DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 2>7 



different points ; it is usually lined with a single layer of cubical 

 cells which contain large nuclei. The Wolffian bodies (wt) are a 

 mass of slightly convoluted tubules that may be traced throughout 

 the greater part of the region through which the Wolffian duct 

 extends. These tubules also vary somewhat in diameter, but they 

 are usually of greater caliber than the duct. No actual nephros- 

 tomes 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. A detailed description of 

 these structures may be given in a subsequent paper. 



Figure 13c is about one hundred and forty sections 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 mesen- 

 tery (ms) in the section drawn was torn across, so that the intestine 

 is not represented. Medial to the Wolffian duct is a tubule (ut ), 

 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 13/, 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 (cc) 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 

 several layers of cells, so that it resembles to a considerable degree 

 the spinal cord of the same region. In the narrow space between 

 the spinal cord and the hindgut is seen the notochord (nt), some- 

 what flattened and relatively and actually smaller than in the pre- 

 ceding figure. A few scattered blood-vessels may be seen in the 

 mesoblast at various places. 



A sagittal section of an embryo of this stage, drawn under the 

 same magnification as were the transverse sections, is shown in 

 figure it>S- The embryo being bent laterally could not be cut by 

 any one plane throughout its entire length, so that only the ante- 

 rior end is represented in the figure. The amnion (a) may be 

 clearly seen except at certain places where it is closely adherent to 



