EMBRYOLOGY OF BDELLODRILUS 145 
the position of some definite future formation. The large macro- 
mere D after the formation of d* divides very unequally, the 
smaller cell becomes the entomere D and the larger cell becomes 
the mesomere d‘ (M). The entire mesoblast is derived from 
the large cell M after its equal cleavage. The primary meso- 
blasts M, M, or meso-teloblasts, are completely grown over 
by the derivatives of X and the cleavage cells of the third genera- 
tion of ectomeres. The descendants of the primary mesoblasts 
are differentiated into two distinct groups of cells. The first 
group becomes the dorsal mesoderm of the adult worm. The 
second the mesodermal germ bands, becomes the ventral and 
lateral mesoderm. The cells of the first group remain undif- 
ferentiated until late development. The latter becomes differ- 
entiated into muscle tissue much earlier than the former. 
3. When completely formed, the germ bands consist of three 
distinct strata of cells: (a) An outer stratum, ectoblast from 
one to two cells thick, which is produced by the three generations 
of ectomeres. This layer persists as the definitive hypodermis 
and secretes the cuticle; (b) A middle stratum, which gives rise 
to the nervous system and the nephridia; (c) An inner stratum, 
mesoblast which gives rise to all of the mesodermal elements, 
blood vessels, septa, reproductive organs, etc. 
4. The middle stratum is composed of eight distinct longitudi- 
nal rows of cells, which at first lie at the surface and form part 
of the general ectoderm (ectoblast), but afterwards become 
completely covered over by the ectoderm. There are four rows 
in each germ band, terminating at the posterior end in a large 
cell or teloblast. The inner or ventral neural row on each side 
gives rise to the corresponding half of the nervous system. 
The three remaining rows of cells (nephridial) on either side, 
give rise to the nephridia. The outer nephro-teloblast often 
proliferates but few cells. 
5. The brain or cephalic ganglia take their origin from the 
extreme anterior ends of the neural rows and are distinctly in- 
dependent of the ectoderm. 
6. The cleavage of the entomeres A, B, C, and D is con- 
tinued to the end without delay. The entire digestive tract, 
