162 
Proceedings of Royal Society of Edinburgh. [sess. 
taneous presence of both the right and left primary epiphysial 
outgrowths. Here, again, the left is by far the more marked of the 
two ; but the right outgrowth is also well developed — more so than 
in the previous instance (fig. 1). This section seems to the author 
to afford distinct proof of the fact that the epiphysis in the chick 
arises in the form of two distinct evaginations. Many other figs, 
of this early stage could have been represented ; but those already 
given amply demonstrate the presence of the right and left 
epiphysial outgrowths. In all the many sections showing paired 
outgrowths the left was better developed than the right. 
A study of the later stages of development of the epiphysis in 
the chick shows that the duration of the bilateral condition is very 
brief — the right and left primary outgrowths blending with one 
another to form the single unpaired epiphysial evagination. This 
is found to take place towards the end of the 3rd day — after 
the 60th hour of incubation. The bilateral condition is thus best 
observed between the 50th and 60th hours of incubation, so that 
it has a very transient existence (just as in Amphibia) ; and this 
probably explains why the bilateral origin has not been previously 
recognised. But it should also be noted that in some instances the 
right or smaller evagination was present, but only faintly distin- 
guishable, so that it was quite possible either to overlook its presence 
altogether (more especially if a single embryo was being examined 
instead of a series), or to consider it was as a small fold of the cere- 
bral wall due to faulty fixation, or, lastly, to look upon it as an 
anomalous condition. All the eggs which were examined in this 
research were incubated under a ‘ broody 1 hen, so that the occur- 
rence of those anomalies which ensue from the use of an artificial 
incubator was avoided. All the embryos were carefully fixed in 
Bles’ fluid, which is an excellent fixative for embryonic tissues, and 
all risks of shrinkage were thus entirely obviated. 
As has been already stated, the bilateral condition of the 
epiphysis ceases to exist about the end of the 3rd day of incuba- 
tion; but one cannot draw a hard-and-fast line of demarcation 
regarding the duration of the bilateral condition, as it is a well- 
recognised fact that chick-embryos vary considerably in their rate 
of growth. In some cases, therefore, the presence of the bilateral 
condition was observed previous to the 50th hour of incubation, 
