1888 - 89 .] Mr D. Hepburn on Dicirtlirodial Joints. 
259 
surrounding mesoblast, except such variations in outline as can be 
accounted for by compression. 
As growth proceeds, digits emerge from the bulbous extremity, and 
a section at the end of the ninth day shows that a process of 
differentiation has taken place in the condensed portions, the result 
of which is the formation of cartilaginous rods separated from each 
other by masses of undifferentiated cells termed the articular disc or 
inter-tissue. 
It is in connection with this articular disc that the future joint 
cavity and its various appendages are developed. Here the joint 
cavity makes its appearance, and may be seen in the wing of the 
chick at the end of the ninth day. The cleft commences within the 
circumference of the articular disc and extends towards the axis of 
the disc, so as to divide it into two segments, each of which is applied 
to the end of a cartilaginous rod, and the segments are held together 
by its undivided periphery. The sides of the cleft are bounded by 
a layer of flattened cells. 
When this cleft does not extend across the axis of the disc, 
material is left for the formation of an interarticular ligament, as 
may be seen in sections taken from the leg of a chick about the 
middle of the second week. 
In the case of some joints two cavities appear, having between 
them a portion of the disc, which ultimately develops into a 
meniscus. Again, when the two cavities fuse in the axis of the 
disc, we have an incomplete meniscus. 
Even at this early stage there is a certain amount of moulding of 
the ends of the cartilaginous rods which foreshadows their future 
shape, and as this occurs at a time when the muscular system is in 
abeyance, it cannot be the result of movement, and neither can we 
ascribe the formation of the cavity to this cause. 
Tracing the changes which take place in connection with the 
"various parts of the now partially divided articular disc, we find that 
the segments applied to the ends of the cartilaginous rods gradually 
become differentiated into hyaline cartilage, until this process has 
affected the whole thickness of the segment, with the exception of 
the row of flattened cells next the cavity of the joint. In the chick 
these are found still persisting at the period of hatching. 
The undivided circumference of the disc has meanwhile under- 
