178 Francis villy. 



The remaining two skeletal elements are closely connected in the 

 adult, though not actually fused. The stapes forms a concavo- 

 convex cartilage, stopping the fenestra ovalis ; whilst the columella 

 is a rod inserted in front of the stapes, between it and the auditory 

 capsule, and expanded distally against the tympanic membrane (see 

 Figs. 17 and 18). Professor Parker calls the proximal end of the 

 columella interstapedial and the remainder the mesostapedial. 



The stapes may be dismissed with very few words. It is formed 

 as a chondrification in the capsular membrane closing the fenestra 

 ovalis, at a period when the remainder of the capsule is well deve- 

 loped, and not long before the tadpole begins to assume the frog's 

 form. No more need be said of it here, as Professor Parker has 

 described this cartilage in his account of the frog's skull. 



The third and last of the skeletal elements mentioned has now to 

 be considered. As far as I can make out, the columella is embryo- 

 logically to be considered, not as a part of either the hyoid or mandi- 

 bular arches, but as similar to the stapes, in that it originates in the 

 membrane closing the fenestra ovalis. In this my results agree 

 with those published in Professor Cope's* recent paper, as they also 

 do concerning the separate origin of the columella and annular 

 cartilage. 



I have found the columella in the same specimen which showed 

 the origin of the annular cartilage. Just in front of the stapes, and 

 in the membrane connecting it with the remainder of the capsule, a 

 small isolated piece of cartilage is present. This ultimately becomes 

 the inner end of the columella, called by Professor Parker the inter- 

 stapedial element. From this cartilage a thin rod of cells extends 

 forwards and outwards close to the inner side of the thymus, till it 

 ends just in front of the last-mentioned organ. This rod will ultimately 

 be Professor Parker's mesostapedial cartilage, and by later growth it 

 will expand against the tympanic membrane at its distal end. At 

 the period under consideration it is very thin and not easy to trace, 

 but notwithstanding this I feel sure that it has no connection with 

 any skeletal element except the interstapedial. Under these circum- 

 stances it is fair to assume that it either grows outwards from the 

 interstapedial cartilage, or else, with less probability, that it is formed 



* "Hyoid and Otic Elements of the Skeleton in the Batrachia," "Journal of 

 Morphology," vol. ii., No. 2, November, 1888. 



