DIFFERENTIATION OF THE NEPHROTOME 199 



female the Wolffian duct shrivels up and disappears. As we shall see later 

 on, these changes in the mesonephros may be modified by the injection of 

 sex hormones. 



The over-all picture of these three successive kidney systems shows a 

 recapitulation of the evolutionary history of the kidney in vertebrates from 

 the lower fishes up through birds and mammals. It is not clear why this same 

 succession of three different types of kidneys should occur in the embryo. 

 Why does the metanephros not develop directly? The pronephros and 

 mesonephros contribute nothing to the final kidney system except for a small 

 part of the Wolffian duct which gives rise to the ureter. The formation of 

 temporary structures is very puzzling, and until further analysis of develop- 

 ment is carried out we can give no good reasons. The factors for the break- 

 down of the mesonephros reside within this structure itself, for if it is 

 transplanted to a foreign environment it still undergoes disintegration. Thus, 

 the mesonephros is not only determined to develop within the embryo but in 

 addition it has a factor within itself which determines its own breakdown. 

 Some mutant genes cause the structure to break down. In one strain of mice, 

 for example, the tail becomes much smaller after forming in the embryo. 



Since the adrenal gland develops in close connection with the kidney, we 

 shall consider its formation at this time. Like the adult pituitary, the adult 

 adrenal is composed of two different kinds of tissue. The cortex of the 

 adrenal originates from the intermediate mesoderm near the metanephros. 

 This cortical portion of the gland secretes hormones which regulate the 

 function of the kidney and so control the composition of the blood. In man, 

 injury of the cortex is associated with Addison's disease. The symptoms are 

 muscular weakness, low blood pressure, and a pigmentation of the skin. On 

 the other hand, the medulla of the adrenal gland arises from neural crest 

 cells which migrate internally to form the sympathetic ganglia. Some of these 

 cells join with the mesodermal component to form the complete adrenal 

 gland. The medulla secretes adrenaline, which has an action similar to that 

 of the sympathetic nerves in that both cause constriction of abdominal blood 

 vessels. This is an extremely interesting situation in which two groups of 

 cells — the sympathetic nerve cells and the cells in the medulla of the adrenal 

 — have a common embryonic origin and, although one group differentiates 

 into nerve cells while the other group forms a gland, both types of cells 

 perform a similar function in the adult. 



