61] THE NASA L ORGAN IN AMPHIBIA— HIGGINS 61 



are well exemplified in Necturus where, even in the adult, the capsule is 

 more nearly entire than in any other genus; a condition which may be 

 explained on the supposition that Necturus is to be regarded, not as more 

 primitive than other Urodelan genera, but as a permanent larva, a view 

 which has been advocated by many in recent years. 



In Amblystoma, as detailed in the foregoing pages, this history is 

 carried farther. There is first a progressive development tending toward 

 a complete capsule, but never approaching completeness as nearly as does 

 the adult Necturus. Then, in conjunction with the process of meta- 

 morphosis, there comes a resorption of parts and a modification of those 

 that persist, resulting in large vacuities in both floor and roof of the cap- 

 sule. These steps are detailed above, and the final result is an envelope 

 for the olfactory organ in which parts are recognizable as homologous 

 with those of the capsules of the adult Anura. 



In the history of the capsules in all Amphibia the following parts are 

 concerned. The two trabeculae are united in the 'ethmoid' region by what 

 Gaupp and others have called an internasal plate. This lies below the 

 tip of the brain and is the 'ethmoid plate' of Winslow or the 'planum 

 basale' of the foregoing description. In front of this planum basale, the 

 trabeculae continue as the cornua trabeculorum to the tip of the skull, 

 supporting the anterior part of the nasal organ. In several Urodeles, 

 where I have studied the early larval stages (this history has not been 

 followed in the Anura with sufficient detail to say whether it holds for them) 

 a bar of cartilage, the columna ethmoidalis, arises on the upper medial side 

 of the olfactory organ (either independently or as an outgrowth from the 

 cornu) and lies parallel to the lower trabecula. By a lateral growth from 

 this ethmoidal column, the planum tectale extends over the dorsal surface 

 of the nasal sac, uniting in the later stages to the cornu trabeculae. The 

 term lamina cribosa, used by Winslow (1898) and Terry (1906) to designate 

 this cartilage is a misnomer, for it is hardly necessary to say that it cannot 

 be homologous with the structure bearing the same name in the mammals, 

 as it lies wholly dorsal to the olfactory nerve. 



In the larvae of several Urodeles, and in my single larva of Rana, the 

 ethmoidal columns of the two sides are connected by a pons ethmoidalis 

 which roofs the fenestra ethmoidalis leading from the cavum cranii to the 

 ethmoid region. This is only a temporary condition in most amphibia 

 where the chondrification of the planum verticale closes the fenestra and 

 unites the floor and roof of the capsule. In Triton and Diemictylus, on 

 the other hand, the planum verticale never develops; so that cavum cranii 

 and internasal space are separated by membranous structures only. 



The last special element entering into the formation of the capsule is 

 the antorbital process, which needs a somewhat longer discussion. In 

 the Urodeles, a process arises from the side of the trabecula just back of 



