182 



UROG-ENITAL SYSTEM 



and an outer part having more of an epithelioid character (nephrogenetic cord, Schreiner). This 

 outer part forms a cord which becomes separated from the coelomic epithelium, and secondarily 

 segmented into a series of solid rounded bodies. These soon develop a lumen and form a 

 series of small vesicles (fig. 224). There are several such in each segment in the rabbit, but it 

 has been asserted (Kollmann) that the organisation is segmental in the human embryo in the 

 early stages. In some cases it can be determined that the vesicle is connected with the coelomic 

 epithelium by a slender string of cells (fig. 224).' From the outer and upper part of the wall a 

 solid sprout, which is soon hollowed out to form a diverticulum of the vesicle grows against, and 

 ultimately opens into, the Wolffian duct. This is the rudiment of the tubule proper, the 

 original vesicle forming the Malpighian capsule. The tubule elongates and becomes S-shaped 



(fig. 225). Into the space between 

 its proximal loop and the upper 

 originally mesial wall of the now 

 flattened, spoon - shaped vesicle, a 

 branch grows in from the adjoining 

 aorta. From this vessel a knot of 

 capillaries is formed which lies in the 

 hollow of the upper wall of the vesicle 

 and becomes the glomerulus. 



The connective- tissue framework 

 of the Wolffian body is derived from 

 the mesenchyme of the intermediate 

 cell mass. 



outer zone 



pelvis- 



Wolffian duct 



bladder, 



TCetanephros : perma- 

 nent kidney, The meta- 

 nephros arises from the posterior 

 part of the same nephrogenetic 

 cord as forms the blastema of 

 the mesonephros. l The tissue, 

 however, remains passive during 

 the time when the tubules are 

 forming in the Wolffian body, 

 and becomes related to a diver- 

 ticulum which grows out from 

 the dorso-mesial aspect of the 

 Wolffian duct immediately in 

 front of its opening into the 

 cloaca (fig. 228). As the diver- 

 ticulum increases in length in a 

 dorsal direction, the nephro- 

 genetic tissue becomes displaced 

 and separated from the Wolffian 

 duct until it comes to lie dorsal 

 and mesial to it. The diver- 

 ticulum becomes expanded at its 

 distal end to form the primitive 

 pelvis of the kidney, while the stalk represents the future ureter. As the ureter 

 elongates the kidney gradually changes its position relatively to the Wolffian body, 

 until eventually it lies above and dorsal to it. The primitive pelvis is surrounded 

 by the nephrogenetic tissue, which now shows a differentiation into an inner 

 epithelioid and an outer mesenchymatous zone (fig. 229). The latter is con- 

 tinuous with the blastema of the mesonephros, but the former is quite separate 



FIG. 229. SECTION OF THE PRIMITIVE PELVIS OF THE 



KIDNEY OF A HUMAN EMBKYO OF FIVE WEEKS OLD. 



(After Schreiner.) 



The inner zone of nephrogenetic tissue is sharply marked 

 off ; the outer zone passes gradually into the surrounding 

 mesenchyme. 



1 This fact, first demonstrated for the chick by Sedgwick, lias been substantiated for all the 

 Amniota, including man, more especially by Schreiner, Zeitschr. f. wissen. Zool. Ixxi. 1902. In 

 recent 

 though 

 which the Wolffian body is the anterior end. 



-**"& "' .tMiij RJJ kj IC.LJ ^jeiLouiii . i. wiootrii, 



paper. Janosik (Arch. f. Anat. April 1907) has again thrown doubt on the ontogenetic fact, 

 he admits the general proposition that the kidney is merely a part of a primitive organ of 





