MAMMALIA 



805 



Splanchnocoel 

 NMochord Nephrot. 



Neurgro e \ Myocoel \ Ao. 



Urinogenital Organs. The pronephros of mammals is a 

 rudimentary formation of the nephrotomes of about the fourth 

 to the eleventh mesodermal somites of the body. The pro- 

 nephric tubules fuse to form a solid mass which grows back- 

 wards as the pronephric duct, and this fuses in turn with the 

 cloaca. The duct becomes hollow, but the pronephros 

 gradually disappears. 



The mesonephros is formed in the solid and fused products 

 of the nephrotomes of succeeding segments. From this 

 nephrogenous tissue the seg- 

 mental tubules arise on the 

 medial side of the pronephric 

 duct. At first solid, the 

 tubules become hollow and 

 open at one end into the 

 pronephric duct, and each at 

 the other end expands into 

 a Bowman's capsule which 

 becomes indented by a 

 glomerulus of a branch from 

 the aorta. The tubules 

 undergo growth and convolu- 

 tion, and secondary and 

 tertiary tubules arise from 

 them as outgrowths. The 

 mesonephros thus forms a 



conspicuous organ in development, but, following the degener- 

 ation of the pronephros, a number of the anterior tubules of 

 the mesonephros disappear even before the posterior ones have 

 been formed. The duct is the pronephric duct, but it may 

 now be termed the mesonephric or Wolffian duct, and it 

 opens, as has been stated, into the cloaca. 



The metanephros, the definite kidney of the mammal, 

 arises as a tubular outgrowth of the mesonephric duct. 

 The tube springs from the lower end of the duct on its 

 dorsal side and grows forward. Distally, it dilates and 

 is produced into a series of kidney tubules which come 

 into relationship with the mesoderm, and a compact organ 

 if formed. The blind ends of the tubules form Bowman's 



Notochord 



Myocoel 



Ao. 

 Proneph. I 



Splanchnocoel 



FIG. 142. Transverse sections to 

 illustrate the development of the 

 pronephros. 



