42 ADAM SEDGTVICK, 



the manner of the segmental duct and head-kidney of the 

 Ichthyopsida. Are the cell cords connecting the duct and peri- 

 toneal epithelium in these segments rudimentary Wolffian 

 tubules, or are they rudiments of a head-kidney ? In the 

 absence of a continuous glomerulus opposite them they differ 

 from the openings of the pronephros. In their development 

 they resemble the latter. If they are Wolffian tubules they 

 develop quite differently from all other Wolffian tubules. If 

 they are rudimentary pronephric funnels, then the chick pos- 

 sesses a rudiment of a pronephros which resembles exactly 

 the hinder developing Wolffian tubules. 



It seems to me that these structures, under the light of the 

 above hypothesis, present no difficulty, and I cannot help thinking 

 that the discovery of their method of development is striking 

 evidence in its favour. They belong, on that hypothesis, to the 

 anterior part of the excretory organ, which has retained the 

 primitive method of development originally characterising the 

 whole organ. They, in some Avian ancestor, have constituted 

 the first developed part of the excretory system, which has been 

 utilised by the larva as its excretory organ. Supposing that 

 Avian ancestor existed now, we should find that its larva possessed 

 an organ which we should call pronephros, having a structure 

 less modified probably from the hinder part of the excretory 

 system than in the case of the Ichthyopsida, i.e. an organ the 

 serial homology of which, with the mesonephros, would no more 

 be disputed than is that of the metanephros with the meso- 

 nephros. 



It may be objected to this view of the anterior part of the 

 Avian excretory system, that it differs in certain marked features 

 from the pronephros of other forms. Of these differences the 

 most important is, perhaps, the fact that there is always found 

 an interval unoccupied by segmental tubes between it and the 

 mesonephros. But in Amphibia Salamandra Fiirbringer^ dis- 

 tinctly states that rudiments, as masses of cells, occupying the 

 same relative position to the segmental duct as do segmental 

 tubes, are found intervening between the two. If these rudi- 

 mentary tubules underwent full development there would be no 

 such gap as that we now find between the pro- and mesonephros 

 of Amphibia. 



But this difficulty is merely part of another difficulty which it 

 seems to me must exist whatever view be taken of the nature of 

 the pronephros, namely, why does this organ, so well developed 

 in the larva and apparently perfectly well performing the func- 

 tions of an excretory organ, atrophy in the adult ? And this 

 difficulty only seems capable of the unsatisfactory explanation, 



* Loc. cit. 



