290 The Alligator and Its Allies 
actual nephrostomes are to be seen, though the 
occasional fusion of a tubule with the peritoneal 
epithelium, as is seen on the left side of the present 
figure, may represent such an opening. 
Figure 13e is about one hundred and forty sec- 
tions posterior to the section just described. The 
embryo is here very slender, so that the contrast 
between this and the first figure (13a) of this stage 
is remarkable. Except in size, this section does 
not differ greatly from the preceding. The spinal 
cord, notochord, etc., are smaller than before, 
but are of about the same relative size. The 
mesentery (ms) in the section drawn was torn 
across, so that the intestine is not represented. 
Medial to the Wolffian duct is a tubule (wi), 
which seems to be the same as those which were 
called Wolffian tubules in the preceding stage, 
but which may be the beginning of the ureter. 
Figure 13f, about two hundred and fifty sections 
posterior to the last, passes through the extreme 
posterior end of the embryo. The section is 
nearly circular in outline and is somewhat larger 
than the preceding. The amnion (a) completely 
encircles the embryo. The ectoderm (ec) is of 
fairly even thickness, and the mesoblast which it 
encloses is of the usual character. The spinal 
cord (sc) is nearly circular in outline, as is its central 
canal. The digestive tract (ent) is larger in section 
than it was in more anterior regions; it is nearly 
circular in cross-section and its walls are made up of 
