212 DEVELOPMENT OF THE MARSUPIAL SKULL, 



rod which lies below the Gasserian ganglion. It afterward* 

 unites with the outer edge of the trabecula. Still later, a 

 process grows up between the maxillary and mandibular branches 

 of nerve v., and gradually it ossifies into the alisphenoid bone. 

 There is thus no doubt as to its later history : the ditBculty is 

 to be certain of its homologies in lower vertebrates. It is 

 undoubtedly not trabecular, and, among reptiles, the only 

 cartilage with which it seems comparable is the palatopterygoid. 

 At first sight this may seem a startling conclusion, but there is 

 much to be said in its favour. In Sphenodon, the most primitive 

 reptile which we can examine, and the one nearest to the mammal- 

 like reptiles from which the mammals sprang, we find a bar of 

 cartilage bearing the same relations to the trabecula as the ali- 

 sphenoid cartilage does in the marsupial. This is the horizontal 

 portion of the palatopterygoid arch. The resemblance is obscured 

 by the development of the upward-passing epipterygoid. When, 

 however, the epipterygoid is aborted, as in Chameleo, we get a 

 short cartilaginous bar almost exactly similar. In the snakes 

 the same cartilage gives rise to the so-called alisphenoid bone. 

 In the Crocodilia we again get an alisphenoid bone; and in the 

 Chelonia a small bone which some regard as alisphenoid, others as 

 an epipterygoid. Even among extinct reptiles we get either an 

 alisphenoid or an epipterygoid, but never both. In the Tliero- 

 cephaliaand Anomodontia there is a typical epipterygoid; in the 

 Cynodontia a fan-shaped bone takes its place, and in the mammal 

 this becomes the bone we call the alisphenoid. 



We may thus conclude that the element which develops into 

 the alisphenoid is a specialisation of a rudiment of the palato- 

 pterygoid arch. 



LITERATURE. 



In Gaupp's paper is given a list of the most important papers on the 



development of the skull of the higher vertebrates. 



[Gaupp, E. — '• Zur Entwickelungsgeschichte und vergleichenden Morphologie 

 des Schadels von Echidna aculeata var. typica." Semon's Zoolo- 

 gische Forschungsreisen in Australien, &c., iii. Band, 4 Lief., p. 782: 

 (1908)]. 



