have appeared on the sides of the head), and as they 

 grow they pass beneath the lateral nasal processes, and beneath the 

 anterior ends of the olfactory depressions, and fuse with the globular 

 processes which also fuse together. Thus the orifice of the stomadseal 

 space is cut into three parts, the two nasal orifices and the mouth. 

 The upper lip is formed by the fused globular and maxillary processes, 

 and contains three lines of fusion one in the middle line between 

 the globular processes, and two more laterally placed between the 

 maxillary processes and the globular processes. In certain cases the 

 fusions do not take, and then clefts are left in the upper lip, and 

 constitute the various forms of hare- 

 lip. From the inner parts of the 

 maxillary processes of opposite sides, 

 palatal ledges grow across the 

 stomadaeal space; and meeting in 

 the middle line, they fuse together 

 and separate the space into an upper 

 or nasal and a lower or buccal 

 space. If the palatal ledges fail to 

 meet, cleft palate results. A cleft 

 may also appear between the nasal 

 orifice and the conjunctival sac, as 

 a result of the absence of fusion 

 between the maxillary process and 

 the lateral nasal process. The lower 

 boundary of the mouth orifice is 

 formed by the mandibular arches. 



Both the tissues of the fronto -nasal process and those of the 

 mandibular arches take part in the formation of the skeleton of the 

 head. The notochord extends forwards as far as the pituitary body in 

 all vertebrates, and in some, protovertebral somites can be traced 

 forwards to a similar point; but in mammals they are only distinct 

 behind the ear in the occipital region, and even there they entirely 

 disappear at an early period. In the lower vertebrates a bar of 

 cartilage appears at each side of the notochord in the head ; these are 

 the parachordal cartilages, and they soon fuse to form a basilar plate 

 in which the notochord is embedded. It becomes the basi-occipital and 

 basi-sphenoid bones. In mammals, the parachordal stage is eliminated, 

 and a basilar plate is formed at once. In front of the basilar plate 

 two trabeculae cranii embrace the pituitary body and extend forward into 

 the fronto-nasal process, where they blend together to form an ethmo- 

 vomerine plate ; and from this, processes extend down on each side, the 



FIG. 643. Diagrams of the cartilaginous 

 cranium. 



A, first stage. Ch, Notochord ; Tr, trabeculae 

 cranii ; P. ch., parachordal cartilages ; P, situa- 

 tion of pituitary body; N, E, 0, situations of 

 olfactory, visual, and auditory organs. 



B, later stage. B, Basilar cartilages ; S, nasal 

 septum and ethmoidal cartilages ; Eth, Eth', 



