9] THE NASAL ORGAN IN AMPHIBIA HIGGINS 9 



of the cartilage cells above the cornu trabeculae in the earlier stage. This 

 columna ethmoidalis (ce) extends posteriorly to the level of the choana, 

 lying medial to the olfactory organ and separating it from the forebrain. 

 As yet it is distinct from all other cartilage structures, although Terry 

 (1906) in a similar stage has described a connection of this bar with the 

 crista trabeculae. 



In this stage is the first appearance of a cartilage roof over the nasal 

 organ. From the posterior tip of each columna ethmoidalis, a small plate 

 has chondrified laterally which partially covers the olfactory sac above the 

 choana. This is the beginning of the planum tectale, which in the later 

 stages completely covers the posterior parts of the nasal organ. In this 

 connection it is of especial interest to note that although the columna 

 ethmoidalis chondrifies from in front backward, the planum tectale arises 

 at the caudal limit of the column and develops anteriorly. 



In the next stage (Fig. 3), several features have been added, to the cap- 

 sule. The crista trabeculae (cr t) is well developed and has united to the 

 posterior end of the columna ethmoidalis (ce), thus forming a wide trough 

 or cavum cranii, which supports the anterior end of the telencephalon. In 

 front of the crista, each trabecula inclines slightly toward the median line 

 to pass into the planum basale (pb), which shows but slight modifications 

 from the earlier stage. 



As yet the dorsal and ventral parts of the capsule are almost entirely 

 distinct from each other, the only connection being by the crests at the pos- 

 terior end (Fig. 4). On the other hand, the columnae of the two sides are 

 connected by a narrow ethmoidal bridge (pe), which lies directly above the 

 anterior margin of the planum basale and is formed by medial processes 

 from each ethmoidal column (Figs. 4, 44). 



Between this bridge and the basal plate are procartilage cells, which in a 

 later stage are to form, with the pons ethmoidalis, a transverse wall of 

 cartilage (to be described later) bounding the cavum cranii in front. Up 

 to this stage there is no complete separation between the cavum cranii and 

 internasal space, a condition which is permanent in Urodeles like Triton. 



Each columna ethmoidalis is now wider than before, except at its 

 extreme posterior end where it joins the crista. In front of the pons 

 ethmoidalis, each column bends obliquely outward and forward from the 

 axis of the skull and ends at the level of the tip of the cornu. A little in 

 front of the cristal connection of trabecula and column, each column bears 

 a lateral process which curves down over the choana. This is the beginning 

 of the planum tectale (pt) forming the posterior part of the more complete 

 roof of the capsule in the later stage. In the literature upon the amphibian 

 nasal capsule, this tectal plate has been called the lamina cribosa; but from 

 its relation to the olfactory nerve it is evident that it is totally different 

 from the cribiform plate of mammalian anatomy, hence the proposal of the 

 name used here. 



