EMBRYOLOGY OF BDELLODRILUS 179 
the embryo are in immediate contact, it is impossible, except 
by longitudinal sections, to determine the exact point of separa- 
tion. The ectoderm of the anterior end of the embryo, which 
is derived from the three generations of ectomeres is continuous 
with the ectoderm derived from the ‘first somatoblast.’ 
The separation of the two ends of the embryo becomes recog- 
nizable in the early formation of the germ bands, as shown in 
figures 59 and 60. The posterior and ventral shifting of the 
neuroblasts (figs. 58-60) continues until all of the teloblasts are 
in a direct line. The small cells between the teloblasts are 
derived from the first somatoblast. In viewing the embryo 
from the upper pole (which now corresponds more to the anterior 
and posterior ends of the future animal) the germ bands extend 
laterally, downward and forward, being curved somewhat. pos- 
teriorly as they pass from the upper to the lower pole (fig. 59). 
The meso-teloblasts in figure 58 are still visible from the exte- 
rior. In figure 59 they are almost grown over, while in figure 
60 they are completely covered. This is due to the posterior 
shifting of the neuroblasts and the growth of the ectomeres 
from above and below. In an embryo viewed from the right 
side (fig. 61, a little older than fig. 60), the position of the neural 
and nephric rows of the germ band are shown. As the rows 
extend anteriorly they are more difficult to distinguish from 
the ectoderm. The neural rows alone can be followed to the 
extreme anterior end. The posterior end of the embryo is 
widely blunt, while the anterior end is more rounded. The 
heavily shaded portion represents the point of separation be- 
tween the two ends. 
Figure 62 represents the same embryo from the upper pole, 
with the ends of the embryo rotated or turned a little posteriorly. 
- In figures 63 and 64 (from right and left sides respectively) 
the embryo is more elongated and the point of separation be- 
tween the two ends is more distinct. The neuroblasts are lag- 
ging in their posterior extension. Their position is median 
ventro-posterior, as shown in figure 65. Their concrescence is 
not yet complete at the posterior end. In the following stages 
of development the cells of the neural and nephric rows divide 
