1 86 Proceedings of Royal Society of Edinburgh. [sess. 
To throw, if possible, some light on polydactyly in the horse, I was 
recently led to study the development of the skeleton of the limbs. 
One result has been the discovery of the long-missing phalanges 
of the second and fourth digits. Their rudiments have been recog- 
nised and their development and subsequent degeneration followed 
until they lose their identity and become inseparably connected 
with their respective metacarpals and metatarsals, to form what are 
familiarly known as the “ buttons ” of the “ splints.” 
It seems to be generally admitted that one of the first steps in 
the formation of the joints of the digits consists in the division of 
the primitive rods of cartilage into segments by discs of embryonic 
cells. The segments eventually give rise to the phalanges, while 
the cellular discs seem to be especially concerned in the formation 
of the various parts of the joint, e.g.^ the articular cartilages, the 
capsule, and the membrane lining the joint cavity. 
I have not hitherto succeeded in finding distinct articular discs in 
the second and fourth digits of the horse, even in very small 
embryos ; but I have observed in some cases clear lines running 
across the rods of cartilage in the position of the joints. It is 
possible that, owing to abbreviation (or to arrest) in development, 
true discs are never formed in these useless digits. Though clear 
evidence of phalanges was not found in a 20 mm. embryo, I made 
out easily enough the position of the metacarpo-phalangeal joint in 
a 25 mm. embryo. In this embryo there was no indication of a 
joint cavity or capsule, but a very thin layer of cells* separated a 
terminal nodule (representing the phalanges) from the metacarpal 
proper. 
This terminal portion (fig. I.), when removed, was seen to present 
at its proximal end a slightly concave surface. The distal end of 
the metacarpal was smooth and slightly rounded, and composed of 
apparently normal cartilage cells. 
The cartilage forming this nodule was found, on further exami- 
nation, to be so invested with cells and fibres that it was impossible 
to ascertain whether either of the phalangeal joints had been 
developed. Partly because of its minute size, and partly because 
of its interest, I did not care to sacrifice it to the microtome. 
* These cells probably represent an articular disc. 
