The Development of the Cartilaginous Skull ete. in Neeturus. 431 
of the cartilaginous capsule bounding the fenestra anteriorly. 
According to Kırrıan (90) in Axolotl, however, the relations are 
similar to those in Necturus, and the operculum arises quite 
independently in the neighborhood of the fenestra, but at the side 
of the cartilaginous ear-case. KILLıan says that in Rana eseu- 
lenta the fenestra is at first closed by connective tissue in which 
later the cartilage of the stapedial plate (operculum) arises, and 
Gaver (93) affirms the same for Rana fusca. Gaupp describes 
the operculum, however, as lying in the posterior part of the 
fenestra ovalis, leaving anteriorly a sickle-shaped space closed 
only by a membrane. In this anterior space the small cartilage of 
the pars interna columellae arises. Now in Triton and Siredon 
the operculum is described by Sr6ur as derived from the anterior 
wall of the fenestra ovalis, and in Necturus the operculum, 
arising outside the wall of the capsule, lies primarily slightly 
anterior to the fenestra ovalis. It is consequently possible that 
the cartilage, called operculum, arising in the membrane closing the 
oval window of the ear in the Anura, is not the homologue of the 
cartilage called »operculum« in the Urodela, which occupies more 
nearly the position of the pars interna columellae as described 
by Gaupp. 
The primary position of the operculum in Necturus is shown 
in fig. 18, pl. XVIII, a sagittal section through the lateral part of the 
cartilaginous ear. At the right, in the lower part of the figure, the 
hyoid cartilage is seen in section, between this cartilage and the 
fenestra which interrupts the continuity of the otic cartilage, lies 
a group of cells more compact than the surrounding mesenchyma. 
In the centre of this group of cells is found the beginning of the 
operculum. I regret that I am unable to trace the origin of these 
closely grouped cells. In slightly earlier stages, scattered cells of 
the mesenchyma occupy the space where in fig. 18 the procartilage 
cells of the operculum lie. 
The cartilage of the operculum is at no time connected with 
the hyoid cartilage, but this fact does not demonstrate that the 
cartilage in question may not be a rudimentary element of the hyoid 
arch, since each element of the cartilaginous arches arises from an 
independent centre of chondrification, and secondary fusions of carti- 
laginous elements do not necessarily show original association. 
In the embryo of 19 mm, not only has the occipital part of the 
basal plate fused with the cartilage of the auditory capsule, but the 
