618 



EMBRYOLOGY. 



cranium, the primary ossifications, and the covering bones, which in> 

 Fishes and Amphibia are easily distinguishable from one another 

 even in the adult animals, are to be recognised as separate parts only 

 in very early stages of development ; later it becomes more difficult 



to distinguish them, at 

 last impossible. This is 

 due to several things : 

 First, the cartila- 

 ginous primordial cra- 

 nium is laid down from 

 the beginning in a rudi- 

 mentary condition; 

 then, too, a large part 

 rf the roof is wanting, 

 the opening being closed 

 by a connective-tissue 

 membrane. 



Secondly, the cartila- 

 ginous primordial cra- 

 nium subsequently dis- 

 appears almost entirely, 

 partly by being dissolved, 

 partly byconversion into- 

 primordial bones. There 

 persist small remnants, 

 which have been retained 

 only in the cartilaginous 

 septum narium and the 

 cartilages of the outer 

 nose connected with it. 

 Thirdly, in the fully developed skull the primordial bones and the 

 covering bones are no longer distinguishable ; for the latter lose their 

 superficial position, become intimately united to the bones derived 

 from the primordial cranium, and with them, filling up the gaps, 

 constitute a firm, closed, bony receptacle of mixed origin. 



Fourthly, in the adult animal, bones which in the embryo are 

 formed separately, and in lower Vertebrates always remain thus, are 

 often fused. There is a fusion not only between bones of like origin, 

 but also between primordial and covering bones, whereby it finally 

 becomes altogether impossible to distinguish them. Many of th& 

 bones of the human cranium are consequently bone-complexes. 



C'ovc 



Fig. 337. Skull of a Frog (Rana esculenta). View frcm 

 beneath. After ECKER. 



The lower jaw is removed. On the left side of the figure 

 the covering bones have been removed from the cartila- 

 ginous part of the skull. 



Cocc, Condyli occipitales ; Olat, occipitale laterale ; GK, 

 auditory capsule ; Qu, quadratum ; Qjg, quadrato- 

 jugale ; Pro, prooticum ; Ps, parasphenoid ; As, ali- 

 sphenoid; Pt, osseous pterygoid; PP, palato-quadratum ; 

 FP, fronto-parietale ; E, ethmoid (os en ceinture) ; 

 Pal, palatinum ; Vo, vomer ; M, maxilla ; Pmx, pre- 

 maxillare ; N, N 1 , cartilaginous nasal framework ; 

 //, V, VI, places of emergence of n. opticus, n. tri- 

 geminus, and n. abducens. 



