What do embryonic eagles look they look like as they develop and grow inside their eggs? Dr. Peter Sharpe from the Institute for Wildlife Studies developed a table of bald eagle embryonic development based on work done by Hamburger and Hamilton (1951). While not all bald eagle eggs hatch in 35 days, the stages of development look something like this…
Development of a chick, drawing from Frank Lillie photos. Artist William Sillin
When we last peeked inside the egg, our embryonic eagle had inner and outer layers, a developing circulatory system, a brand new heart, and an anterior-to-posterior template in place. It looked more like a tadpole than an eagle but, at four days old, the stage was set for it to begin developing limbs, a head, and eyes.
Day Four to Day Twelve
Between day four and twelve, our eaglet roughly doubled in size and differentiated even further. It curled up and sprouted a rough head and tiny buds that began developing into legs, wings, and a tail. Limbs extended, toes and wings began forming, eyes appeared, and a tiny beak, complete with tongue, sprouted from the embryo’s head. It requires a little imagination to see a bird, but at two to three centimeters long, our eaglet is visibly taking shape.
A seven-day old chicken embryo. Eagle Math tells us that it is roughly equivalent to a 12-day old eagle embryo..
Vital Membranes
As the embryo grew and changed, three vital membranes formed around it. Like a mammalian placenta, they cushioned and protected it while managing waste, gas exchange, and nutrient transport. The amniotic membrane enclosed the embryo in a fluid-filled sac, providing protection and space for movement and growth. The allantois and chorion arose as extensions of the embryo’s circulatory system, fusing to create a vital exchange network for respiration, waste storage, and calcium absorption from the eggshell. By the twelfth day, the vitelline membrane had fully enclosed the yolk, which keeps it from diffusing into the membranes and albumen that surround it.
What about the albumen? Although it is widely considered unimportant, it suspends the yolk, keeps the yolk and developing embryo in place, and keeps microbes at bay. How? It contains few nutrients that support microbial life and those are locked up by proteins that keep them inaccessible to bacteria. Birkhead compares a microbe’s journey from the eggshell to the embryo to a human trying to walk across the Atacama Desert: there is nothing there to sustain life. It’s also slightly alkaline, which microbes generally don’t like, and the antimicrobial proteins it contains are thought to be most effective at the temperature the egg is incubated. The egg white might be my least favorite part of the egg, but it is critical to embryonic survival.
At eleven days, the tiny embryonic eagle is about 27% of the way to hatch and still doesn’t look especially birdlike. It has a head, the beginnings of an eye, stiff differentiated limbs that are just beginning to bend, the beginning of a beak, rudimentary digestive organs, and a defined sex. The stage is set for further organization. Next up? An egg tooth, true eyes, and feather germs.
Illustrations were taken from Popular Science Monthly/Volume 71/September 1907/The Problem of Age, Growth and Death III: Link. Thanks to artist William Sillin for allowing us to use his lovely illustrations: http://www.willsillin.com/ (check it out – his illustrations are very cool!). Also take a look at this cool plate by Keibel and these lovely photos of chicken embryos: http://www.microscopy-uk.org.uk/mag/artnov04macro/mlchicken.html.
Things that helped me learn about this subject:
Image Credit
The Raptor Resource Project