228 THE DEVELOPMENT OF THE SKULL. [CHAP. 



tinuation of the base of the skull, and such they were con- 

 sidered to be by Rathke, their discoverer. Their different 

 mode of development, and (in the lower vertebrates) primitive 

 independence of the investing mass, prove that this is not the 

 case, and that they are almost undoubtedly to be looked on 

 as paired appendages. Their primitive independence of the 

 investing mass is clearly shewn in the skulls of the Marsipo- 

 branchii, in which the trabecute are formed of a dense 

 fibrous tissue and the investing mass of hyaline cartilage. 

 (W. Muller, Ban der Hypophysis u. des Processus infundibuli 

 cerebri. Jenaische Zeitschrift, Vol. vi.) A very probable 

 view, first put forward by Huxley in his Hunterian lectures 

 (vide Anat. Vertebrates, p. 77), and subsequently adopted by 

 Parker as well as other investigators, is, that they are homo- 

 logous with the rods of cartilage developed in the visceral 

 arches, and that they are therefore the remnants of an 

 anterior pair of arches. 



Gegenbaur (Vergleichende Anatomic der Wirbelthiere, in. Heft) looks upon 

 the trabeculse and their coalesced extremities simply as a prevertebral portion 

 of the cranium, and not as paired appendages of the investing mass. Their 

 paired condition seems to militate against this view ; it must be admitted how- 

 ever that our present knowledge of them does not permit us to state with 

 certainty more than that they are not morphologically a continuation of the 

 investing mass. 



In the bird and apparently many higher vertebrates they 

 are never independent of the investing mass, and it was this 

 fact which led to the erroneous view of their nature, formerly 

 adopted, that they were forward continuations of the axial 

 skeleton. 



In them and in the plate of cartilage formed by their 

 coalescence in front appear the ossifications of the whole of 

 the sphenoid, the ethmoid and nasal regions. 



6. The remainder of the paired series of appendages 

 are developed in the visceral arches. The foremost of 

 these are the cartilaginous rods developed in the first visceral 

 or mandibular arch. In our account of the face we men- 

 tion that the mandibular arch on each side produces a 

 bud known as the superior maxillary process, which goes 

 to form the superior boundary of the mouth. In this 

 process, as well as in the primitive arch, rods of cartilage 

 appear. 



The segmentation, so to speak, of the first visceral arch 



