256 STUDY OF PIG EMBRYOS. 



thelium 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 ccelom. The Wolffian bodies, W.b, are voluminous organs pro- 

 jecting from below the aorta on either side of the large intestine, Reel, and extend- 

 ing far into the abdominal cavity. At the lower ventral edge of the Wolman body 

 appears the Wolffian duct, W.D, a wide, longitudinal canal into which the Wolffi- 

 an 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 mesenchyma between the blood spaces and 

 the walls of the tubules. On the median side of the Wolman 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 propor- 

 tions and in the details of their structure. It is not difficult to make a reconstruc- 

 tion 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. Each 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 glomeru- 

 lus 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, wrien the latter first become distinct, lie near together. As development 

 con tin" ^ 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 modification of the cardinal vein, and morphologi- 

 cally 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, card, may be conve- 

 niently 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 sub-cardinal 

 vein. It is a vessel of great morphological importance, since on the right side of 

 the embryo it acquires a connection with the liver which renders it possible for the 

 blood of the right sub-cardinal vein to pass through the blood spaces of the liver 

 directly to the heart. This makes a very direct channel, a more direct one than 

 existed previously, when the blood from the sub-cardinal came to join that of the 

 cardinal, passing up to the common cardinal and then back to the heart. The new 



