406 WILSON. (Vo. III. 
toplasmic branches of the muscle-cells, but I have not fully 
satisfied myself on this point. JI have searched in vain for 
nerve-cells in this network. 
The migratory mesoblast of- the trunk-region is gradually dis- 
placed by the trunk-mesoblast of the germ-bands, which as de- 
velopment proceeds, steadily extends both dorsally and ventrally, 
forcing its way between the ectoblast and entoblast as it grows. 
The two bands finally meet and fuse with one another, first along 
the median ventral line from before backwards, subsequently 
along the median dorsal line in a similar manner. From it (v. 
znfra) are developed the permanent muscles of the trunk, which 
are differentiated from the mesothelial walls of the ccelomic 
cavities and are unbranched (as may be clearly seen in prep- 
arations macerated in Hertwig’s fluid). The longitudinal fibres 
are the first to appear, forming on each side a longitudinal bun- 
dle that lies just opposite the boundary between the nephric 
and neural cords (Figs. 72, 90, 91). A second bundle appears 
later just outside the nephric cord. I have been unable to deter- 
mine whether the larval branching muscle-fibres disappear or 
remain embedded in the adult body-wall. 
In the head-region the mesoblastic cells become amceboid and 
branched and ultimately arrange themselves so as to enclose a 
single median cavity (prostomial or head-cavity) traversed by 
branching contractile cells. 
From the foregoing account it appears that the trunk-meso- 
blast is a “mesothelium,” the migratory mesoblast of the trunk 
a typical ““mesenchyme,” while the cephalic mesoblast is of an 
intermediate character. All these cells have, however, a common 
origin, and the distinction between “mesothelium ”’ and “ mesen- 
chyme” in the embryo as originally made by the Hertwigs (No. 
25) is here of wholly secondary importance (as has been shown 
to be the case in other annelids by Kleinenberg and Hatschek). 
The migratory mesoblast of the trunk appears to be a special 
larval musculature precociously developed in order to enable the 
embryo to manage the enormous mass of albumen with which 
its body is distended. 
2. Origin of the Celom. — The trunk-cavities appear in the 
mesoblastic bands in regular succession from before backwards 
(Figs. 50, 63, 97). In longitudinal sections the bands are seen 
to be posteriorly but one cell in thickness; further forward 
