No. 3.] THE EMBRYOLOGY OF THE EARTHWORM. 4OI 
V. GENERAL HISTORY OF THE GERM-BANDS, 
I shall use the term germ-band to mean the composite struc- 
ture extending along either side of the body from the primary 
mesoblast to the dorsal side of the mouth, and consisting in its 
middle portion of the three strata of cells referred to at p. 389. 
1. The Mesoblast. —In its earliest form, as it occurs in the 
blastula, each germ-band consists only of the primary mesoblast 
with the corresponding row of mesoblastic cells extending 
forwards from it and joining its fellow in front (Figs. 30, 31, 
32). As the blastula flattens shortly before the invagination, 
the mesoblastic rows are carried out towards the sides of the 
embryo and their anterior ends become widely separated. As 
the invagination proceeds the mesoblastic rows undergo a two- 
fold change. First, they grow forward and upwards, as already 
described, so as to approach one another above the anterior lip 
of the blastopore (Fig. 43); and second, their middle portions 
are carried still further downwards so as to lie on the ventral 
side, though they are still widely separated. Meanwhile the 
ectoblast becomes thickened immediately over the mesoblastic 
cells and thus forms the outer or ectoblastic stratum of the 
band. This thickened area of the ectoblast everywhere accom- 
panies the underlying mesoblastic band, but is somewhat wider. 
Dorsally and ventrally it passes rather suddenly into the large- 
celled, flattened epithelium that forms the upper and lower in- 
vestment of the embryo (Fig. 41). Sections at this stage show 
the ectoblast to be still everywhere composed of a single layer 
of cells. In front the thickened ectoblastic areas fade away into 
the pavement epithelium of the dorsal surface (surface views in 
Figs. 33, 35, 36, 41, 43; sections in Figs. 52, 53, 67,68; see ex- 
planation of plates). 
It is necessary to refer at this point to the group of enlarged 
ectoblastic cells known as “ Schluckzellen,’”’ which appear at an 
early stage at the anterior part of the body in the embryos of 
some Hirudinea, and in a number of the Oligocheta, including 
some species of Lumbricus. These cells, whose mode of origin 
has been carefully studied by Hatschek in Criodrilus (No. 18), 
and by Vejdovsky in a number of Oligochzeta (No. 46) have 
been shown by the last-named author to be larval excretory 
organs. They are at first enlarged ectoblast cells which lie at the 
