ABSTRACT
Since most eggs are small, the manipulative difficulty of experimenting on them is considerable, and the older experimental embryologists had fotmd themselves restricted almost entirely to the expedient of cutting the egg into fragments, which were then allowed to develop in isolation. The result of such an experiment was normally either that the fragment developed into the fate which would have been in prospect for it if it had remained untouched in the egg, or that it developed into a complete embryo. In the former case, the egg was called a mosaic egg, in the latter a regulation egg; and, if a further step in theoretical analysis were called for, the fragments of the former type might be called 'unipotent' and those of the latter 'totipotent', these words implying that the former had only one potency for development while the latter had all the potencies required to produce a complete organism. Where it was possible to perform experiments on a series of younger and older stages, it was commonly fotmd that while fragments from an early stage might be totipotent, those from a later one had become tmipotent.
