Re: How does a bird or chicken develop inside of an egg?

Hi Carol!

Thanks for your question! Understanding how various organisms (e.g. chickens, mice, fish, and humans) develop is the major focus of the field of developmental biology. Development is a very complex series of highly coordinated events that result in the transformation of a fertilized egg, known as a zygote, into an adult organism. Unfortunately, I cannot tell you specifically how a chicken forms things like a brain, a heart, and bones, while inside the egg because we do not have all the answers yet. But, I can discuss some of the general processes that must occur in order for development to proceed normally.

The development of nearly all multicellular organisms begins with a single cell, the fertilized egg or zygote, which undergoes a series of cell divisions to generate all the cells of the adult. In the mature organism, there are several different types of cells--muscle cells, skin cells, and blood cells, to name just a few. The process through which a single cell can produce such a diverse array of cell types is known as differentiation. Once generated, these different kinds of cells also need to be organized structurally, into tissues and organs. This process is known as morphogenesis (the creation of form and structure). While these two processes are occurring, the developing organism must also undergo extensive growth (increase in size). So, to summarize, there are three principal features that characterize development: growth, differentiation, and morphogenesis. It is the details associated with each of these processes that developmental biologists are beginning to uncover and understand. Further, it is these details that are responsible for creating a complex, multicellular organism from a single cell.

I would like to emphasize that, within the shell of a chicken egg, there is a large amount of yolk. However, that is not all there is inside. Situated on top of the yolk is the zygote, which is formed by the union of an oocyte and sperm. As mentioned above, this is the cell from which all future cells of the organism are derived. Therefore, the zygote is essential for the development of a new adult organism. The yolk is important too, as it provides nutrients for the developing chick until it hatches from its shell.

I hope this information is helpful. Please feel free to contact me with any further questions! If you would like to read more, the information I have covered can be found in any general developmental biology text. I recommend Developmental Biology, fifth edition by Scott F. Gilbert. Below, I have listed some fun developmental biology web sites.

-Nikki
nmdavis@fas.harvard.edu

Zygote
This site is an addendum of sorts to Gilbert's developmental biology text.

The Virtual Embryo
This is a fantastic site that contains a wealth of information.

Morphing Embryos
This is a really neat site from Odyssey of Life, Nova Online, showing time-lapse movies of developing human, pig, chick, and fish embryos.