178 GEORGE W. TANNREUTHER 
spindle, is a common occurrence in Bdellodrilus. In some in- 
stances it is necessary to employ sections, in order to determine 
the origin of cells. The transverse axis of the embryo at this 
stage is often greater than the longitudinal (figs. 49-50). This 
condition persists for a brief period only, during the formation 
of the teloblasts. As the embryo increases in length the neuro- 
blasts are carried more and more posteriorly (figs. 56-57). 
In order to get a better understanding of the origin and orienta- 
tion of the neuroblasts—X right and X“” left—with reference to . 
the other teloblasts, the figures of plate 4 are so arranged that 
the left side of the developing embryo corresponds to the reader’s 
left. In figure 45 the upper pole is turned a little posteriorly, 
to show the upper outer edges of the entodermal cells. Figure 
47 (from ventral pole) shows some of the ectodermal cells. The 
remaining figures are either turned forward or backward on their 
transverse axes. The ectomeres x*® and x® right and left serve 
as good points for orientation (figs. 46-53). After the formation 
of the teloblasts, bilateral symmetry is fully established. The 
meso-teloblasts, however in some instances, are still a little to 
the left of the median axis. This variation in the symmetry of 
the mesoblast does not in any way change the end result. In 
the early history of the germ band formation the teloblasts X 
and X™) are slightly separated, while X™ and X® are widely sepa- 
rated from the corresponding teloblast on the opposite side 
(figs. 56, 58). The neuroblasts and the nephroblasts begin their 
proliferation of cells to form the germ bands, about the same 
time (fig. 58). At this stage of development, the exact orienta- 
tion of the embryo is distinct. Since the embryo is completely 
turned on itself, the further use of the terms, apical and ventral 
poles, is significant only as being convenient in description. The 
mouth, as stated above, is formed in the center of the apical 
pole and the anus in close proximity on the dorsal side of the 
tenth segment. Figure 59 (upper pole view) shows the com- 
plete curvature of the embryo. The heavily shaded portion 
represents approximately, the boundary between the anterior 
and posterior ends. This figure shows that the teloblasts are 
coming more and more in a straight line. Since the two ends of 
