WITH NOTES ON THE WEST INDIAN SPECIES. 313 



Thus, if the above comparisons are correct, Amphioxus possesses at some period of 

 its life vestiges of the three pairs of regional or archimeric excretory canals, whose 

 function has been superseded (by substitution) by the nephric tubules. 



I have suggested above (p. 281) that the primordia of the nephric tubules may 

 actually be recognised at the dorsal medial angles of the gill -pouches of Enteropneusta 

 (cf. PI. XXXII. Fig. 55 dgp). 



In any case it is quite certain that the topographical coincidence of the nephric 

 tubules with the gill-clefts as described by Weiss and Boveri in Amphioxus is not an 

 accidental association. It evidently means what it appears to mean, namely, that the 

 nephric tubules and the gill-clefts were primarily coextensive 1 . 



The nephric tubules of Amphioxus have superseded the regional pores as the 

 essential organ of excretion but in a very different way from that in which the latter 

 are superseded in the Enteropneusta by the glomerulus, which is an organ sui generis. 

 The nephric tubules belong to the same cycle of changes as the regional pores and 

 originate from the same blastema. They replace the regional pores by true substitu- 

 tion, just as in the Vertebrata the mesonephros replaces the pronephros and the meta- 

 nephros the mesonephros. 



Whether or not the regional pores arose as such or have been differentiated from 

 a more indefinite condition as seen in the multiple madreporites of many Echinoderms 

 is not an easy question to decide. The analogy with other cases would lead us to 

 suppose that the ideal condition indicated in the adjoining table is not the primordial 

 condition. We may at any rate formulate provisionally the following sequence of 

 phyktic changes relating to the evolution of the Vertebrate kidney. 



i. ii. in. 



Nephric ti 

 tensive wit 

 [Cephalochorda] 



Water-pores Regional pores Nephric tubules 



[Indefinite] [Definite; archimeric] [Coextensive with gill-clefts] 



Hydrotrema 



IV. V. VI. 



Pronephric tubules Mesonephric tubules Metanephric tubules 



[Emancipated from gill-clefts] [Opening into coelom] [Emancipated from coelom (Wiedersheim 2 )] 

 [Craniate embryos [Anamnia] [Amniota] 



and larvae] 



The following table expresses in brief the conclusions to which we have arrived, 

 but it should be taken in conjunction with the text to avoid misunderstanding. 



It is intended to show the origin, by substitution, of the Vertebrate excretory- 

 system from the archimeric system of excretory canals. Of course the table will only 



1 By realising this truth, Paul Mayer's discovery of the six connecting vessels between dorsal aorta and 

 suhintestinal vein in embryos of Pristiurua and Eiickert's discovery of their topographical coincidence with the 

 pronephric tubules, to which they furnish rudimentary glomeruli, will have assigned to them the importance 

 which is their due. 



2 B. Wiedersheim, " Ueber die Entwicklung des Urogenitalapparates bei Crocodilen und Schildkroten," 

 Arch. f. mikr. Anat., Bd. xxxvi., 1890, p. 410. 



43—2 



