60 ILLINOIS BIOLOGICAL MONOGRAPHS [60 



CONCLUSIONS BASED ON THIS STUDY 



PHYLOGENY OF THE NASAL CAPSULE IN AMPHIBIA 



The cartilaginous nasal capsule of Amphibia must have been derived 

 from that of some lower vertebrate, be it Dipnoan or be it Crossopterygian, 

 and that in turn from the Elasmobranch. So far as our knowledge goes, 

 the Elasmobranch capsule is a continuous cartilaginous structure with no 

 gaps or openings except that for the naris and the small foramina for the 

 passage of nerves. The amphibian capsule, on the other hand, has inter- 

 ruptions in its walls; some of which are due to the fact that in the adult, 

 bones appear in the nasal region and thus do away with the necessity of 

 the persistence of protecting cartilages in certain places. Some are caused 

 by the development of a complete olfactory duct with its choana, in addi- 

 tion to the naris. 



Within the class of Amphibia there are many differences between the 

 various capsules, caused by differences in the size and shape of the cover- 

 ing bones, by the changes in the development and shape of the organ of 

 Jacobson, the modifications of the lacrimal duct, as well as by the difference 

 in the size and shape of the olfactory sac. It follows, therefore, that in the 

 history of the amphibian capsule, two stages should be recognized; one a 

 progressive process in which the capsule is built up, a repetition of the 

 ancestral (piscine) history, and the other in which this capsule is reduced 

 and changed into the definitive form. 



In following these two stages, we are hampered in our conclusions by 

 the fact that we have no accurate and detailed account of the develop- 

 ment of the capsule of any fish, and expecially in those groups universally 

 recognized as ancestral. Neither Parker nor Sewertzow give any details 

 of the development of the Elasmobranch capsule which can be used in this 

 connection. The studies made in this laboratory of the development of 

 the capsule in Acanthais only go far enough to show that it consists, when 

 it first is recognizable, of a rudimentary floor, and an even more incomplete 

 roof, and that these arise independently of the trabeculae or other cranial 

 elements. 



The foregoing studies show, that in those forms where the develop- 

 ment has been followed with any detail from one stage to another, that 

 there is first a progressive development, apparently tending toward the 

 formation of a complete capsule like that of the adult Elasmobranch. 

 Then alterations occur, largely degenerative in character, which result in 

 the partial reduction of the floor and roof. These progressive changes 



