THE ABDOMEN • 829 



This constitutes what is known as the fibrous or interstitial stiooia, 

 of wliich there is a very small amount. 



Early Condition of the Kidneys. — The kidneys of a child at the period of 

 birth are lobulated, each lobule representing a Malpighian pyramid sur- 

 rounded by cortical substance. In this respect they resemble the permanently 

 lobulated kidneys of certain animals, e.g., the ox. The lobules first become 

 apparent towards the end of the third month of intra- uterine life, and aU 

 traces of them have usually disappeared by the eighth or teaith year. They 

 may, however, remain persistent to a greater or less extent throughout life, 

 though this condition is of very rare occurrence. It is, however, not un- 

 common to find the surface of an adult kidney marked by faint grooves 

 indicative of its original lobulated condition. The disappearance of the 

 lobulated condition is brought about by the adjacent lobules coalescing, this 

 being accompanied by an increased development of cortical substance at the 

 surface. The portions of cortical substance which remain in the spaces 

 between the medullary pyramids (except their apical parts) represent the 

 septula renum or columnae Bertini. 



Development of the Kidney and Ureter. 



The kidneys are of mesodermic origin, and are developed in connection 

 with the intermediate cell-mass, which appears along the groove separating 

 tHe mesodermic somites of the paraxial mesoderm from the latereil plate of 

 mesoderm. The intermediate cell-mass, Uke the paraxial mesoderm, under- 

 goes segmentation, its segments being known as nephrotomes, each of which 

 is a sohd cord of cells. 



In the course of the development of the urinary system in vertebrates, 

 three sets of organs have to be considered — namely: (i) the pronephros, 

 (2) the mesonephros. and (3) the metanephros. 



The pronephros, fore-kldiiey, or head-kidney, which is functional in lower 

 vertebrates, originates on either side as a thickening of the lateral part of 

 the corresponding intermediate cell-mass, called the Wolffian ridge, which 

 projects into the coelom or body-cavity. The Wolffian ridge consists of soUd 

 cords of cells, which soon become tubular and give rise to a longitudinal 

 canal, called the pronephrie or s^mental daet. This duct extends from the 

 region of the heart anteriorly to tibe cloaca posteriorly, into the ventral com- 

 partment, of which latter it opens. Associated with the cephahc end of 

 this duct there are two epithelial tubules, known as the pronephrie tnboles, 

 which he at right angles to it. By one end these tubules open into the 

 pronephrie duct in those vertebrates in which the pronephros is functional, 

 but in the human embryo they do not communicate with the pronephrie duct. 

 By their other extremities they open into the ccelom or body-cavity (peri- 

 toneal ca\ity), the openings being known as nephrostotnata. In the neigh- 

 bourhood of the nephrostomata, but independent of them, there are tufts 

 of capillar^' bloodvessels, covered by epithehum. These are called the 

 pronephrie glomeruli. 



In those vertebrates m which the pronephros is well developed — e.g., cer- 

 tain Fishes, and Amphibia during the larval stage, the urinary secretion 

 makes its way from the pronephrie glomeruU into the coelomic cavity, from 

 which it passes' through the nephrostomata into the pronephrie tubules, and 

 thence into the pronephrie duct. The latter duct conveys it to the cloaca. 



In most vertebrates the pronephros is a rudimentary structure, and its 

 tubules soon atrophy. The pronephrie duct, however, persists as the Wolffian 

 duct, which belongs to the mesonephros. 



The mesonephros, mid-kidney, or Wolffian body consists of (i) a series of 

 transverse tubules, called the mesonephric or Wolffian tubules, and (2) the 

 Wolffian duct. 



The Wolffian tubules are developed from the intermediate ceU-mass, which, 

 as stated, is broken up into segments, called nephrotomes. Each solid 



