The first questions to be determined were as to their nature and origin.

How were they produced?
Did they come from bodies of the same sort according to the general laws governing the production of living things, or did they arise spontaneously?
a question which could not be solved by speculation but by experiment.

The first experiments, by Needham, 1745, pointed to the spontaneous origin of the organisms.

He enclosed various substances in carefully sealed watch crystals from which the air was excluded, and found that animalculi appeared in the substance, and argued from this that they developed spontaneously.

In 1769, Spallanzani, a skilled experimental physiologist, in a brilliant series of experiments showed the imperfect character of Needham's work and the fallacy of his conclusions.