REGION OF PRONEPHROS 



r REGION OF OPISTHONEPMROS — 1 



REGION OF MESONEPHROS REGION OF MET4NEPHR0S 



MESONEPHRIC RENAL UNITS 

 ^7 ARISING BY CONDENSATION OF 

 GROUPS OF CELLS WITHIN 

 15 NEPHROGENIC CORD 



- PRONEPHRIC 



IMESONEPHRIC) 



DUCT 



VESTIGIAL 



PRONEPHRIC 



TUBULE 



SECRETORY 

 PORTION 

 OF TUBULE 



MALPIGHIAN 

 BODY 



COELOM 



NEPHROSTOMAL / SECRETING 



CANAL MESONEPHRIC TUBULE 



DUCT 



Fig. 344. Regions of kidney origin within the vertebrate group; types of renal units 

 formed. (A) The regions in the body where the diflferent types of vertebrate kidneys 

 arise. The pronephric tubules and the pronephric duct are shown in black to emphasize 

 the fact that this part of the developing renal system is a fundamental and necessary 

 primordium without which later kidney development is distorted. (B) Differentiation 

 of the anterior portion of the nephrotomic plate and the common method of origin of 

 the pronephric duct. In the anterior region (toward the left in the figure) the nephrotomic 

 plate segments into individual nephrotomes from each of which a renal tubule arises 

 (see tubules 1 to 5). Tubules 6-9 is a vestigial area of tubule development. The anterior 

 mesonephric region indicated by tubules 10 to 15, etc. In the anterior mesonephric area, 

 e.g., tubules 10 and 11, the individual tubules show a tendency to arise segmentally, 

 but in more posterior mesonephric regions, e.g., tubules 12 to 15, etc., the tubules arise 

 through condensation of cellular masses within the nephrogenic cord. Hence, primitive 



(Continued on facing page.) 



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