DEVELOPMENT or THE AMERICAN ALLIGATOR — REESE S7 



distance anterior to this point the aorta has divided into three, or it 

 might be said that it has given off. two, large branches. These two 

 branches, one on either side near the posterior cardinal vein, pass 

 toward the ventral side of the embryo on each side of the cloaca and 

 end at about the region represented by the present figure. The 

 small portion of the aorta that remains after the giving off of the 

 two 'branches just described continues, as the caudal artery (ca), 

 into the tail; it is a small vessel just under the notochord, and gives 

 off small, paired branches at regular intervals toward the vertebral 

 region. The posterior cardinal veins (pc), posterior to the open- 

 ings of the Wolffian ducts into the cloaca, unite to form a large 

 caudal vein lying just ventral to the caudal artery. 



Stack XY1II 



Figure 21 (Plate XXII) 



This embryo, as may be seen, for example, by the form of the 

 appendages, is slightly further developed 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 ex- 

 tends 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 XXII) 



Figure 22 is a photograph of a somewhat older embryo, removed 

 from the egg and freed of the fcetal membranes. The appendages 

 show the position of both elbow and knee joints, and in the paddle- 

 shaped manus and pes the digits may be faintly seen. The tail is 

 very long and is spirally coiled, the outer spiral being in contact 

 with the frontal region of the head. The jaws are completely 

 formed, the upper projecting far beyond the lower. The elliptical 

 outline of the eyes is noticeable, but the lids are still too little devel- 

 oped to be seen in this figure. The surface of the embryo is still 

 smooth and white. 



