ix. j OF THE DOG. 125 



Pig" {Philosophical Transactions, 1874), of which the fol- 

 lowing is a brief summary. 



The notochord (see p. 16) extends into the basicranial 

 axis only as far as the hinder border of the pituitary body, 

 corresponding with the middle of the future basisphenoid 

 bone. 



The skull is formed from — 



• \ A cartilaginous basicranial plate, embracing the 

 irior extremity of the notochord. 



b. A series of five paired descending cartilaginous arches, 

 developed in the visceral laminae, constituting the sides of 

 the face and neck ; of which two are in front of, and three 

 behind the mouth. 



c. A pair of cartilaginous auditory capsules. 



d. A pair of cartilaginous nasal capsules. 



The basicranial plate grows up as an arch over the 

 occipital region of the skull, and coalescing with the auditory 

 capsules laterally gives rise to the primordial skeleton of 

 the occipital, periotic, and basisphenoidal regions, of the 

 skull : the parietal and frontal regions being afterwards 

 completed by ossification in membrane surrounding the 

 cranial cavity. 



Of the arches, the first pair, constituting the trabecules 

 cranii, pass forward from below the front end of the basi- 

 cranial plate, enclosing the pituitary body, in front of which 

 they coalesce, and together with the olfactory capsules, give 

 rise to the presphenoidal, and ethmoidal regions of the 

 cranium (see Diagram on p. 106, I.). 



The second arch (pterygopalatine) 1 gives rise to the ptery- 

 goid, palatine, maxillary and malar regions, (IT.). 



The cleft between this and the next arch is the mouth. 



1 This is more properly an outgrowth from the third, than an inde- 

 ndent arch. 



pendi 



