390 WILSON. [Vor. III. 
middle stratum which lies between (a) and (4) and agrees in 
general histological character with the ectoblast (from which 
it is indirectly derived). From the middle stratum arise (a) the 
nervous system, (4) the outer part of the nephridia, (¢) the 
setigerous glands and the sete. 
(4) The middle stratum is arranged in a series of distinct 
longitudinal cell-rows which, in early stages, lie at the surface 
and form part of the general ectoblast, but afterwards sink 
below the surface and are completely covered by the remaining 
ectoblast. Of these rows there are either three or four in each 
germ-band. The inner (or ventral) row gives rise to the cor- 
responding half of the nervous system, and will therefore be 
called the xeural row, or cord. The two succeeding rows on 
each side give rise to a portion of the nephridia and to the 
inner series of setigerous glands, and will be called the xephric 
vows. The fourth or outer row, which I have found as a dis- 
tinct structure in one species only, gives rise to cells which 
form the outer or dorsal part of the middle stratum. Their 
precise fate is still in some doubt, but it is probable that they 
give rise to the outer series of setigerous glands, though perhaps 
to other structures as well. 
(5) Until a comparatively late stage each row of the middle 
stratum terminates behind in a large cell (teloblast), which is 
the parent of the entire row, and thus of all the structures to 
which the row gives rise. The inner teloblast will therefore 
be called the zeuvoblast, those at the ends of the nephric rows 
the zephroblasts; the fourth, owing to the uncertainty of its 
fate, will be called the /ateral or outer teloblast. These four 
teloblasts may be called the anterior teloblasts to distinguish them 
from the posterior or mesoblastic teloblasts which lie at the 
extreme hinder ends of the germ-bands. 
(6) The teloblasts of the middle stratum are at first ordinary 
ectoblast cells which cannot be distinguished from the adjoining 
cells. They first appear shortly after the completion of gas- 
trulation, and at this time lie at the surface, like the rows (now 
very short) to which they give rise. At a later period they 
sink below the surface and ultimately break up into groups of 
cells lying at the hinder ends of the corresponding rows. 
(7) The cephalic lobe (prostomium) arises by the union of 
the anterior ends of the germ-bands. The mesoblastic part 
