198 GEORGE W. TANNREUTHER 
the inner stratum of the germ bands. The plane of division 
in the formation of the primary mesoblast is at right angles to 
that of the secondary. 
In Dinophilus, M is formed at the twenty-nine-cell stage, by 
an unequal division of D. M is much larger than D, as in 
Bdellodrilus, and is in front and below X, slightly to the left of 
the median plane. The division of M is now delayed until the 
seventy-two-cell stage, when two small cells are budded off 
anteriorly towards the vegetal pole, close to the line of junction 
of the two mesoblasts. At the next division two small cells 
are budded off, one on either side of the first pair. The follow- 
ing cleavages are teloblastic and produce the mesoblastic bands. 
The mesoblasts do not move into the cleavage cavity as in many 
other annelids, but remain on the surface until covered by the 
ectoderm. 
In Unio, at the thirty-two-cell stage, M is formed by a very 
unequal division of the macromere D. The first division of M 
is equal and bilateral. Their position is immediately behind 
the entomeres. The next division of the two mesoblasts is very 
unequal, two small cells, m, m, are budded off at the posterior 
lip of the blastopore. Later the mesoblasts are included within 
the segmentation cavity, where they take up their final position 
behind the archenteron and give rise to the definitive meso- 
blastic germ bands with lateral teloblasts. 
From the forms compared above it is very evident that there 
is a remarkable similarity with Bdellodrilus, not only in the early 
cleavage stages, but in the establishment of the germ bands as 
well. Thus cells having the same origin and lineage have the 
same final result in a wide series of forms (d‘ the mesoblasts). 
Again, cells of unlike origin have a different fate (first and second 
somatoblasts); or cells of a different origin may have the same 
fate (d‘ of annelids and the second and third generation of ecto- 
meres in polyclades). Then cells of the same origin may havea 
different fate (a2? in Unio and Bdellodrilus). These contradic- 
tions, however, are far less striking than the resemblances. The 
‘first somatoblast’ in each of the above four forms gives rise to 
the ventral plate and all or nearly all of the trunk ectoderm, while 
