EMBRYO OF p MM. 225 



mesothelium represented in the engraving as a continuous line. With a higher 

 power the mesothelium is seen to consist of a single layer of cells, but varying 

 somewhat in thickness in different regions. By following the contour of the 

 mesothelium the student will recognize at once that all of the viscera are, in the 

 anatomical sense, outside of the crelom. The Wolffian bodies, W. b, are vol- 

 uminous organs projecting from below the aorta on either side of the large intes- 

 tine, Red, and extending far into the abdominal cavity. At the lower ventral 

 edge of the Wolffian body appears the Wolffian duct, W. D, a wide, longitu- 

 dinal canal into which the Wolffian tubules open. The large size of the duct is 

 characteristic of this stage. In later stages it is smaller. The tubules are very 

 large, contorted in their course, and appear, therefore, variously cut. They are 

 formed by a cuboidal epithelium and are provided with a sinusoidal circulation. 

 The endothelium of the blood spaces can generally be seen fitting closely against 

 the epithelium of the tubules. Here and there, however, there is some mesen- 

 chyma between the blood spaces and the walls of the tubules. On the median 

 side of the Wolffian body are the glomeruli, which are of large size, and similar 

 in structure to the glomerulus of the permanent kidney, though differing from the 

 renal glomeruli in their proportions and in the details of their structure. It is not 

 difficult to make a reconstruction of the course of a single tubule by following it 

 through a few neighboring sections. The general course of a tubule is in the 

 transverse plane, but it is much contorted. Bach tubule begins at one of the 

 glomeruli, with which it is in open communication. It then bends so as to make 

 a somewhat irregular S-shaped figure, and finally opens into the Wolffian duct. 

 After leaving the glomerulus it widens somewhat, but before it joins the Wolffian 

 duct it again diminishes in diameter. The changes in diameter are gradual. 

 The blood spaces or sinusoids of the Wolffian body are derived from the posterior 

 cardinal veins. The veins and tubules, when the latter first become distinct, lie 

 near together. As development continues both enlarge and encroach upon one 

 another's territory; hence there is an intimate intercrescence of the blood-vessels 

 and of the tubules, resulting in the formation of sinusoids. The whole of the 

 Wolffian body might from one point of view, therefore, be regarded as a modifi- 

 cation of the cardinal vein, and morphologically all of the blood spaces between 

 the tubules belong to that vein. There remain typically two portions of the 

 cardinal vein which are more or less open and distinct. The one on the dorsal 

 side of the Wolffian body, car d, may be conveniently regarded as representing 

 the original cardinal vein. The other, on the ventral side of the Wolffian body, is 

 at first not a very distinct channel, but gradually becomes more and more so, and 

 is known by the distinctive name of subcardinal vein. It is a vessel of great mor- 

 phological importance, since on the right side of the embryo it acquires a connec- 

 15 



