ABSTRACT
Low water-living vertebrates apparently need only little and simple provision for regulating the developmental environment. A comparative survey of all classes of vertebrate animals will show that a most striking evolution of arrangements has come about which tends to insure for the developing egg and embryo a properly regulated environment. The birds have retained much or all of the reptilian method and have only improved upon this arrangement by providing the temperature which guarantees the development of the egg. The temperature regulation is very probably uniformly adequate in the mammal since the embryos of animals which may be studied experimentally are actually capable of withstanding a wider range in temperature variation than would be possible within the body of the mammalian mother. The parent animal has evolved no mechanism or habit for effectively avoiding or greatly lessening these dangers.
