from the GNU version of the Collaborative International Dictionary of English
- n. A germ in that stage of development in which its form is simply that of a non-nucleated mass of protoplasm. It precedes the one-celled germ. So called from its likeness to a moner.
from The Century Dictionary and Cyclopedia
- n. In embryology, a name given by Haeckel to a supposed non-nucleated stage of an impregnated ovum, when it has the form-value of a simple cytode, or moner.
Sorry, no etymologies found.
The monerula was the fertilised ovum after the disappearance of the germinal vesicle;  it was the equivalent of the primordial anucleate Monera which are the ancestors of all animals.