204. GEORGE W. TANNREUTHER 
the second and third quartette is mesodermal. The behavior 
of d‘ in the polyclad Discocoelis, is also very suggestive. Here 
the division of d‘ is equal and gives rise to two symmetrically 
placed cells at the posterior end of the embryo, comparable to 
the primary mesoblasts found in annelids and molluscs. Some 
investigators have even suggested that these two posterior cells 
in the polyclads may give rise to the mesoblast bands in this 
particular group. This latter point, however, has never been 
verified. 
Table 3 shows that the first, second, third and fourth generation 
of micromeres, in a series of widely separated forms, may con- 
tribute to the formation of the mesoderm. 
TABLE 3 
Ist GEN. | 2D GEN. 3D GEN. 4TH GEN. 
Annelids: | | 
Thalassema | part of a, b | none 1 cell each of | d* part mes. 
/and ¢ quad’s | a, b and -c | 
quadrants 
Bdellodrilus | none none none d‘ all mes. 
Molluses: | 
Unio none | ar? none dé all mes. 
| (larval) 
Crepidula none | ea ler ner none d‘ part mes. 
Physa none | none beme d‘ part mes. 
Podarke none | none eseeed2 d* part mes. 
| @3-2-1-2, 
(3:2-2-2 
Polyclads: | 
Discocoelis none | all mes. allmes. | none 
Leptoplana none _part of each none none 
quadrant 
In case of the ecto-mesoblast a complete series could be ar- 
ranged, in which all of the cells of certain quartettes contribute 
to the mesoblast, to those forms in which only a small part of cer- 
tain quartettes is mesoblastic. Again in case of the coelo-meso- 
blast we have a wide range of variation, in which all of d* 1s meso- 
dermal, to those in which only a small part of d‘* is mesodermal. 
As far as records show, Capitella is the only annelid in which 
