328 The Alligator and Its Allies 
STAGE XVIII 
FIGURE 21 (PLATE XXVII.) 
This embryo, as may be seen, for example, by the 
form of the appendages, is slightly further devel- 
oped than the one represented in Figure 20. The 
figure is from a photograph of a living embryo as 
it lay in the egg, a portion of the shell and shell 
membranes having been removed. The embryo, 
which lies on its left side, is rather faintly outlined 
because of the overlying allantois. The allantois 
has been increasing rapidly in size, and is here so 
large that it extends beneath the cut edges of the 
shell at all points except in the region in front of 
the head of the embryo, where its border may be 
seen. Its blood-vessels, especially the one that 
crosses the head just back of the eye, are clearly 
shown in the figure, and in the living specimen, 
when filled with the bright red blood, they form a 
most beautiful demonstration. As in the chick, 
the allantois lies close beneath the shell membranes 
and is easily torn in removing them. 
STAGE XIX 
FIGURE 22 (PLATE XXVII.) 
Figure 22 is a photograph of a somewhat older 
embryo, removed from the egg and freed of the 
fetal membranes. The appendages show the posi- 
tion of both elbow and knee joints, and in the 
