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|
|Chicken embryo at roughly two days incubation: equivalent to an eagle at about 3.5 days|
From 3.5 to 9 days: The amnion closes, sealing the developing embryo inside the egg's innermost membrane. The allantois forms to sequester liquid waste and exchange gases through the porous eggshell. Wings, tail, and leg buds form. Eyes develop pigment. Leg buds grow larger than wing buds and limbs begin to form. Elbow and knee joints are distinct by roughly 8 days and digital grooves - the precursors of toes and wing structures - are distinct by roughly nine days. The embryo's beak and tongue begin to form.
|Chicken embryos roughly 23 to 25% of the way to hatch|
From 11 to 17 days: The egg tooth and two scleral papillae form on the 11th-12th day. Limbs are bent. Dorsal feather germs form on the 12th day. A nictitating membrane is visible on day 13. Ventral feather germs develop, the eyelids begin closing, and flight feather germs develop.
|A chicken embryo roughly 50% of the way to hatch|
From 18 to 23 days: Leg scales, tiny talons, and plantar food pads appear. The eyelids are almost closed and the eyes are no longer quite as large in proportion to the rest of the head.
We are 65% of the way to hatch! Other than the closing of the body cavity, most major morphological changes are done. The eyelids will close, the eyes will grow into their sockets, the eaglet will develop natal down, it will internalize its egg yolk sac, its body cavity will close (leaving behind an egg yolk sac scar) and it will position itself for hatching!
So what happens right before hatch? Just a few days from external pip, the rapidly growing embryo is taking up nearly all the space in the egg. It...
- Turns so that its head is at the large end of the egg next to the air space.
- Pierces the internal membrane - the internal pip - and begins to breathe air with its lungs. Hatch has started!
- Takes the yolk sac into its body as it consumes most of the remaining albumen and yolk.
- Grows enough to contract the hatching muscle, pointing its head up and positioning its egg tooth against the shell of the egg. The eggshell is thinner and weaker than when it was laid, since the growing embryo absorbed calcium from the shell for its bones.
- Rubs its egg tooth against the shell, which cuts a small hole. We have an external pip!
- Rotates its body, slowly cutting a ring around the shell.
- Pushes its body against the shell, forcing the shell apart.
- Works itself free of the shell membranes and halves. The eaglet has landed and hatch is complete!
We are looking forward to hatch later this week! Curious about what's in store? Watch this 2015 video of the very final stages of hatch in N2.
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:
- Guide to Bald Eagle Egg Incubation and Chick-Rearing: http://www.iws.org/bald_eagles/Incubation%20Manual.PDF
- The Poultry Site: Embryonic development day by day
- The brood patch, the hatching muscle and what’s occurring inside our peregrine eggs
- Gastrulation and Neurulation: http://biology.kenyon.edu/courses/biol114/Chap14/Chapter_14.html
- What is a primitive streak? https://en.wikipedia.org/wiki/Primitive_streak
- What is a somite? https://en.wikipedia.org/wiki/Somite
- What are blood islands? https://en.wikipedia.org/wiki/Blood_islands