466 ADAM SEDGWICK. 
A cause of abbreviation is so clear in this case that I need not 
waste time in stating it. 
But the whole details of the development of the secondary, 
&c., dorsal tubules needs reworking, for, with the exception of 
the observation of Mr. Balfour’s for Elasmobranchs, we have no 
real knowledge of their exact method of development. The result 
of such an investigation cannot but be exceedingly interesting 
from a phylogenetic standpoint. 
I cannot help thinking, as before stated, that the development 
of the external glomeruli in the chick may have some interest in 
this relation. 
The modification of the mesonephros of the Amniota is, on 
the above hypothesis, due to the fact that some Avian ancestor 
possessed a larva in which the anterior part of the excretory 
system was early developed, the development of the hinder part 
being deferred, and consequently modified, just as we see to be 
the case now in the Ichthyopsida. 
The stiJl greater modification und retardation of the develop- 
ment of the metanephros or true kidney of the Amniota, and 
the great size which the Wolffian body reaches in the embryo, 
are striking facts which demand consideration in any discussion 
of the Vertebrate excretory system. 
In my paper on the “ Development of the Kidney” I have stated 
my views on the relation of the Amniote kidney to the mesone- 
phros. But one point in that paper is left untouched. 
Why does the kidney appear so late? and also why does the 
Wolffian body become so large and complex—so much larger 
than the small-sized chicks, in which it is fully developed, can 
need ? 
And, further, why should this organ, apparently so well 
adapted to serve as the excretory organ of the adult chick, 
atrophy ? 
It may be said, in answer to the latter question, that only 
those tubules of the mesonephros which open into the cloaca 
independently of the Wolffian duct can function in the adult, as 
those which have not so changed their course would interfere 
with the function which the Wolffian duct later acquires—the 
carriage of semen. 
It seems to me that the only answer which can be given to 
the first of these questions is this: 
The kidney is thrown back in development for the same 
reason that the mesonephros of the Amphibia is, viz. because 
the ancestor of the chick underwent part of its development out 
of the egg, at which stage of development the testis, not being 
developed, did not interfere with the excretory functions of the 
Wolffian tubules, or vice versd. The large size of the mesone- 
