444 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
to continue development when the releasing mechanism was delayed. 
With the protection of the eggs, the need for enormous numbers 
would be lessened, and the female could concentrate on producing a 
fewer number of large-yolked eggs. A continuation of the trend 
toward the prolongation of development within the egg would lead 
eventually to metamorphosis before hatching. With the elimination 
of the free-living larval period, the typical larval characters would 
tend to drop out, resulting in direct development. All these stages 
are represented in the life histories of modern amphibians. They 
are thus, we can be sure, all practicable ways for an amphibian to 
reproduce its kind. Furthermore, each step would seem to have a 
selective advantage over the one before. 
Somewhere along the line, these amphibian ancestors of the rep- 
tiles must have adopted the practice of internal fertilization with 
copulation. This, too, is found in modern amphibians and would 
seem to be advantageous for animals breeding on land. 
The stage would now be set for the development of the typical rep- 
tilian cleidoic egg with its fluid-filled amnion and protective shell. 
It is hard to visualize these structures evolving in an aquatic egg, 
but in one that was already terrestrial, anything that reduced the 
dependence of the developing embryo on environmental moisture 
would be decidedly advantageous. 
It is impossible to determine, of course, how these changes were 
correlated with the anatomical changes by which the amphibian 
body plan turned into the reptilian body plan, or even to know where 
we should draw the line separating the two orders. In the present 
state of our knowledge, it is at least permissible to hypothesize that 
by about the time the shelled amniote egg had developed, the animal 
that hatched from it had progressed far enough to be called a reptile, 
and this seems as good a place as any to draw the line. It has the 
advantage of being a line that separates the modern forms as well. 
It seems probable, though, that we should revise our conception of 
the significance of the cleidoic egg. Perhaps it was not the first 
reptiles that broke the bond yoking the vertebrates to the immediate 
vicinity of water for reproduction. It seems more likely that these 
reptiles descended from amphibians that were already fully terres- 
trial in their breeding habits, that practiced internal fertilization by 
copulation, and laid large-yolked eggs in sheltered spots on land, 
from which the young developed directly with no intermediate aquatic 
larval stage. 
As for the other amphibians, it is obvious that many of them have 
acquired independently one or more of the reptilian breeding charac- 
teristics listed above, but presumably not since the days when their 
ancestors paddled about in the Paleozoic puddles has any amphibian 
acquired them all and thus developed into an amniote. The rest of 
