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 



