v CHONDROCRANIUM 307 



evolution of some of the lower Vertebrates (Elasmobranchs, 

 Sturgeons) there is still going on a process of spreading backwards 

 of the hinder limits of the skull with the incorporation into it of 

 additional vertebrae. 



As regards the evolutionary origin of the prechordal part of the 

 cranium we have so far no clue. 



The primary cartilaginous cranium does not remain by itself in 

 any Vertebrate. There become inseparably fused with it the cartilagin- 

 ous capsules which surround and protect the nose (olfactory capsule) 

 and the ear (otic or auditory capsule). Cartilage may also develop 

 in the sclerotic of the eyeball but owing to mobility of the eyeball 

 being necessitated by its having to be turned towards the direction 

 from which impressions are received, the cartilage in this case does 

 not undergo fusion with the chondrocranium. 



There are also associated with the cranium, and more or less 

 closely connected with it, the series of hoop-like cartilages of the 

 visceral arches l and finally in many of the subdivisions of the 

 Vertebrata important bony elements become added on to the chondro- 

 cranium. The description of the development of the skull will 

 therefore fall naturally into three sections : (1) The Chondrocranium 

 including the sense capsules, (2) The Visceral Arches, and (3) The 

 Bony Skull. 



The Chondrocranium. — The chondrocranium shows many 

 differences in detail in the various groups of Vertebrates. The 

 epichordal portion commonly makes its appearance as a pair of rods 

 of cartilage — the parachordal cartilages — lying one on each side of 

 the front part of the notochord. The prechordal portion similarly 

 takes its origin in a pair of trabeculae which lie dorsal to the buccal 

 cavity on each side of the infundibulum. Important differences are 

 seen in the relations of these primary cranial cartilages in different 

 members of the Vertebrata. Thus in Elasmobranchs, Ganoids, 

 Teleosts, Eeptiles and Birds the trabeculae are at first quite isolated 

 from the parachordals while in Lampreys, Amphibians and Lung- 

 fishes they are continued at their hind ends into the parachordals 

 (Sewertzoff). 



Again in many Vertebrates, apparently in correlation with the 

 great development of the eyes 'in early developmental stages, the 

 cranial cavity no longer extends forwards between the eyes. Its 

 walls have come together to form an interorbital septum and fore- 

 shadowing this, the trabeculae are closely approximated or even fused 

 in the median line. Gaupp applies the term tropibasic to such a 

 type of cranium and contrasts it with the platybasic type in which 



1 In the neighbourhood of the margin of the mouth there frequently develop in 

 the lower Vertebrates (Fishes and Amphibians) isolated pieces of cartilage (labial 

 cartilages). These are sometimes termed the precranial skeleton, and various specu- 

 lations have been made as to their possible evolutionary significance. Up to the 

 present there appears to be no convincing evidence that they are other than mere 

 secondary developments and consequently they will not be referred to further in this 

 hook. 



