EAR-BONES OF OPPOSSUM. 107 



As regards the anatomical relations of the malleus, this in- 

 vestigation serves merely to confirm the now almost universal 

 opinion that the malleus is a derivative of the mandibular arch. 

 As in the higher mammalia, it continues Meckel's cartilage 

 without a break, the cartilage throughout being well developed. 



The relation of the head of the malleus' with the body of the 

 incus is, however, entirely different from anything yet found in 

 the embryos of the higher forms. The process of segmentation 

 of the incus from the malleus has already begun, but cellular 

 continuity is in certain places still plainly to be made out. This 

 continuity is not by means of histologically indifferent tissue, 

 but by tissues sharply separated from the neighboring undiffer- 

 entiated mesoblast. The connecting cells are, in fact, cartilage 

 cells, though only in the first stage of their development (see 

 Plate III., Fig. i, also Plate II). 



It is apparent, therefore, that the incus is to be regarded, as 

 the Germans have so long contended, in the light of a descend- 

 ant of the mandibular arch. With the hyoidean arch it has ab- 

 solutely no relation. The derivation of the short crus from the 

 labyrinth wall, already noticed by Dreyfuss in some of the 

 higher forms, is present also in the Opossum (Plate II.). Its 

 significance is still problematical. 



This investigation was suggested to me by Professor Osborn, 

 and was carried on during the year 1896 in the Zoological 

 Laboratory of Columbia University. 



Annals N. Y. Acad. Scr., XII, July 8, 1899—8. 



