464. ’ ADAM SEDGWICK. 
to the commencement of the secondary glomeruli. With this 
division of the glomerulus segmentally, and of each segment of 
it into further secondary glomeruli, each lying in a specialised 
part of the body cavity, the openings of the segmental duct 
began to fold and divide, incompletely at first, into special open- 
ings, one for each secondary glomerulus. Finally, this division 
was completed, and the segmental duct communicated by a 
number of openings in each segment with specialised parts of 
the body cavity containing a portion of the original aortic ridge. 
The specialised parts containing these glomeruli being still open 
to the body cavity, and the glomeruli being still all distinctly 
attached by a common stalk to the walls of the body cavity, and 
the intermediate parts of the original continuous ridge having 
completely vanished, now the capsules enclosing the glomeruli 
became more and more completely marked off from the body 
cavity. The openings putting them in communication with the 
segmental duct elongated into tubules which became coiled, and 
the glomeruli themselves gained a greater independence of each 
other by a development of intermediate tissue. 
A trace of the original state of things has descended to the 
present time in the pronephros, with its continuous glomerulus 
opposite the opening of the segmental duct, and placed in a 
specialised part of the body cavity. Differences in structure 
from the supposed primitive state of things have of course arisen, 
in consequence of the specialisation of the pronephros as the 
larval excretory organ. 
In the same way a trace of the division of the primary 
glomeruli into primary, secondary, &c., glomeruli, is left 
in the curious development of the external glomeruli of the 
anterior part of the Avian mesonephros. Only in this case no 
cause can apparently be given for the retention of this primitive 
feature of development. 
An examination of an early stage in the development of the 
Avian Wolffian tubules, when the primary and secondary 
tubules are both fairly well established, but not very compli- 
cated in structure, points very distinctly to the fact that the 
glomeruli of the two tubules are parts of one primitive glome- 
rulus. ‘They appear to be continuous, and while one looks 
ventrally, .e. the so-called primary glomerulus, the other looks 
dorsally. A glance at the accompanying woodcut will make 
this clear. 
If this drawing of a section through the Wolffian body of a 
chick in a part with primary and secondary tubules, be compared 
with fig. 24, which is from the anterior part of the same chick 
where there are no secondary tubules, it will be seen that the 
