AMPHIBIANS—GOIN 443 
III. Eggs retained in the oviduct of the female and the young “born alive”: 
Caecilidae, Proteidae, Salamandridae, Plethodontidae, and Bufonidae. 
Perhaps the most obvious thing that can be ascertained by a study 
of the above outline is the fact that there is so little correlation 
between the life histories of the amphibians and their evolutionary 
relationship. Here we find a form of copulation in the caecilians 
and frogs, and internal fertilization without copulation particularly 
common in the salamanders. We find both small eggs with indirect 
development (with an intermediate, free-living larval stage), and 
large eggs with direct development, in the salamanders and in the 
frogs and toads. It has been shown by Moore (1942) that in certain 
groups of frogs there is a tendency to have larger eggs toward the 
Temperate Zones and smaller eggs toward the Tropics but even here 
there are exceptions (Moore, 1949), for Rana pipiens, the leopard 
frog, lays larger eggs in Mexico than it does in the northern United 
States. But this does not explain the tendency toward large 
eggs and direct development in the leptodactylids, hylids, and 
rhacophorids, all of which are essentially tropical groups. 
It appears then that there is nothing about the diverse life his- 
tories of modern amphibians to give a sure clue to the development 
of the land egg. Since this is so, perhaps we had best look at rep- 
tilian reproductive habits and see if it is possible to figure out the 
combination of factors that made possible the reptile pattern of 
reproduction. 
CHARACTERS OF REPTILIAN REPRODUCTION 
1. Internal fertilization. This is necessary because the sperm must enter the 
egg before the shell is deposited around it by the glands of the oviduct. 
2. Copulation, an effective and apparently efficient method of achieving internal 
fertilization. 
3. Eggs laid on land in a protected spot. (Sometimes the young are “born 
alive,” but this is certainly secondary in the reptiles. ) 
4, Hgg with yolk mass large enough to carry the embryo through development, 
until the adult body form is reached. 
. Development direct without an intermediate larval stage. 
6. Egg cleidoic, that is, with a fluid-filled amnion and protective shell. 
On 
From a consideration of this list, we can trace a probable course 
for the evolution of breeding habits in the amphibian stock leading 
to the reptiles. The first step away from the primitive pattern of 
laying a large number of unprotected eggs in open water was prob- 
ably to lay the eggs in a sheltered spot away from the main body 
of the pond or stream. Next would come deposition of the eggs in 
a humid, protected spot on land, depending on some mechanism 
(heavy rains, flooding, falling from overhanging leaf, or transporta- 
tion by parent) to release and return the newly hatched larvae to 
the water. Once this stage was reached, an increase in yolk supply 
in the egg would be advantageous since it would allow the embryo 
