WOLIFFIAN DUCT AND BODY IN THE CHICK. 461 
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, 
1 Loc. cit. 
