196 EMBRYOLOGY 



which the vertebrae form. In a similar manner the main contribution of the 

 pronephros to the embryo is the establishment of the mesonephric, or 

 Wolffian, duct. 



The Wolffian duct receives the mesonephric tubules, which end in the 

 renal capsules (Figs. 117 and 118). As Figure 117 shows, the nephrotome 

 first forms mesonephric vesicles. These vesicles then elongate and take on an 

 S shape. One end of the S indents to form the renal capsule, while the 

 opposite end connects with the mesonephric duct. This looks like a clear case 

 of an attraction between the duct and the tubules. The cup-shaped renal 

 capsule is soon filled with blood capillaries which form a glomerulus. The 

 glomerulus connects with the dorsal aorta. The efferent blood vessels coming 

 out of the glomerulus spread over the surface of the tubules (Fig. 118) and 

 unite with the veins draining the mesonephros. The renal portal circulation 

 is established by connections between the posterior cardinal and the sub- 

 cardinal vein, which forms a part of the inferior vena cava. The details of 

 these circulatory changes have already been considered. 



The general mechanism of excretion, on the basis of information from 

 other animals, is a filtration of substances through the renal capsule, followed 

 by a partial recovery of substances by absorption into the walls of the tubule. 

 The remaining excretory products, such as urea, then pass from the tubule 

 into the Wolffian duct, down to the cloaca, and finally into the cavity of the 

 allantois. The mesonephros functions throughout most of the period of 

 incubation, but it is gradually replaced by the developing metanephros, which 

 differentiates from the nephrotome of more posterior regions, opposite 

 somites 30 to 33 (Fig. 117) . A new excretory duct, the ureter, develops from 

 an outgrowth of the Wolffian duct just where it enters the cloaca. This 

 ureter grows forward and sends out short outgrowths in the direction of the 

 nephrotome. The nephrotome at the same time forms metanephric vesicles 

 which become converted into renal capsules and tubules. These structures 

 form by the same process as dp the mesonephric renal capsules and tubules. 

 Connections of the tubules with the outgrowths from the ureter complete the 

 pathway for the excretions. 



Although the metanephros is similar in structure to the mesonephros, its 

 circulation is quite different. The blood after passing through the glomeruli 

 is picked up by the renal veins which flow into the inferior vena cava. There 

 is no renal portal circulation through the metanephros. The functioning of 



