444 



COMPARATIVE ANATOMY 



(e.g. Elasrnobranchii and Amniota) it is more or less shortened 

 and consists of only a small number of canals : this is doubtless 

 due to a secondary reduction, which also affects the glomeruli. 

 The anterior part of the pronephric duct is formed by the 



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Hill's 



fusion of the distal ends of the pronephric tubules : the middle 

 and posterior parts of the duct arise in various ways in different 

 Vertebrates, and are developed from before backwards. 



It is very probable that the pronephros at one time extended 



