No. 3.] ZHE EMBRYOLOGY OF THE EARTHWORM. 405 
blastic bands form a complete ring which encircles the archen- 
teron longitudinally with its middle part drawn somewhat 
towards the ventral aspect. Posteriorly the bands are a single 
cell in width ; anteriorly they soon widen into flat plates which 
are widest in the middle region, narrowing somewhat as they pass 
upwards at the sides of the stomodzeum. It should be borne in 
mind that the cephalic mesoblast (which forms the anterior 
median portion of this ring) is formed mainly by an actual migra- 
tion of cells from the anterior ends of the mesoblastic bands. 
At a somewhat later period some of the cells situated at the 
edges of the mesoblastic bands lose their connection with the 
others, pass upwards between the ectoblast and entoblast, and 
thus give rise to what I have called at page 389 the mzgratory 
mesoblast ; the remaining portion of the mesoblastic bands (in 
which the coelomic cavities afterwards appear) constituting the 
“trunk-mesoblast.” Anteriorly the migratory mesoblast of the 
trunk-region is continuous with the cephalic mesoblast (Fig. 50) ; 
toward the ventral side it passes suddenly into the mesoblast of 
the germ-bands. 
The cells of the migratory mesoblast do not form a continu- 
ous layer, but remain scattered, become branched, and form a 
beautiful network (Fig. 89) that encloses the entire archenteron. 
In this network at least two kinds of cells can be distinguished. 
The first are enormously long branching muscle-fibres arranged 
in a longitudinal and a circular set which cross at right angles 
and form an open network. The substance of the fibres stains 
deeply with haematoxylin and is nearly homogeneous; the nu- 
cleus is situated near the middle of the fibre, attached to its 
side, and surrounded by asmall quantity of granular protoplasm. 
The fibres are sometimes simple, but more usually subdivide 
towards their extremities into excessively fine twigs, and occa- 
sionally a cluster of branches is given off near the middle of 
the fibre at right angles to its general course. The granular pro- 
toplasm surrounding the nucleus in some cases also gives off 
branches, but these are easily distinguishable from the muscular 
branches. 
Cells of the second kind are much less numerous than the 
others. They are granular branching cells having the general 
appearance of large connective-tissue cells. In some cases their 
branches appear to join those of other similar cells and the pro- 
