158 
ADAM SEDGWICK. 
behind this point. But it does not all undergo the above 
development into Wolffian tubules. It breaks up into 
Wolffian tubules as far back as the thirtieth segment. Behind 
this pointy ^. e. from the thirty-first to the thirty-fourth 
segments inclusively, the Wolffian blastema undergoes quite 
a different fate. It remains for some time almost quite pas- 
sive and ultimately gives rise to the epithelium of the per- 
manent kidneys. In consequence of this I have called that 
part of the Wolffian blastema between the thirty-first and 
thirty- fourth segments the kidney blastema ; and in future 
shall refer to it by that name (figs. 12, 15, 16, 17, hh). It 
is important to notice that this kidney blastema develops 
in an exactly similar manner to the Wolffian blastema. 
It is not until well into the fourth day, when the ureter 
has appeared, that it is possible to draw the line between 
the two. 
Fig. 1^ is taken from the thirty-second segment of a chick 
with thirty-four protovertebrae ; it shows a blastema of cells 
lying just internal to the Wolffian duct. Fig. 10 is taken 
from the tw’enty-ninth segment of the same chick. It shows 
the hindermost trace of a Wolffian tubule 1 could find at 
this stage. In all the sections between figs. 10 and 12 there 
is present, just as in fig. 12, a blastema of cells lying just 
internal to the Wolffian duct. 
In a slightly older embryo the hindermost trace of a 
Wolffian tubule would be in the thirtieth segment. In still 
older embryos secondary tubules would have appeared in the 
thirtieth segment, but no trace of a primary tubule in the 
thirty-first, and so on in later stages, Wolffian tubules never 
appearing in the thirty-first segment. In the embryo, from 
which figs. 13 to 17 were taken, the ureter had not appeared. 
In examining a series of sections from the posterior part of 
this embryo, some of which are figured (figs. 13 to 17), the 
following points are noticeable, illustrating what has just 
been stated. 
A primary and secondary tubule are present in fig. 14, and 
it is almost the last section in which any trace of a Wolffian 
tubule can be seen (the two above tubules are cut in the 
next two sections). The tubules adjacent anteriorly to these 
are three in number (fig. 13), consisting of primary, secon- 
dary, and tertiary. Supposing the Wolffian body were going 
on developing in the region behind that from which fig. 14 
was taken, we ought at the least to find at this stage primary 
tubules in that region, for the formation of primary and 
secondary tubules is always separated by an interval of time. 
But no such primary tubules are seen. Behind fig. 14 (figs. 
