104 WEIL. 



evidence for this theory. On the other hand, German authority 

 since the time of Reichert has been unanimous in declaring the 

 incus to be derived from the first, or mandibular arch. With 

 reference to these long-standing and apparently irreconcilable 

 discrepancies, the following statement of Fraser, who made sec- 

 tions in three planes of a large number of forms, is deserving of 

 particular attention. He says : "I soon learned that the incus 

 was quite as distinct from both cartilages (mandibular and 

 hyoidean) when they could properly be called so, as it was at 

 birth or at adult age, so that I had to work upon embryos at a 

 stage preceding the true cartilaginous one, that is, at a stage 

 between that in which there was not the slightest trace of carti- 

 lage to be detected, and that in which the cartilages of the 

 arches were sharply and clearly defined, and in which the 

 cartilage cells had acquired a characteristic hyaline structure. 

 But here again the difficulty arose that although the cartilages 

 could be roughly distinguished, yet they were not limited by 

 any sharp line of demarcation, but faded gradually away into 

 the adjacent mesoblastic or embryonic tissue, from which they 

 differed only in the greater aggregation of round cells." 



Two years ago, as I reported to the Academy, an investiga- 

 tion of sections of a full series of Pig embryos, induced me like- 

 wise to conclude that it was impossible from the material em- 

 ployed to decide with certainty as to the origin of the incus. 



DiDELPHYS MURINA. 



Up to the present time, no investigation into the ossicula of 

 the Marsupialia by means of the newer methods has been re- 

 corded. In the Berliner MonatsbericJite for 1 867 there is a note 

 to the effect that Professor Peters found in certain young Mar- 

 supialia, including Didelpliys, that the os tympanicum was con- 

 tinuous with Meckel's cartilage. This was in support of his 

 theory that the quadrate of lower vertebrata is to be sought in 

 the tympanic bone of Mammals. Upon this point, however, 

 Peters was led astray, doubtless on account of the minuteness of 

 the object under observation and the lack of the more perfect 

 modern method of sectioning. In my sections I find that the 



