Placental matrotrophy is nourishment of developing offspring by transfer of maternal material across a placenta. It supplements or replaces reliance on reserves packaged in the ovum, such as yolk. Viviparity specifies retention of developing offspring and live birth; placental matrotrophy additionally specifies a route of nutrient supply.
A striking example comes from the lizard described as Mabuya heathi in a 1984 study. Its very small ovum could not account for the large increase in offspring dry mass; the authors estimated that placental transfer supplied more than 99 percent of the near-term fetus's dry mass. Examination of the placenta linked the growth measurements with an anatomical route of exchange.[ref]
Dry mass matters because uptake of water alone can greatly increase wet mass. In a synthetic comparison, an initial 1 mg of dry mass and a final 10 mg imply a net addition of 9 mg of dry material. Measuring that increase supports transfer beyond the original reserves; determining the particular nutrients and transport mechanisms requires further measurements. The lizard example also makes placental matrotrophy a comparative character across vertebrates, rather than a synonym for being a mammal.
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