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 {i. 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. In the 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 



