The Development of the Alligator 289 
vessel. The mesoblast on each side of the body 
is here differentiated into a distinct muscle plate 
(mp). These muscle plates have very much the 
appearance of the thickened ectoderm seen in the 
younger stages of development. At about its 
middle region (7. e., at the end of the reference line 
ec) each muscle plate is separated from the over- 
lying ectoderm by an empty space; this space is 
still more marked in some other series. Ventral 
to the aorta, and supported by a well marked 
though still thick mesentery (ms), is the intestine. 
It is a small, nearly cylindrical tube with thick 
walls; the splanchnic mesoblast which surrounds 
it is more dense than the general mass of mesoblast; 
it was somewhat torn in the section and is so 
represented in the figure. The urinary organs 
have made considerable progress since the last 
stage. Inthe figure under discussion they are seen 
as a group of tubules on either side of the aorta. 
The tubule most distant from the middle line, on 
each side, is the Wolffian duct (wd). It extends 
through the posterior two thirds of the embryo and 
varies in diameter at different points; it is usually 
lined with a single layer of cubical cells which con- 
tain 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 
19 
