26 ALFRED GIBBS BOURNE. 
fig. 33, and show that the mode of development alters in the 
nephridia which are late in appearing. 
I have been unable, owing to the advanced stage of develop- 
ment of all the other structures, to fully trace out the nephri- 
dium of fig. 39; but there is sufficient to show that the neck 
has undergone great elongation, so that the glandular loop lies 
very dorsally. The loop itself has become very complicated, 
and the excretory duct extends away beyond the portion 
drawn. The great importance of the stage is that it shows 
the developments which are taking place in the neck region. 
The cells here are giving rise to the secondary loops (z. 1, x. 
2, n. 3). 
Fig. 88 shows that in a nephridium developing at this late 
stage the growth in the neck region commences at a relatively 
much earlier stage. The neck is here already much elongated, 
while the primary loop is still very undeveloped and the 
excretory duct has not yet acquired its proper lumen. It is 
clear that the primary loop is the earliest to develop of a 
series, and that the whole structure is the homologue of the 
nephridium of Lumbricus, and, as these secondary loops form 
or give rise to all the scattered nephridia on the one side of 
each segment, these are taken as a group, but not individually, 
homologous with the Lumbricus nephridium. 
Fig. 40 is taken from an old embryo ; 2 2 is a portion of the 
original neck region, now much attenuated; a to f are secondary 
loops which have arisen from it; of these @, at any rate, has 
acquired an excretory pore. These loops show that the same 
sort of difference obtains between a secondary loop which de- 
velops late and one which develops early as between a ne- 
phridium which develops late and one which develops early. 
The secondary loops give rise to tertiary loops as outgrowths 
from their own neck region. In this way some fifty or more 
loops develop which ultimately become separated from one 
another, while each develops its own excretory duct and becomes 
a micronephridium. 
There is no elongation in the neck region and no develop- 
ment of secondary loops in the nephridia of the most anterior 
