The eye, adapted for focussing objects under water, has to be so changed in its proportions as to see its surroundings through air.
That part of the ear whose function is to determine the position of the animal with respect to gravity and to recognize changes in position, can remain unaltered, but the lagena, which, with its associated structures, the columella, middle ear and tympanic membrane, is concerned with hearing in the ordinary sense, neces sarily undergoes changes on account of the very different specific gravity of the mediums, water or air, through which sound waves come to it.
Aquatic Amphibia have, in common with fish, a special sense, whose organ is the lateral line, which is concerned with the recog nition of movements in water; with the transition to land this sense is entirely lost.
Any animal living in water is so nearly floating that the pro portion of its weight which has to be supported by the limbs is extremely small. As soon as it comes out of water practically the whole of its weight falls on the legs. Thus the skeleton and mus culature necessarily become more powerful.
The most serious changes, however, are those in the mode of reproduction. An amphibian which lays its eggs in the water can fertilize them there, but a terrestrial animal can only lay an egg if it be included in a shell which will protect it from mechanical injury and, a matter of more importance, from drying up. Such a shell cannot be perforated by a spermatozoan, so that fertiliza tion must take place within the body of the mother before the shell is formed. The uro-genital organs of both sexes have to be so modified as to allow this to take place, and the oviduct of the female must be provided with the glands that are necessary for the formation of the shell.
The amphibian egg may be comparatively small; it only needs to contain a food supply sufficient to maintain the developing embryo to a stage when it hatches as a small larva capable of feeding on the abundant food present in the water in which the egg was laid. The animal, which hatches from an egg laid on land, must make its appearance at a stage in development when it can main tain itself under conditions similar to those in which its parents live. The time taken in reaching such a stage of development is
considerable, and the egg included in its shell has no opportunity of obtaining food or water from outside. Thus, when laid, the egg must contain everything necessary for the development of the embryo up to the time of hatching.
The reptile ovum, the yolk of the egg, contains the great bulk of the food materials, whilst the albumen, the white which sur rounds it, is mainly a water store. The egg-shell is porous and transmits gases. The character of the development of the large egg is necessarily modified by its bulk, much of the yolk remain ing undivided into cells until it is absorbed and converted into part of the animal's own tissues. Special embryonic membranes, the amnion and allantois, are produced during the development for the protection of the embryo and for its nutrition and respiration, the allantois serving also as a reservoir for the nitrogenous waste products produced by its metabolism.
The great majority of these changes, including all those which are of the greatest importance, cannot be determined from fossil material, and we are driven back for the discrimination between fossil reptiles and fossil amphibians to the use of technical points mainly of little functional importance to the animal. The break between the Amphibia and Reptilia was regarded by Huxley and other early workers as the most important in the vertebrate phylum ; such contrasted terms as Ichthyopsida and Sauropsida, Anamniota and Amniota emphasize its importance. None the less we now know an animal, Seymouria, from the lowest Permian of Texas, which is regarded by one group of students as an am phibian and by another as a reptile. As the osteology of this animal is very completely known, the doubt which exists as to its systematic position illustrates vividly the completeness with which the gap between these two divisions has been bridged.