354 The American Naturalist. [April, 
The true cleavage cavity appears in the 13-cell stage. In the 16-cell 
stage two mesenchyme cells were found lying in the cleavage cavity, 
near the macromere, and Stauffacher thinks they are derived from it. 
At about the 30-cell stage the last primary micromere is formed. 
Ziegler thought it formed the two large primary mesoderm cells, but 
Stauffacher thinks it enters into the formation of the ectoderm along 
with all the previously formed micromeres, 
The macromere next divides into equal right and left halves, From 
each of these a large cell is segmented off into the cleavage cavity, one 
slightly before the other, agreeing with Rabl’s account for Unio. These 
two cells last formed are the primary mesoderm cells, The two small 
remaining macromeres form the endoderm.—C. P. SIGERFOOS. 
