246 Evolution? The Fossils Say YES!
The embryo is also aided by multiple specialized systems. It is surrounded by a mem-
brane called the amnion, which is filled with amniotic fluid to buffer it from shock and tem-
perature change. Attached to the gut of the embryo but outside the amnion is the yolk sac,
which provides food for the embryo so it can hatch out relatively well developed and ready
to face the world. A second sac off the hindgut of the embryo is known as the allantois, and
it collects wastes and aids in respiration. Finally, the entire assembly—amnion, yolk sac, and
allantois—is surrounded by another fluid, albumin (the “egg white”), which fills the rest of
the volume of the egg. Just beneath the shell is a porous membrane known as a chorion, which
helps hold in the fluids while allowing oxygen to enter and carbon dioxide waste to escape.
The amniotic egg has other implications. Each egg is more costly to produce, so fewer
can be laid, and each embryo will be more completely developed and independent when it
hatches. In addition, the egg cannot be fertilized (as is done in most fish and amphibians) by
the male swimming near the egg cluster in the water and spraying them with sperm. Instead,
the amniotic egg requires internal fertilization, usually involving sexual intercourse. Males
and females must copulate so that the sperm can reach the eggs inside the female, and the
eggs undergo much of their development inside her, not in the water. Internal fertilization
has evolved more than once, of course. Most land-living arthropods (insects, spiders, scorpi-
ons, and so on) must copulate for the same reasons that amniotes do—they can’t spread their
eggs and sperm around in water. In the sea, sharks are among the few non-amniotes that use
internal fertilization, and they lay smaller numbers of large eggs, or some even give birth to
live young that are fully developed.
How did the transition from primitive tetrapods to the amniotes occur? We cannot tell
which extinct organisms known from fossils laid an amniotic egg, because eggs are rarely
preserved with their parents. Only a few fossil eggs from the time of the first amniotes are
known anyway. Instead, we must work with the features of the skeleton to decide which
FIGURE 11.2. Diagram showing the parts of the amniotic “land egg.” (From Romer 1959; used by permission of
the University of Chicago Press)
Yolk in yolk sacShellAmnionChorionAllantois