555 



ON SOME DEVELOPMENTS OF THE MAMMALIAN 

 PRENASAL CARTILAGE. 



By R. Broom, M.D., CM., B.Sc. 



(Plate XLiv.) 



The prenasal element has been shown by Kitchen Parker* to 

 be a median cartilaginous development found in the anterior part 

 of the head and arising in connection with the front part of the 

 cranial trabeculse. Though this element is represented in all the 

 groups from the Elasmobranchs to Mammals, in the majority of 

 forms it is either rudimentary or only found in the young, and in 

 the adult it rarely attains any great degree of development. 



In the Elasmobranchs the prenasal cartilage is a well-developed 

 structure, and forms the axis of the large rostrum in the Skate 

 and other fishes. The rudimentary prenasal found in most higher 

 forms is probably an inheritance from these cartilaginous fishes. 



In the bony fishes with the development of the premaxillary 

 bones an agent was provided which served the double purpose of 

 cutting the water and of giving a firm support for the teeth, and 

 the need for the prenasal being thus gone we find it reduced to a 

 mere rudiment. 



The premaxillary bones in almost all the higher forms with 

 their important tooth-bearing function prevent the development 

 of the prenasal cartilage, which though sometimes an element of 

 importance in the very young animal, in most cases becomes 

 obliterated by the developing premaxillaries. 



In birds the prenasal forms the framework of the beak in the 

 early embr3^o, but as development advances it gradually becomes 

 encroached upon by the premaxillaries on either side. In the 



* W. K. Parker. Various Monographs on Development of the Skull, 

 Phil. Trans. 



