IV 



PRONEPHROS 



229 



duct rudiment. As development goes on however the tubules 

 belonging to nephrotomies I, III, and IV, those labelled A, 

 C and D in the figure, become reduced in size and finally 



£"■ 



h 



Fig. 126. — Dorsal view of pronephros of Polypterus at stages 20, 23, 24 + , 25 

 and about 28. 



a.n.d, archinephric duct. The tubule rudiments are indicated by letters, the nephrotomes by 



Roman numerals. 



disappear, while B and E on the other hand increase in length 

 and become the functional tubules. The anterior end of the 

 archinephric duct becomes gradually modelled out of the solid 

 rudiment already referred to in the way indicated in the figure. 



:~~an.di 



Fig. 127. — Part of a longitudinal vertical section through the series of nephrocoeles 

 in the pronephros of Polypterus — stage 24 + . 



a.n.d, archinephric duct ; ect, ectoderm ; end, endoderm ; nc.B, nephrocoele " B " ; 

 nc.F, nephrocoele "F." 



After it has assumed its definitive tubular form this front part of 

 the archinephric duct commences to grow actively in length, it 

 becomes thrown into complicated coils and forms a large fraction 

 of the entire bulk of the pronephros in its early functional stages. 



