EMBRYOLOGY OF BDELLODRILUS 199 
the ‘second somatoblast’ produces the definitive mesodermal 
elements of the adult animal. 
3. VARIATIONS IN THE METHOD OF MESODERMAL FORMATION 
All annelids and molluses which have been carefully studied 
show that the ectoderm arises from the three generations of 
ectomeres, the mesoderm from M and the entoderm from the 
remaining cells. There aré, however, a few minor variations 
in forms like Clepsine, Crepidula and Nereis. In polyclades 
the mesoderm is directly associated with the ectomeres. The 
second and third generations of ectomeres, as in Discocoelis, 
produce the mesoblast, and the macromeres the entoblast. In 
molluscs and annelids the mesoderm is more closely associated 
with the macromere D. There is considerable variation in the 
cleavage of D in the formation of the ‘second somatoblast;’ 
in some forms D is given over entirely to the mesoderm; in other 
forms it shares equally, or in still others it contributes but little 
to the mesoderm. 
Forms in which the mesoblast has two sources 
(a) Ecto-mesoblast and (b) coelo-mesoblast. In Thalassema, 
Torrey distinguished between ecto-mesoblast, from the ectoderm 
and. coelo-mesoblast from M. He states that the coelomeso- 
blast is present in two bands, each consisting of five sub-equal 
cells. These are closely applied to the body wall and lie in the 
usual position on each side of the neural rows, but are more widely 
separated than in most annelids. The ecto-mesoblast on the 
other hand, derived from the first and second quartette of ecto- 
meres, is present in great abundance and many of the cells have 
already undergone considerable differentiation in the formation 
of the muscles. He further states that the two mesoblast cells, 
M, M, are the last to sink in at gastrulation, instead of the first, 
as in the case where development is more direct (Nereis and 
Amphitrite). The two coelo-mesoblast bands of five cells each 
are shown to have the same origin and early history as the meso- 
blast bands in other annelids. The coelo-mesoblast, which is 
meagerly developed in the trochophore, is clearly correlated with 
