EMBRYOLOGY OF BDELLODRILUS 169 
4. THE SECOND SOMATOBLAST 
Immediately after the formation of X from the posterior 
macromere D, d? is budded off from D (figs. 24-25). Next D 
divides very unequally in an oblique direction and gives rise to 
d‘ (M) the ‘second somatoblast,’ as previously described. Fig- 
ures 37 and 85 show the position of D and M after the cleavage 
of the macromere D. The twenty-five-cell stage is reached by 
the bilateral division of M (figs. 37-88). The cells M, M at 
first are a little to the left of the median plane, but later in 
course of development they become symmetrical tothe longi- 
tudinal axis of the adult worm. 
Scon after the bilateral division of M, the ‘second somato- 
blast,’ each cell M,M right and left buds off five or more small 
cells directly beneath the first generation of ectomeres (figs. 
86-87). It is impossible to detect these cells except by means 
of sections, hence the uncertainty as to their exact number. 
They are characterized by their large nuclei with homogeneous 
staining chromatin and they contain but little yolk material. 
These small cells divide once or twice soon after their formation 
from the primary mesoblasts and then remain inactive until 
late embryological development, at least until after the germ 
bands are completely formed and the embryo has undergone 
considerable differentiation (as the formation of the lumen of 
the digestive system, etc.). 
These undifferentiated mesodermal cells occupy the region 
which becomes the central dorsal side of the embryo, at the 
point where the developing worm is completely turned on it- 
self (figs. 92, 98-99). The history of these cells can be readily 
followed through their different stages of development, so that 
there can be no question as to their exact origin and history. 
When the embryo begins to straighten, the progeny of these 
small cells extend toward either end and form the splanchnic 
and somatic mesoderm on the dorsal side of the worm. This 
secondary mesoderm later becomes continuous with the primary 
mesoderm, which forms directly from the mesoblasts M,M. 
