The Columella Aiiris in Amphibia. 555 



the structures under consideration pass. The results of the study 

 in this species were given in concise form in our first or preliminary 

 paper. The evidence upon which the conclusions were based was 

 not submitted at that time, however, and may be given now, together 

 with such further details as are important in this connection. 



The first definite trace of the columella is to be found in embryos 

 of 11-13 mm. in length, a short time before hatching. At this 

 period while the cupola of the otic capsule is chondrified, the basal 

 plate is still in the pre-cartilage (chondroblastema) stage so that 

 the ventral boundary of the primary fenestra is not yet sharply 

 defined, and the difficulty of delimiting its ventro-medial side is 

 further increased by the fact that the cells of the future floor as well 

 as those occupying the site of the future membrane are equally rich 

 in yolk granules, in" which respect they differ from the cells of the 

 surrounding mesenchyma. The squamosum is just appearing as a 

 minute scale of bone above and behind the otic process of the 

 palatoquadrate. 



Below and beneath the squamosum, filling in the space between 

 otic capsule, squamosum and otic process of the palatoquadrate, and 

 extending down upon the outer side of the vena petroso-lateralis to 

 the yolk-bearing cells occupying the site of the future membrane, is 

 a dense tissue, with numerous cells and homogeneous matrix, which 

 embodies the proton of the columella, stilus and squamoso-columellar 

 ligament. The derivation of these cells was not definitely deter- 

 mined. Younger embryos lend some support to the view that they 

 migrate down around the vein. The different appearance of the cells 

 of the otic capsule adjoining the lower end of this cell-group argues 

 against their origin from this portion of the otic capsule (membrane 

 or floor). 



In a 13-14 nun. specimen the demarcation of the columellar tissue 

 is sharper and the portion against the fenestral membrane denser. 

 The condition at this stage is reproduced in a photograph, Fig. 39, 

 PI. IV, col. At about this time, chondrification begins and in 

 the larva 14-15 mm. in length there is a small cone of cartilage, 

 whose base is against the membrane and whose apex projects out into 

 the dense tissue still connecting with the cell-group between squa- 



