CHAP, x.] THE DEVELOPMENT OF THE CAT. 337 



canal. The first visceral cleft persists on each side as the external 

 auditory meatus, the tympanum and Eustachian tube. The other 

 visceral clefts all successively close up and disappear. The primitive 

 roof of the mouth is formed by the floor of the cranial cavity ; 

 subsequently a lateral growth from each maxillary process extends in- 

 wards so as to form a partition across the upper part of the primitive 

 buccal cavity, thus constituting the palate, and separating the hinder 

 parts of the nasal passages from the mouth. The posterior nares 

 open at first (before this growth of the palate) into quite the anterior 

 part of the primitive buccal cavity. 



The cartilaginous basis of the true future cranium is laid at 

 the anterior end of the chorda dorsalis, which becomes surrounded 

 by a solid, flattened mass of cartilage called the basi -cranial plate or 

 pamchordal cartilage, which is the precursor of the basilar part of 

 the occipital bone. 



Continuous with this on each side is a rounded portion of cartilage, 

 which encloses the primitive ear (formed by a depression of the 

 surface, as before mentioned) and is the precursor of the petrous 

 part of the temporal bone (Fig, 154, au}. The basi-cranial cartilage 

 grows up on each side, and these lateral parts meet together above 

 and enclose the foramen magnum with a cartilaginous ring. The 

 basi-cranial plate, together with the enclosed chorda, stops short 

 anteriorly at the hinder margin of what later becomes the sella 

 turcica. 



From the front of the basi-cranial plate two cartilaginous rods, 

 called the trabeculce cranii (Figs. 153 and 154, tr), extend forwards. 

 They at first diverge, but afterwards converge and meet, thus 

 enclosing between them the place of the future pituitary fossa 

 (Fig. 154, py). The trabeculso at their anterior junction form a 

 cartilaginous expansion called the ethmo-vomerine plate, which sends 

 downwards three other plates to form the median and lateral 

 ethmoids (thus embracing the two primitive olfactory sacs,) in- 

 growths of the lateral ethmoids forming the future maxillo-turbinals. 

 Also two small cartilages appear, on each side of the cranial cavity, 

 as forerunners of the all- and orbito-sphenoids. 



The walls and roof of the skull are otherwise completed by 

 membrane only, until the commencement of that membrane's 

 ossification. 



There are certain other cartilages, however, which play an 

 important part in forming certain parts of the bony skull, namely, 

 the cartilages of the visceral arches. 



The first of these descending rods of cartilage, is connected above 

 with the cartilaginous auditory capsule, and is called " Meckel's 

 cartilage" (Figs. 153 and 154, mn). What has the appearance of 

 being merely an outgrowth forwards from this first descending rod 

 (the maxillary process of the first visceral arch) lays the cartilaginous 

 foundation of the bony upper jaw, including the pterygoid and 

 palatine bones (pps), while the rest of Meckel's cartilage (beyond the 

 point where such process is given off forwards) lays the cartilaginous 



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