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 ¢) 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. 
