200 GEORGE W. TANNREUTHER 
the long duration of its free swimming and almost stationary 
larval existence. In all forms where there is a tropchophore stage 
of long duration, as is the case in all annelids with equal cleavage, 
the two coelo-mesoblast cells do not, in the early stages at least, 
bud like teloblasts. This is tue in Hydroides, some species of 
Polygordius, in Thalassema and Podarke. 
Many of the annelids and molluscs show that the so-called 
ecto-mesoblast (designated. as larval mesoblast or as mesenchyme 
by some authors) arises from certain ectodermal cleavage cells 
of the second or third quartette of ectomeres and is entirely 
independent of the coelo-mesoblast. In Thalassema (Torrey) 
ten. large ectodermal cells sink in from the ectoderm and. give 
rise to all the mesenchyme. Three of these cells are from the 
a, c and d quadrants of the third quartette and seven from the 
first quartette of ectomeres. The most important source of 
mesenchyme in Thalassema is from the three cells of the third 
quartette (3d2-2-2-1, 3c?121, and 3a?-22). The first two sink into 
the cleavage cavity, Just before gastrulation and at first le close 
to the two coelo-mesoblast cells. They soon migrate laterally 
and bud off simultaneously small cells toward the M cells. They 
divide like teloblasts, but in the reverse order to the ordinary 
direction. So close is the connection of these cells with the 
coelo-mesoblast that one would be certainly led to think that they 
formed a part of these bands, unless their cytogeny had been 
carefully followed. Similar conditions are described by Tread- 
well in Podarke obscura. The progeny of these two cells form 
almost the entire mesenchyme of the post throchal region and be- 
come differentiated for the most part into muscles of the diges- 
tive tract. The progeny of the other ectodermal cell migrates 
to the mid-ventral line. The ecto-mesoblast cells of the first 
quartette sink into the primary body cavity later than those 
of the third; their exact cell lineage has not been traced, but 
probably give rise to gut musculature. This mesoblast has 
commonly been considered as purely larval and transitory. 
In some instances it is possible to determine its exact origin, 
but in many others merely the general region from which it 
arises. 
