EMBRYOLOGY OF BDELLODRILUS V7 
figure 51 (a little later stage) the blastopore is nearly closed. 
This early closure of the blastopore in Bdellodrilus, is due largely, 
to the ventro-anterior shifting of the macromere D over A, B 
and C in the formation of the somatoblasts (figs. 18, 85). 
The closure of the blastopore, in some of the annelids, occurs 
at a very late stage of development. In Clepsine the telo- 
blasts give rise to rows of cells, which pass anteriorly around the 
entomeres A, B and C beneath the edge of the blastodise or cap 
of cells. The blastodise with these rows of cell cover about 
half of the entomeres. By the downward growth of the blas- 
dodisc and the concrescence of the germ bands, the closure of 
the blastopore is completed. The closure of the blastopore 
in Clepsine occurs on the ventral side, nearer the anterior end. 
In Bdellodrilus, the germ bands are not formed until later and 
take no part in the closure of the blastopore. Text figures 
10 to 13 show the position of the ectoderm, entoderm, and the 
first and second somatoblasts, at different stages in the closure 
of the blastopore. The region of closure is similar to that of 
Clepsine. 
At the time of the formation of the secondary mesoblast 
just beneath the first generation of ectomeres, the entire ento- 
derm is situated in the anterior half of the embryo. But soon 
after the formation of the m cells (text fig. 12 and fig. 86), the 
entodermal cells by a rapid proliferation extend posteriorly 
between the m cells and the primary mesoblast. During the 
formation of the primary mesoblast, the meso-teloblasts them- 
selves are carried posteriorly, ahead of the entoderm. The 
entoderm, thus becomes situated between the m cells or second- 
ary mesoderm above and the mesoblast bands or primary meso- 
blast below. The entoderm in reality never reaches the posterior 
limit of the meso-teloblasts, as shown in figures 98 and 99. 
The interior of the developing embryo, now consists of a solid 
mass of small entodermal cleavage cells (figs. 95-99), heavily 
laden with yolk. These cells are readily distinguished from 
the surrounding mesodermal cells, by their deeper cytoplasmic 
stain. Figure 99 (a vertical longitudinal section near the median 
plane) shows the pcesition of the entodermal cells in the embryo. 
