598 Proceedings of the Royal Society of Edinburgh. [Sess. 
IY. 
A brief summary of the principal conclusions to which I think my 
preparations lead is as follows : — 
1. (a) The malleus, stapes, and incus are derivatives of the first visceral 
arch. In the case of the malleus and incus, this is evident from their 
position and relations with other structures. In the case of the stapes, the 
exact correlation of parts is more obscure, but it has been shown that, 
owing to twisting and compression, the stapes anlage has become shifted so 
that it lies dorsal and medial to the re-bent portion of the first pharyngeal 
pouch, but that it lies also cranial to the point of flexure. Inasmuch as in 
an early stage this point of flexure is a cardinal point in the topography of 
the region, the position of the stapes with regard to it, in addition to the 
relationship shown by the stapes with other structures, precludes the 
possibility of the stapes having been derived from the hyoid arch. But 
there is nothing in the position of the stapes nor in its other relation- 
ships to preclude the possibility of its having been derived from the 
first arch. And as the stapes has been shown to be independent of the 
auditory capsule, it is evident that it must be a derivative of the 
mandibular arch. 
(6) The stapes and incus are at all times structures distinct from the 
Meckelian bar, being chondrified independently. The malleus, on the 
other hand, is continuous with the proximal end of Meckel’s cartilage, and 
the connection between them is of a jDrimary nature. 
2. The stapes cannot be homologous with any structure found in the lower 
Vertebrata, for it is derived from the first arch, whilst the amphibian and 
reptilian stapes is either derived from the auditory capsule or from the 
second arch, and the hyomandibula of fishes is also derived from the 
second arch. Thus the mammalian stapes must be regarded as a structure 
peculiar to that group. The malleus and incus are, however, homologous 
with the articulare (in part) and the quadrate of the lower groups. 
My thanks are due to Professor Cossar Ewart for the material for these 
investigations, and for many helpful suggestions, and it is from his 
laboratory that this communication is offered. It is a great pleasure to 
acknowledge my indebtedness to him and to Drs Beard and Ashworth of 
the Edinburgh University Zoological Department. 
A grant towards the expenses of the investigation has been made from 
the Earl of Moray Endowment Fund of the University of Edinburgh. 
