80 
ments as well as the time data are at once apparent by glancing at 
Figs. 145, 166, 147 in the Manual. 
This note is written to aid in securing new data, especially those 
relating to the size and form of young embryos. The two other points 
required before we can ascertain the age of embryos, namely, the 
time of fertilization and the time of ovulation may be secured by 
standardizing the corpus luteum anew and by collecting more young 
embryos following a single copulation. 
As to measuring young human embryos it is to be emphasized 
again that the crown-rump measurement, or sitting height, should be 
made from the crown, over the middle of the mid-brain, to the lowest 
point of the rump while the embryo is fresh or, what amounts to the 
same, after it has been fixed in formalin. This measurement should 
be exact as it represents the living and nod the shrunken embryo. 
In recent years I have measured a large number of embryos, first, 
fresh or in formalin, again after dehydration; finally they will be 
measured several times in the various fluids while they are being 
imbedded in paraffin. 
But even with this care the size of the embryos of the same 
stage of development seems to vary considerably. However, if all the 
suitable specimens in the literature are compared with the best in my 
collection, the variation in size of a given stage is still very pronounced. 
In order to give the question a test, I had photographed all of the 
good profile illustrations in the literature so that the pictures of the 
embryos gave a crown-rump measurement of about 50 millimeters. 
Then I had prints, enlarged to this size, made from my own nega- 
tives. These various photographs could now be compared from different 
standpoints, that is, they could be arranged according to their menstrual 
age, according to their greatest length, or according to their degree 
of development. When the photographs were arranged according to 
their menstrual age, no satisfactory classification could be reached. 
When considered according to size the results were still quite unsat- 
isfactory. Finally I arranged them according to their stage of devel- 
opment, and this proved to be more satisfactory, as I had anticipated. 
At first I arranged the two hundred and sixty-six photographs 
of embryos, ranging from two to twenty-five millimeters, in twenty 
stages, taking the external features— branchial arches, arms and 
legs—as my guide. As all of the photographs had been marked 
arbitrarily to conceal the length of the specimens, and as there were 
