82 
The stages are written across the columns and the length of the embryos 
in millimeters, for each stage, form the columns. Thus, for stage L 
there are three specimens that are four millimeters long, nine that 
are five, five that are six millimeters long, etc. It is now seen at a 
glance that there are variations in the length of embryos for each 
stage. A line drawn through the table to determine the probable 
mean for each stage gives us the following: 


|Stages /H|1I|J|K|/L|M|N|/O|P|/Q|R|S|T\U 
‘Mean ie een. 
length? 2 3/4 |5|6 8 |10 12/14 16 18 20|22 24 
a les eh | 

The probable mean as thus determined shows that with embryos 
up to seven millimeters long stages may be made that are on an 
average one millimeter apart while with those above seven millimeters 
the stages are two millimeters apart. 
I intentionally do not illustrate the stages I have selected for I 
do not wish to view this grouping as a final classification. Also the 
number of embryos at the beginning and end of the series is much 
smaller than the material permits. I have not included any of my 
own specimens here. From the literature I have taken only profile 
drawings and not all of them. The photographs were taken from the 
illustrations of Broman, Bonnet, Bryce, His, HoCHSTETTER, KEIBEL 
and Erze, KEIBEL, KOLLMANN, Minot, RaBL and Rerzıus; unpublished 
photographs were also received from BARTELMEZ, ROBERT MEYER, 
Gack, KEIBEL, MCMURRICH, PIERSOL, STREETER and THompson. These 
latter anatomists, having learned of this study, most generously sent 
me prints before I had thought it proper to ask for them. 
I have selected the letter H to represent the stage that includes 
embryos about two millimeters long, reserving the letters A to G for 
earlier stages like that of Bryce. This stage includes embryos from 
the first appearance of the myotomes to the beginning of the limb 
buds. Thus an embryo with fourteen myotomes, 1.8 millimeters long 
and with two branchial arches could be represented by this formula, 
H.1:8,.m 14) Br;. 
Stage I begins with the appearance of the arm bud and has 
three pronounced branchial arches; J has four arches; in K they are 
reduced to three, that is, they are receding, so in stage K they may 
be expressed with a minus (—) sign, thus, K, 5, m (?), Brs—. 
