EMBRYOLOGY OF BDELLODRILUS 173 
Balfour, Hatschek, Goette, Kowalevsky and many others 
made use of the term ‘mesoblastic bands’ as the equivalent of 
the germ bands. In Hirudinea, according to Whitman, the 
germ bands are composed of three distinct layers; the ectoblastic, 
mesoblastic and the neuroblastic elements. Wilson gave the 
same interpretation in his studies on ‘‘The embryology of Lum- 
bricus.” In Bdellodrilus the term ‘germ bands’ includes the 
three strata of cells as in Hirudinea and Lumbricus. 
1. INNER STRATUM OF THE GERM BANDS 
After the formation of the teloblasts, five on either side of 
the median axis (one neuroblast, one mesoblast and three nephro- 
blasts), the mesoblasts or meso-teloblasts are the first to begin 
the formation of the germ bands by a forward. proliferation 
of cells near the posterior lip of the blastopore (text figs. 12, 13, 
17). The plane of division is nearly at right angles to the forma- 
tion of cells in the secondary mesoblast (text figs. 10, 18). The 
cells of the mesoblast bands are considerably smaller than the 
teloblasts from which they originate. They grow forward 
between the entoderm and the ectoderm and finally meet at 
the anterior end of the larva. As these bands grow forward 
they become several cells broad, but seldom more than two cells 
deep. Their differentiation begins anteriorly and progresses 
backward. The first cells of the mesoblast bands, when formed, 
are on the surface, but soon become covered by the ectodermal 
cells. As the mesoblast band extends forward below and around 
the entoderm, it forces its way to the extreme anterior end of 
the embryo beneath the ectoderm. It finally encloses the 
digestive tract on the ventral and lateral sides and becomes 
continuous with the secondary mesoblast on the dorsal side. 
The two mesoblast bands fuse first at the anterior end along 
the median, ventral side and subsequently with the dorsal second- 
ary mesoderm. In figure 99 (a longitudinal section) the meso- 
blast is differentiated into splanchnic and somatic layers, with 
the coelom between. The longitudinal muscles become differ- 
entiated before the circular. At the extreme posterior end the 
