iv EENAL OKGANS 261 



sations occur which give rise to definitive nephrotomes and these 

 also are no longer strictly segmental, there being about 2 to each 

 segment from segment 25 to 30. 



The definitive nephrotomes pursue the normal course of develop- 

 ment. The first to appear are towards the ventral edge of the 

 nephrotomal tissue but later other subsequent units appear in 

 succession more dorsally. 



General Morphology of the Eenal Organs of Verte- 

 brates. — The main problem connected with the morphology of the 

 renal organs is that which deals with the serial homology of its con- 

 stituent elements. Lankester (1877) clearly implied this homology 

 when defining his terms archinephros etc. while, looking at the 

 matter from a more strictly embryological standpoint, Sedgwick 

 (1881) formulated the view that pronephros, mesonephros and meta- 

 nephros are simply successive portions of a single elongated ancestral 

 excretory organ possessing a duct and segmentally arranged, serially 

 homologous, tubules. 



In discussing this archinephros theory it is necessary to bear in 

 mind the following points : — 



(1) The names pronephros, mesonephros and metanephros accord- 

 ing to their original definitions signify three sets of renal structures 

 forming a succession along the length of the body in a tailward 

 direction : — (a) an anterior or headward set, (b) a middle set and (c) 

 a posterior or tailward set respectively. It is inadmissible by the 

 terms of the original definition to use them in any other sense and 

 to do so is bound to lead to confusion. 



(2) In addition to the anteroposterior series of renal units there 

 may develop a sequence of elements within the same body segment — 

 i.e. the development of the primary unit may be followed by the 

 production of a series of subsequent units, secondary, tertiary, 

 quaternary and so on, probably derived originally from the primary 

 nephrotome by a process of budding. The extent to which such 

 subsequent units may develop differs greatly in different animals and 

 in different segments. In the pronephric region there are commonly 

 none, in the opisthonephros of Hypogeophis there may be as' many as 

 twenty in a segment, while it is possible that the metanephros of the 

 Bird is to be looked on as a gigantic mass of subsequent tubules 

 belonging to a single segment. 



It is obvious that in comparing renal elements of different parts 

 of the series care must be taken that the comparisons are made 

 between elements of the same order, and it is further obvious that a 

 danger to be guarded against is involved in the theoretical possibility 

 of the suppression of the tubules of one- order — say the primary 

 tubules — in some particular region. 



The comparison of mesonephros with pronephros involves then 

 these two fundamental questions: — 



(1) Does the mesonephros contain a set of units of the same order 

 as those of the pronephros ? — i.e. in this case primary elements and 



