EMBRYOLOGY OF BDELLODRILUS ETe 
can be followed step by step. The mesoblastic and neuroblastic 
portions of the germ bands can be traced to the anterior end of 
the embryo. The meso-teloblasts are the last to disappear. 
They are distinct until after the formation of the stomodaeum 
and its connection with the pharynx. The concrescence of 
the germ bands begins anteriorly and progresses posteriorly. 
THE ECTODERM AND ITS PRODUCTS 
The three generations of ectomeres are given entirely to the 
formation of the ectoderm, which later becomes differentiated 
into the definitive hypodermis, with its glands, the cuticle and 
the anterior and posterior ends of the digestive tract. The 
ectoderm includes, in addition to the above, all of the telo- 
blasts, except the two larger and deeper ones which represent the 
mesoblasts. The reason for regarding the eight teloblasts and 
their derivatives as a part of the general ectoderm, is on account 
of their origin and position. In position, they are superficial 
at first and can not be distinguished from the general ectoderm, 
except by their arrangement in rows. Small cells are budded 
off from the teloblasts, which form the trunk ectoderm. In 
Clepsine these teloblasts are at first superficial at the posterior 
end of the embryo. In Lumbricus they are found directly in 
the general ectoderm, and beyond question form a part of it. 
1. THE NERVOUS SYSTEM 
The nerve chain in Bdellodrilus first appears as a double row 
of cells, nearly uniform in size. Each row of cells originates 
from a single cell, the neuroblast. The neuroblasts, when first 
formed, are widely separated, but symmetrical to the median 
axis of the body. Figures 47 and 48 (in ventral view) show their 
position when first formed by an equal division of the protelo- 
blasts X and X. They take up their position on either side of 
the mesoblasts (figs. 47, 48, 93). When first formed the neuro- 
blasts are turned somewhat anteriorly as shown in the horizon- 
tal section of figure 93. This movement of the cells to their 
final position, independent of the former position of the cleavage 
