Egg size, number, and development
Water-breeding frogs and toads lay smaller eggs and in larger numbers than the land-breeding midwife toads. Based on this observation Kammerer expected midwife toads to develop the same adaptations, when forced to breed in water. Egg size and female fecundity are heritable traits, hence selection on egg size in experiments is possible. However, in Kammerer’s experiments there was no direct selection on egg size and number. It is hard to see how an increase in temperature could have selected for smaller eggs and higher fecundity.
Kammerer describes morphological changes of the eggs: they develop more voluminous gelatinous coats and yolk decreases with subsequent breeding cycles and generations. Embryonic development in water is heterochronous in comparison to eggs developing on land. This change involves a suite of adaptations, of which it is unlikely that they can be brought to expression by a mild increase in temperature.