DEVELOPMENT AND ANATOMY OF SOME EARTHWORMS. 21 
matrices in P. pellucida does not, however, take place in 
regular succession, beginning at the ventral and ending at the 
dorsal end of the band. It is quite clear that in P. pellu- 
cida new secondary matrices are produced between existing 
ones, either by matricial cells which separate from the existing 
matrices and travel along the intermatricial band, or by the 
division of any matrix into two. There is obviously no essen- 
tial difference between these two methods, nor is it possible to 
draw any definite line of demarcation between them, but the 
latter method prevails in the earlier and the former in the later 
stages of the production of the full number of secondary 
matrices. 
Fig. 17, from a transverse section of an embryo of P. pel- 
lucida, shows a secondary matrix dividing intotwo. Fig. 18, 
from the same series, shows two secondary matrices connected 
by the intermatricial band; one of these is the most ventral 
of the series belonging to one side of a segment, while the 
other one shows the band connecting it with the next matrix 
beyond. 
Figs. 19—25 are views of portions of matricial bands of 
embryos of P. pellucida which had been slit open and 
flattened out; they are arranged, with the exception of fig. 23, 
in the order of their age. Fig. 22 shows the entire matricial 
bands on one side of three consecutive segments. 
In P. pellucida the two secondary matrices first produced 
from a primary matrix come to lie at the two ends of an entire 
band, all the other matrices in the band being subsequently 
produced. I have deduced this law from the order of appear- 
ance of the setee, assuming, as we fairly may, that the matrices 
form these setz on arriving at similar ages. 
When all the secondary matrices have been developed they 
come to lie at nearly equal intervals from one another, and the 
intermatricial band gradually disappears (fig. 25). 
Development of the Embryonic Sete. 
In this, as in all other matters relating to the segmentally 
arranged organs, any segment presents during development 
