83 
Stage L also has three branchial arches, the elbow is beginning 
and the nasal pit is at its maximum. 
Stage M has two branchial arches, the leg is dividing into upper 
and lower leg, the lachrymal groove is pronounced and the hand plate 
is well formed. 
Stage N—Two branchial arches, thumb beginning, and the foot 
plate, ear and sinus cervicalis are well formed. 
Stage O—Slightly advanced over N, the great toe is beginning. 
P—The branchial arches have disappeared, the ear is well 
formed and the toes outlined. 
Q@—The arms reach to the ventral midline. (Doubtful stage.) 
R—More advanced than Q. 
S—The hands touch each other, the leg is extended and the 
great toe is pronounced. 
T—The hands are crossed and the foot is formed. 
U—Fetal form, the head is well developed. 
These provisional stages will now be thoroughly tested by com- 
paring within each stage the degree of development of an organ 
which is critical for this stage. Thus I have found that in its develop- 
ment the heart corresponds with the stage of development and not 
with the length of the embryos as given in my collection. BARDEEN 
has found the same for the development of the skeleton. However, 
the first appearance of ossification centers corresponds much better 
with the crown-rump length of the embryo studied than is indicated 
by the variation in length of the embryos as shown in the stages 
given in the table. This I have tested by the SCHULTZE method in 
100 specimens ranging from fifteen to one hundred millimeters in 
length. But in them, all of the measurements of length were made 
in the glycerine and caustic potash mixture which no doubt allowed 
the embryo to swell and regain its normal dimensions. The order of 
development of the earliest ossification centers is remarkably regular 
with the exception of the lower ribs and the upper and lower verte- 
brae. It is therefore probable that the embryos of a given stage, if 
measured fresh, will be much nearer a common length then is indi- 
cated in my table. 
A comparison of the profile illustrations of young human embryos 
shows clearly that those from His supplant those which precede 1880, 
and in turn these are fully displaced by those of HOoCHSTETTER and 
Keiser. The value of photographs is fully demonstrated by those 
6* 
