THE MESOMERE (INTERMEDIATE CELL MASS) 



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The Pronephros or Head Kidney. 



The somatic wall of the nephrotomal region at the level of the 

 second, third, and fourth somites thickens and projects laterally be- 

 tween the hypomere and the body ectoderm. This longitudinal bulge 

 is known as the pronephric shelf, because it is due to the development 

 of the pronephros or head kidney and can be seen readily from the 

 exterior. This is noticed first from the external view at the tail bud 

 stage (2.5 mm. body length) as the pronephric ridge or bulge just 

 posterior to the gill plate. Within the lateral extensions of these 

 nephrotomal masses there appear cavities, the nephrocoels, which 

 run together longitudinally to form a common tube. This tube and 

 its lumen grow posteriorly along the lateral border of the nephro- 

 tomes, to join the cloaca and be known as the paired pronephric or 

 segmental ducts. Actually the duct is not in any way segmented, but 

 it will receive segmental kidney tubules. Posterior to the fourth somite 

 this duct develops independently of the nephrotomal tissue and be- 

 comes joined to the cloaca at the 4.5 mm. stage. 



Three pairs of pronephric tubules, one at the level of each of the 

 somites II, III, IV, appear between the segmental duct and the 

 coelom. The original connection of the nephrotomes with the somites 

 is lost in favor of a nephrogenic cord. Finally each tubule acquires an 

 opening, the nephrostonie, into the adjacent coelom. This opening 

 shortly acquires cilia and becomes funnel-shaped, by the time it 

 reaches the 5 mm. body length stage. The pronephric tubules lengthen 

 and become coiled (convoluted) and at the 6 mm. stage are embedded 

 in the mesenchyme and sinuses of the developing posterior cardinal 



Internal glomerufus 

 Pnmory kidney tubule 

 Pronephrrc duCt 



Ciliated 

 nephrostome 



External 

 glomerulus 



Development of the pronephric tubule. (Schematized after Felix.) 



