HISTOLOGY. 



invaginated into its globular distal extremity (Fig. 277). The tubule is 

 said to form the capsule of the glomerulus, consisting of an outer and an 

 inner layer between which is an extension of the lumen of the tubule. 

 The layers are continuous with one another at the stalk of the glomerulus. 

 There the efferent vessel may be found near the afferent artery as in the 

 figure, or, as has been described in the pig, several radiating efferent ves- 

 sels may leave the capsule at different points. Whether these all emerge 

 through one crescentic aperture in the capsule, or whether, by coalescence 

 of its edges between the vessels, they leave through separate openings, has 

 not been determined. The stalk and its tubule may both be on one side of 

 the capsule, and not at its opposite poles as in the figure. From the blood 



circulating through the glomer- 

 ulus, fluid "filters" into the tu- 

 bule, forming the greater part of 

 the urine. 



The tubules, starting from 

 the ventro-medial glomeruli, fol- 

 low a convoluted course to the 

 Wolffian duct. In the pig two 

 tubules have been found to unite 

 before entering the duct, and 

 near the glomeruli they may fork 

 so as to connect with two cap- 

 sules. A blind diverticulum is 

 shown in Fig. 277. The tubules 

 are lined throughout with sim- 

 ple epithelium. It is flat in the 



capsule where, in the pig, it is said to be thinner in the outer layer; 

 the reverse condition has been figured for the human embryo. The 

 remainder of the tubule may be divided into conducting and secretory 

 portions. The latter, found in the middle part of the tubule, has low 

 columnar epithelium with dark basal protoplasm and a clear vacuolated 

 appearance toward the lumen. These cells are supposed to excrete a 

 portion of the urine. The conducting tubules have a cuboidal epithelium 

 without indications of glandular activity. The secreting and conducting 

 portions of the kidney tubules have been more thoroughly studied than 

 those of the Wolffian tubules. 



Veins of the Wolffian body. Early in embryonic life two vessels arise 

 from the vitelline veins close to their entrance into the atrium and grow 

 forward into the head, one on either side. These are the anterior cardinal 

 veins, and from each of them a posterior cardinal vein grows along the 



FIG. 277. RECONSTRUCTION OF A WOLFPIAN TUBULE 

 PROM A HUMAN EMBRYO OP 10.2 MM. (Except 

 the glomerulus, after Kollman.) 



c., Inner layer, and c. a., outer layer of the capsule of 

 the glomerulus; div., diverticulum; gl., glomer- 

 ulus; W. d., Wolffian duct. 



