906 
MR. A. FRASER ON THE DEVELOPMENT OF THE 
Pig embryoes. —1 centim., 1*5 centim., 2 centims., 2‘3 centims., and at 2 - 6 centims. 
Dog embryoes. —1 centim., 1*3 centim., and at 2*5 centims. 
Sheep embryoes. —For a series of tliese I have to thank Dr. Allen Thomson. 
1 centim., 1*4 centim., 1*6 centim., 2 centims., and at 4 centims. 
Rabbit embryoes. —1 centim., and at 1’5 centim. 
Human embryoes .•—1 centim., fully, and at 4 centims. The head and body of the 
last embryo measured about the length stated. 
In addition to these I have made observations upon the embryoes of the Mouse and 
Calf, but these were not complete. 
In the embryoes, and at the stages mentioned above, I have made complete series 
of sections (in many cases several complete series), in the long vertical (the plane 
being either parallel to the middle line of the head or along the obliquity of the 
post-oral arches, in order to cut their cartilages along their whole length), transverse 
vertical (with reference to the ossicula), and obliquely transverse approaching the 
horizontal planes. These were numbered so that I had corresponding sections at 
different stages for study and comparison. During the course of the work I soon 
learned that the incus was quite as distinct from both cartilages when they could 
properly be called so, as it was at birth, or at adult age, so that I had to work upon 
embryoes at a stage preceding the true cartilaginous one, that is, at a stage between 
that in which there was not the slightest trace of cartilage 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 the characteristic hyaline appearance. In order that 
I should have abundance of material for the study of this particular stage, I cut in the 
long vertical direction the heads of seven embryo Pigs, five Dogs, five Babbits, four 
Sheep, seven Pats, and one Human embryo. 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 greater aggregation of round 
cells ; hence it is that there is room for difference of opinion among rival homologists, 
especially those who, resting their claims solely upon the condition of the embryonic 
cartilages at this immature stage, fail to recognise the value of a knowledge of their 
form and relations throughout their entire distribution in the vertebrate series, and of 
the great importance of the relation of the nerves in determining the true homology 
of the parts in dispute. As I shall show further on, the value of the mandibular 
branch of the seventh nerve (chorda tympani) in the solution of this particular question 
is considerable. In the endeavour to make the embryonic history of these parts clear, 
I shall first of all describe the parts coming into more or less close connexion with 
the ossicula auditus, but more in a morphological than in a histological sense ; then, 
secondly, I shall give a description of the proximal extremities of the first two post¬ 
oral cartilages, which is in effect a description of the malleus and incus, and compare 
the embryonic condition of these ossicles with their form in the adult. I shall also 
