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54 Hand-Rearing Birds

which keep the yolk suspended in the egg and prevent the embryo from sticking to the shell. In
addition to serving as a source of water and protein for the embryo, the albumen contains antimi-
crobial proteins that are barriers to infection. Surrounding the albumen are two nonliving mem-
branes: the inner and outer shell membranes. The air cell forms between these two membranes, at
the blunt end of the egg in most species, initially as the egg contents cool and contract after laying
and during incubation through evaporation of water through the pores of the shell. The shell pro-
vides not only physical protection but the primary source of calcium and other minerals for the
developing embryo. The shell is covered by the cuticle layer which ranges from glossy to chalky to
barely visible in various species. This coating helps regulate water loss and, along with the shell
and shell membranes, provides a mechanical barrier to infection. The embryo begins to develop
before the egg is laid and does so initially in a flat plane, with the four extraembryonic membranes
extending outward from the body wall.
The yolk sac is the first of these living membranes to develop. It is a highly vascular structure,
initially forming a roughly circular pattern of vessels as it begins to envelope the yolk. Before other
membranes have formed and because, along with the embryo, it is in contact with the inner sur-
face of the shell membranes at the uppermost point of the egg, it serves as the first respiratory
organ for the embryo, providing a limited amount of gas exchange. As it grows, it becomes thicker
and develops folds lined with villi to increase surface area for the uptake of nutrients, much
like the mature intestinal wall. The yolk is the primary nutrient source for the embryo, and for the
chick even after hatch for a few days, and contains maternal antibodies, providing some passive
immunity to the embryo, which has no active immune function.
The amnion is the next membrane to develop, also extending outward from the body wall. This
nearly transparent membrane is minimally vascular. It grows up and over the embryo, eventually
closing and filling with fluid that will cushion the embryo during development. The amnion also
contains fine muscle fibers that contract rhythmically to prevent the embryo from sticking to it
until embryonic muscles are developed enough to perform this function.
The chorion is formed by the extension of the amnion folding back on itself from the seam
where it sealed over the embryo. It extends out to line the entire inner shell membrane, fusing with
it and with the allantois, which is the last membrane to form.
The allantois is highly vascular. It emerges balloon‐like from the hind gut of the embryo, con-
tinuing to expand and coming in contact with the chorion, with which it fuses, forming the chorio­
allantoic membrane (CAM) or chorioallantois. The fluid that expands and is contained by the
allantois is urinary waste, with uric acid crystals held in suspension until shortly before hatching
when the concentration causes them to precipitate out of suspension into strands of opaque, white
urates. The chorioallantois is the primary respiratory organ for the embryo throughout most of
incubation, allowing transpiration of oxygen, carbon dioxide, and water vapor from its dense vas-
cular network across the shell. It also forms the albumen sac, which ruptures its contents into the
amniotic fluid just after the middle of incubation, allowing the embryo to consume the albumen
protein that facilitates its final growth stage. Lastly, the chorioallantois transports calcium and
trace minerals, etched from the shell, to the developing embryo.

­ Hatchery­Facilities­and Equipment

Before the first egg arrives, the hatchery must be well organized and equipped. While few facilities
have the resources to build and equip an ideal hatchery, most of the following principles can be
achieved in any facility with some creativity and attention to detail.