206 GEORGE W. TANNREUTHER 
it is reduced until it is little more than a mere nucleus, while 
in others it has completely disappeared as an entomere, and is 
given over entirely to the formation of mesoderm. 
In annelids, in a gradually decreasing series, D (Nereis) is 
the same size as the entoblast cells A, B and C. In Dinophilus 
it is about half the size of these cells. In Bdellodrilus D is little 
more than a mere nucleus; while in Clepsine D is given over 
entirely to the formation of the mesoderm. In molluscs it is 
a fairly common condition to find the entoblast cell D smaller 
than A, B and C, or even greatly reduced. In Crepidula it is 
very little reduced; in Unio it is more than half reduced, while 
in Ischnochiton, D is often little more than a mere nucleus. 
The second somatoblast, M, may contribute tc the formation 
of entoderm as well as mesoderm. In forms like Crepidula M 
is mostly entodermal. In Fiona (Casteel) the division of M 
in the formation of the entoderm is very similar to that in Crepi- 
dula. In Unio two small cells are budded off from M, which 
lie near the entoderm, and are probably concerned in the forma- 
tion of that layer. 
In some of the annelids the primary mesoblasts bud off small 
cells directly posterior to the macromeres. This number varies; 
in Nereis there are six to ten, and in Aricia there are but two. 
In many of the other annelids and also in some of the molluscs, 
where their cell lineage has been traced, it is found that these 
small cells give rise to entoderm. There are at least sixteen to 
twenty species of annelids and molluscs in which similar cells 
have been found (small cells from the primary mesoblasts.) 
Diverse accounts of their behavior and fate have been given 
by different investigators. Table 4 shows the fate of these small 
cells in a few of the annelids and molluscs. 
In the molluse Alpysia, according to Carazzi, each primary 
mesoblast buds off four small cells. Three of these are meso- 
blastic and one is entoblastic. This interesting condition might 
be considered as a transitional form or as a connecting link be- 
tween. those forms in which these small cells are entirely mesoder- 
mal and those in which they are entodermal. Again we could 
arrange a series of annelids and molluscs in which at one extreme 
