ABSTRACT
The origin of truly terrestrial vertebrates was associated with the development of a skin which is relatively impermeable to water. The skins of fishes and of most amphibia are freely permeable to water, and water is readily lost when the animals are exposed to ordinary atmospheric conditions. The skin of a lizard on the other hand is relatively impermeable to water and the animal is therefore able to withstand exposure to air without serious inconvenience.
Complete adaptation to terrestrial conditions involved the capability of laying and incubating eggs on land. The eggs of fish and of typical amphibia absorb, during development, considerable quantities of water from the external environment. The embryos of reptiles and of birds derive the equivalent of this water from the albumen layer secreted round the ova by the walls of the oviduct.
Evidence is presented to show that the reptilian type of egg is to be derived from that of a dipnoan or amphibian, where the egg is surrounded by a mucilaginous or albuminous secretion of the walls of the oviduct.
The mammalian egg can be derived from that of a reptile by supposing that the aqueous secretions of the oviduct are passed direct into the embryo instead of forming a separate phase round the ovum. An intermediate type is found in Monotremes where the secretions are passed into the ovum itself before the egg is laid.
Weldon incubated eggs in such a way as to replace the water normally lost by evaporation without interfering with the processes of evaporation or respiration. In such eggs the amnion failed to develop normally.
