650 J. PLAYFATR MOMURRICH. 



SumTnary. — The most noticeable feature in the configura, 

 tion of the embryonic cranium is the bending up of the facial 

 portion against the front of the skull, this being due to the 

 bending up of the coalesced trabecule cranii at their extremity. 

 As development proceeds this bending up does not diminish- 

 but the mouth becomes carried forward by the growth of the 

 horizontal portion of the trabeculse, and it is not until Stage d 

 is arrived at that the elongation of the snout is dependent upon 

 the straightening out of the rostral cartilage. 



In the adult the first thing to be noticed is the forward 

 extension of the occipital region and the compactness of the pro- 

 otic region. The absence of any cartilaginous sphenoid bone, 

 the wide opening for the passage of the orbital muscles and 

 nerves consequent upon this, tlie absence of an osseous inter- 

 orbital septum as in the Siluroids and Cyprinoids, the want of 

 nasals, and the structure of the ethmoids, are also points of 

 considerable importance. 



II. The Visceral Skeleton. 



In the youngest stage observed (a) most of the visceral 

 arches were apparently fully formed, and consisted of seven 

 cartilaginous bars, some, however, being more differentiated 

 than others, the first three already showing a specialisation 

 into their future parts. 



The first postoral or mandibular arch consists, on either 

 side, of a well-developed mandibular portion (fig. 1, Mck.), 

 bounding the gape below, extending forwards and upwards, 

 and curving slightly inwards towards its fellow of the opposite 

 side. Articulating with the proximal extremity of each of 

 these Meckelian cartilages is a single rod-shaped portion 

 (Ft. Qu.), extending backwards and slightly upwards, but 

 lying quite free in the tissue of the prevertebral portion of the 

 skull, except for the articulation with Meckel's cartilage. 

 Subsequent development shows this to be the rudiment of the 

 pterygo-quadrate portion of the jaw, and hence it may be 

 denominated the pterygo-quadrate cartilage. 



The second postoral or hyomandibular arch is represented by 



