From Egg To Chick:
Development and Preservation of
Embryos
STAGES OF
EMBRYONIC DEVELOPMENT
Shortly after the ovum has been released from the
hen's ovary, it is picked up by the funnel or
infundibulum. Sperm from the male are found in the
folds of the infundibulum. Soon after the ovum is
picked up by the infundibulum, many sperm contact
the germinal disc, but only one unites with the
germ. Thus fertilization occurs about 24 hours
before the egg is laid.
Since the fertilized germinal disc, or blastoderm,
spends about 24 hours in the warmth of the hen's
body (about 107o F (42 C) while the egg
is being completed, certain stages of embryonic
development occur during that time. About three hours
after fertilization the newly formed single cell
divides and makes two cells. Then there are four,
eight, sixteen, and more. Cell division continues
until there are many cells grouped in a small, whitish
spot visible on the upper surface of the egg yolk.
When the egg is laid and its temperature drops
below about 80o F (27 C), cell
development ceases. Cooling at ordinary temperatures
will not kill the embryo, and it will begin to develop
again when the egg is placed in the incubator. Keeping
eggs at temperatures above about 80o F (27
C) prior to incubation will cause a slow growth which
leads to a weakening and eventual death of the
embryo.
During incubation various processes occur. They are
mainly respiration, excretion, nutrition, and
protection. Extraembryonic membranes are membranes
outside the embryo's body which make these functions
possible (Fig. 8). The extraembryonic membranes are
the yolk sac, amnion, allantois, and chorion.
The yolk sac is a layer of tissue growing over the
surface of the yolk. Its walls are lined with a
special tissue which digests and absorbs the yolk
material. The amnion is a transparent sac filled
with a colorless fluid. The amnion and amniotic
fluid provide protection from mechanical shock and
permit the developing embryo to exercise.
Respiration by the embryo is made possible by the
allantois. Blood vessels in the allantois bring oxygen
to the embryo and take carbon dioxide away. The
allantois also stores excretions, absorbs albumen used
as food by the embryo, and absorbs calcium from the
shell for the structural needs of the embryo. The
allantois ceases to function when the chick punctures
the air cell and starts to breathe on its own.
A fourth membrane (not shown in Fig. 8), the
chorion, surrounds both the amnion and yolk
sac. Initially the chorion has no apparent function,
but later the allantois fuses with it to form the
chorio-allantois membrane. None of these
extraembryonic membranes become a part of the
chick.
STUDYING EMBRYONIC DEVELOPMENT
You can study the external form of chicken embryos
at various stages of development. Beginning with the
3rd or 4th day of incubation you can, by carefully
breaking open an egg each day, observe many events in
the development of embryos. And it is easy to preserve
the embryos for display or additional study.
To study the early embryonic development from the
beginning to the 4th day of incubation, you will need
to prepare a set of whole mounts and observe them
under a microscope. This is a rather exacting
procedure which requires considerable time. It should
not be attempted by the younger student or 4-H
member.
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Seven-day old embryo with its embryonic
membranes a embryonic blood vessels. (Slightly
modified from Duval, A. L. Romanoff, Cornell
Rural School Leaflet, September, 1939.) (Fig. 8)
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Bone formation and growth can be studied by
clearing and staining the embryo. This process
requires advanced and exacting techniques,
considerable time and equipment, and some financial
outlay. Therefore, clearing and staining of embryos
should be at tempted only by the advanced student
under the supervision of an experienced person.
If you plan to study the different stages of
embryonic development by one or more of the above
methods, be sure to incubate enough eggs for this
purpose. The stages of embryonic development are
listed in the table below (see also Figs. 9 and
10). You will not be able to see all of these with the
naked eye. But just by breaking and observing an egg
each day you can identify many of the
stages.
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Successive changes in the position of the
chick embryo and its embryonic
membranes. (From A. L. Romanoff, Cornell Rural
School Leaflet, September, 1939.) (Fig. 9)
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Preserving embryos. You can either break
open an egg and preserve the embryo each day, or you
can set eggs on succeeding days and do them all at
one time. Most students prefer to do one egg each
day, because doing all stages at one time will
require several hours.
Break the egg gently in the region of the air cell,
which is at the large end of the egg. Use blunt
tweezers to peel away the upper portion of the shell
and shell membranes and to separate the embryo from
the remainder of the egg. Remove all extraembryonic
membranes, and with small scissors sever the
umbilical stalk near the body wall. Wash the embryo
gently and thoroughly in running tap water to insure
a nice, clean appearance. Be careful not to lose the
embryo, especially those in the early stages of
development.
If you want to make close-up pictures or slides,
place the embryo in a small petri dish or similar
container. Then to preserve the embryo, place it in
a 10 percent solution of formalin (1 part of 37
percent formaldehyde with 9 parts of water) in glass
jars with screw caps. Small baby food jars are
excellent for this purpose. Use a gallon jug or
similar container to prepare the formalin - never
use a metal container.
Label each jar with pertinent information. This
might include species, variety, age of embryo, date
and similar information.
Preparing whole mounts. Early embryonic
development between zero and four days of incubation
can be studied by preparing whole mounts of embryo
and examining them under a microscope. This is
difficult procedure, so you will need to arrange
with an experienced person for guidance and
counsel.
The first step in preparing whole mounts is fixing
the embryo, that is, preserving the actual form of
the embryo and producing optical differences in its
structure or making it possible to produce these
difference in subsequent treatment.
The chick embryo, albumen, and yolk must be removed
moved from the shell and the embryo then separated
from the albumen and yolk before the embryo can
fixed. Since the early stages of embryonic
development are the most difficult to work with, you
should start with a 4- or 5-day-old embryo and work
back to the 1-day embryo.
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Daily changes in the weight and form of the
developing chick embryo - in this case White
Leghorn. (Based on A. L. Romanoff, Cornell Rural
School Leaflet, September, 1939.) Illustration
Copyright © 1998, Jill Hixon and
the University of Illinois. Not the same
illustration as appears in the original
text, but close and more colorful. (Fig. 10)
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Advanced students who want to prepare whole mounts
of embryos can find the procedures in such works as
Animal Micrology, Introduction to Vertebrate
Embryology, and Handbook of Microscopal
Technique (see the reference list on page
16).
Clearing and staining embryos. Bone
formation and growth in chick embryos can be studied
by clearing and staining the embryos. First the
embryo must be fixed. Then it must be cleared, that
is, made more I transparent. Finally it is stained
with a color such as alizarin red S, so that the
skeletal structure can be readily seen.
As is true of preparing whole mounts, clearing and
staining requires advanced techniques. So it should
not be attempted except by the advanced student
under the supervision of a qualified
person. Procedures for clearing and staining embryos
can be found in such references as Introduction
to Vertebrate Embryology, Staining the
Skeleton of Cleared Embryos with Alizarin Red
S, Alizarin Red S and Toluidine Blue for
Differentiating Adult or Embryonic Bone and
Cartilage, and Bone Development in the
Chick Embryo. (See the reference
list.)
IMPORTANT EVENTS IN EMBRYONIC
DEVELOPMENT
(From A. L. Romanoff - Cornell Rural School
Leaflet, September 1939.)
BEFORE
EGG LAYING
- Fertilization
- Division and growth of living cells
- Segregation of cells into groups of special
function
BETWEEN LAYING AND
INCUBATION
- No growth; stage of inactive embryonic
life
DURING INCUBATION
FIRST DAY:
16 hours - First sign of
resemblance to a chick embryo
18 hours - Appearance of
alimentary tract
20 hours - Appearance of
vertebral column
21 hours - Beginning of
formation of nervous system
22 hours - Beginning of
formation of head
23 hours - Appearance of
blood islands - vitelline circulation
24 hours - Beginning of
formation of eye
SECOND DAY:
23 hours - Beginning of
formation of heart
35 hours - Beginning of
formation of ear
42 hours - Heart begins to
beat
THIRD DAY:
50 hours - Beginning of
formation of amnion
60 hours - Beginning of
formation of nose
62 hours - Beginning of
formation of legs
64 hours - Beginning of
formation of wings
70 hours - Beginning of
formation of allantois
FOURTH DAY:
Beginning of formation of
tongue
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FIFTH DAY:
Beginning of formation of
reproductive organs and differentiation of
sex
SIXTH DAY:
Beginning of formation of
beak and eggtooth
EIGHTH DAY:
Beginning of formation of
feathers
TENTH DAY:
Beginning of hardening of
beak
THIRTEENTH DAY:
Appearance of scales and
claws
FOURTEENTH DAY:
Embryo turns its head toward
the blunt end of egg
SIXTEENTH DAY:
Scales, claws, and beak
becoming firm and horny
SEVENTEENTH DAY:
Beak turns toward air
cell
NINETEENTH DAY:
Yolk sac begins to enter
body cavity
TWENTIETH DAY:
Yolk sac completely drawn
into body cavity; embryo occupies practically
all the space within the egg except the air
cell
TWENTY-FIRST DAY:
Hatching of chick
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