EMBRYOLOGY OF BDELLODRILUS 191 
It divides into two, four and finally eight large cells called the 
teloblasts; the middle stratum of the germ bands is derived from 
them. These eight teloblasts are arranged into two groups of 
four cells each. Each group, which later is composed of four 
rows of cells, produces the middle stratum of the germ band on 
the corresponding side. The inner row of each band lies ulti- 
mately near the median ventral plane and gives rise to the 
corresponding half of the nervous system. The adjoining rows 
—‘nephroblasts’—give rise to the nephridia. The derivatives 
of the outer row are still in doubt, but probably take part in the 
formation of the ectoderm. 
In Dinophilus (Nelson) d? (X) is formed by a laeotropic divi- 
sion of the macromere D (text fig. 25); D is much smaller than X. 
Immediately after the formation of X, x! is budded off to the 
right at a low level. Next x? is budded off to the left at a higher 
level than x'; x is next formed by a dexiotropic division from 
the dorsal side, a little to the left. Next X divides equally and 
produces X and X, right and left. These two large cells cor- 
respond to the proteloblasts in Bdellodrilus. Finally X on either 
side divides equally, and produces the two teloblasts on each 
side of the median plane. These four cells, according to Nel- 
son, correspond to the posterior teloblasts of Nereis. They also 
correspond to the neuroblasts and nephroblasts of Bdellodrilus. 
The division of X in Dinophilis and Nereis differs no more than 
do the corresponding divisions in Nereis and other annelids 
(Amphitrite, etc.). At the time of the closure of the blasto- 
pore in Dinophilus, the descendants of X are distributed dorsally 
and laterally to the posterior stem cells. In Neries the main 
bulk of the descendants of X lay on the vegetal side of the stem 
cells. 
In Unio (Lillie) the ‘first somatoblast’ X is formed by an un- 
equal division of D (text fig. 24) in a median posterior position; 
x! is budded off from X, just behind C on the vegetal pole; 
next x? is budded off from X symmetrically with x! on the right 
side, just posterior to d’; next x* is formed from X towards the 
apical pole, posterior to d'2; then x‘ is budded off from X an- 
teriorly, towards the vegetal pole. This division of X does 
JOURNAL OF MORPHOLOGY, VOL. 26, No. 2 
