408 



Gaupp that the course of the chorda tynipani proves that the colu- 

 mellar chain of the Reptilia cannot be homologous with the ossicular 

 chain of the Mammalia, because in the Reptilia this chain lies behind, 

 in the Mammalia in front of the chorda. This sounds formidable, since 

 it appears at first sight impossible for the chorda to change from 

 the Reptilian position into that of the Mammalia, unless the chorda 

 had slipped through between incus and malleus, or rather between 

 incus and stapes, i. e. through the gap between quadrate = incus 

 and columella = stapes in their phraseology. But a simple wire- 

 model and a string, each fixed at either end into two boards standing 

 at a right angle will make it clear, how easily this change can be brought 

 about. Reptilian condition first: Chorda passing in a loop above the 

 columella (remaining median to the extracolumella) then passing for- 

 wards and down along the quadrate, crossing laterally over the extra- 

 columella-MECKEL connexion, and then into the mandible. — Now slip 

 the chorda over the extracolumella, letting its distal arm pass between 

 the extracolumella and tympanum (with which the extracolumella is 

 only secondarily connected) and we have the Mammalian condition : 

 Chorda passing over the stapes -incus connexion, behind and to the 

 inside of the malleus-xMeckel connexion, and into the mandible. Noth- 

 ing prevents the distal part of the chorda from being laterally overlaid 

 by the Meckel-malleus cartilage, especially if we take into consideration 

 the great reduction in length of the shaft of the quadrate so that in 

 the Mammalia the Meckel-malleus cartilage forms almost a straight 

 line, instead of a right angle as it does in the Reptiles. 



The same effect is produced if the chorda be assumed to be 

 stationary, and if the extracolumellar mass (= incus -\~ malleus), i. e. 

 the angle of the model, be pushed from below upwards and forwards 

 against the quadrate. That this is not a phantastic explanation is 

 shown by Sphenodon in which at any rate the returning half of the 

 chordal loop passes from behind and laterally over the extracolumella 

 and hyoid, cf. Versluys. The forward move of the whole chain, or 

 rather of its middle third, has actually taken place in the Mammalia, 

 and it is now ontogenetically developed as what Kingsley calls a 

 pretrematic part in opposition to the post-trematic chain of Reptilia. 

 At the best this is no fundamental difference since in either case the 

 foot of the stapes and the Meckelian connexion are indisputably fixed 

 and absolutely homologous points. 



In conclusion let us compare the two theories with each other. 



Evolution of the whole ossicular chain out of the extracolumella + 

 columella and these again out of the hyomandibula has the following 



