Laboratory Activity 6: Early Chick Development

Question 1

At the blunt end of the chick egg is an air space. What do you think its functions might be?

Answer 1

The air space at the blunt end of the chick egg forms as the egg cools after being laid, due to the contraction of its contents.

This space serves as a vital respiratory area, allowing gas exchange between the embryo and the environment.

As the embryo develops and its oxygen demand increases, the air space enlarges to accommodate this need. This ensures the growing embryo has access to oxygen, particularly before hatching.

Question 2

Why do you think the initial cleavages of the chick egg are meroblastic (incomplete) rather than holoblastic (complete)?

Answer 2

Because of the large amount of yolk present.

The dense yolk inhibits complete cleavage, restricting cell division to the small blastodisc at the animal pole. As a result, only the blastodisc undergoes cleavage, leading to an incomplete or partial division.

This type of cleavage allows the embryo to develop while still utilizing the yolk as a nutrient source.

Question 3

What is the primitive streak, and how is it formed?

Answer 3

The primitive streak is a structure that serves as the main site of cell migration during gastrulation, establishing the body’s longitudinal axis.

It forms through the thickening of epiblast cells at the posterior end of the embryo. These cells proliferate and migrate toward the midline, elongating into a linear structure.

This streak guides mesodermal and endodermal cells inward, playing a crucial role in forming the three germ layers and determining the body plan of the developing chick.

Question 4

During chick gastrulation, what germ-layer regions remain external and do not move inward?

Answer 4

Ectoderm (outermost germ layer)

• gives rise to structures such as the epidermis (skin, hair, and nails) and the nervous system (neural tube and neural crest cells). Its external position allows it to contribute to surface structures and neural development.

Question 5

What regions of mesoderm can be identified in the 24-hour chick embryo?

Answer 5

The mesoderm in the chick embryo differentiates into three
distinct regions, each giving rise to specific structures.

A. The paraxial mesoderm, located adjacent to the neural tube, forms somites that later develop into the vertebrae, skeletal muscles, and dermis.
B. The intermediate mesoderm, positioned between the paraxial and lateral plate mesoderm, gives rise to the kidneys, gonads, and adrenal glands.
C. The lateral plate mesoderm contributes to the formation of the cardiovascular system, the lining of body cavities, and limb structures.

Question 6

Parts of the Egg

Answer 6

Egg white (Albumin)
Germinal vesicle (Blastodisc)
Egg yolk (Chick ovum)
Chalaza

Question 7

Shell membranes:

Answer 7

Cuticular membrane
Spongy layer
Mamillary layer
Inner shell membrane

Air space

Question 8

24-HOUR CHICK EMBRYO

Area vitellina
Proamnion
Foregut
Headfold
Neural tube
Lateral plate mesoderm
Open neural plate
Hensen’s node
Primitive streak
Anterior neuropore
Area vasculosa
Somites
Unsegmented somitic mesoderm
Area pellucida

Question 9

characterized by a shell and extra-embryonic membranes (amnion, allantois, chorion, and yolk sac) that protect and support the developing embryo, allowing amniotes (reptiles, birds, and mammals) to lay eggs on land or retain them within the mother.

Answer 9

Amniote egg

Question 10

_______ protects against physical damage and dehydration while still allowing for gas exchange

Answer 10

outer shell

Question 11

Packed within is enough food and water to survive the long journey to hatching. In the chick, it takes ____ days.

Answer 11

21

Question 12

Seasoned smoked sausage made of beef and pork

Answer 12

Frankfurter

Question 13

In a medium-size chick egg (about 60 gm), these provisions include more protein (7.2 gm) than there is in a frankfurter; as much fat (6 gm) as is in 2 teaspoons of butter but with about seven times the cholesterol (300 mg); and about 8.5 teaspoons of water (40 gm).

Question 14

Which end of an egg marks the region of an air space

Answer 14

Blunt end

Question 15

Most of the bulk in the chick ovum is not cytoplasm, but ___________ suspended in a sea of ________

Answer 15

spheres of yolk lipid;
yolk protein

Question 16

What is the classification of chicken egg in terms of yolk content?

Answer 16

macrolecithal

Question 17

98% of eggshell is ________, which is __________.

Answer 17

calcite;
calcium carbonate

Question 18

_______ is the major component of bone as well (in the form of ________, or ________).

Answer 18

Calcium;
calcium phosphate;
apatite

Question 19

The calcium that goes into the shell comes both from the hen’s diet and from her bones.

Question 20

3 layers of eggshell: 2 calcified layers, the inner _______ and
the outer _________; and outermost the thin shiny ________.

Answer 20

mammillary layer;
spongy (or crystalline) layer;
cuticular membrane

Question 21

The outer cuticular membrane is made of ________.

Answer 21

glycoprotein

Question 22

The outer cuticular layer, prior to laying, it is wet and slippery, which helps with _________, the process of laying eggs, but soon after laying it dries out and serves to protect against invasion by microorganisms.

Answer 22

oviposition

Question 23

The pores in the eggshell are more abundant in this area

Answer 23

Over the air space

Question 24

If air could not pass through the shell, what would happen to the embryo?

Answer 24

the embryo would lack oxygen and build up carbon dioxide, leading to respiratory failure and ultimately, death.

Question 25

Function of shell membranes

Answer 25

retard water loss give mechanical support provide a defense against microbes

Question 26

Function of egg albumen

Answer 26

88% water = major source of water for the developing embryo The rest is glycoproteins, the most abundant of which is ovalbumen

Question 27

Some of the proteins in the albumen serve antibacterial functions:

Answer 27

1. Lysozyme = enzyme that disrupts bacterial cell walls 2. Ovotransferin = binds iron 3. Avidin = binds the vitamin biotin 4. Ovomucin and cystatin = thought to have antiviral activity

Question 28

the albumen has a pH that prevents bacterial growth...

Answer 28

Alkaline pH ~10

Question 29

Very dense and is coiled into 2 cordlike structures attached tightly to the yolk membrane. These are the _______. They suspend the yolk in the middle of the albumen and allow it to rotate.

Answer 29

chalazae

Question 30

The yolk will rotate, always orienting to gravity so that ________.

Answer 30

its animal pole (and the embryo) face upward

Question 31

Do the fibers twist the same way? The twisting occurs during egg formation as the yolk is slowly rotated during its descent through the oviduct

Answer 31

No, the fibers of the two chalazae in an egg twist in opposite directions, anchoring the yolk and keeping it centered within the egg.

Question 32

Where does the yolk sit in relation to the albumen? Does it float, or is it submerged?

Answer 32

The yolk floats in the albumen The albumen is slightly denser than the yolk

Question 33

It is here that the egg nucleus is found

Answer 33

Blastodisc

Question 34

If you see a spot of blood in the yolk, this can be either an indication of _______ or __________

Answer 34

a developing embryo; simply the incorporation of a blood clot from the ovary at the time of ovulation

Question 35

The yellow color of the yolk is due to ________, the same pigments seen in carrots and other yellow vegetables. The ---------- are absorbed from the hen’s food and deposited rapidly in the yolk.

Answer 35

carotenoid pigments

Question 36

Why is there concentric layers of dark and light yolk?

Answer 36

Greater amounts of carotenoid pigments are deposited during the day when the hen is eating than at night when the hen is sleeping. Each sequence of a light and dark ring equals one day.

Question 37

The female chick has only one fully developed ovary and oviduct (usually the left), while the other remains vestigial.

Answer 37

likely due to evolutionary adaptations for flight, reducing weight.

Question 38

Fertilization takes place high in the oviduct, in the region called the _______.

Answer 38

infundibulum

Question 39

Path of egg in the female reproductive system of a chicken

Answer 39

Mature ovum in Ovary Fimbriae Infundibulum Magnum Isthmus Uterus (shell gland) Vagina Cloaca

Question 40

After ovulation, the egg remains fertilizable for only about ________. This does not mean, however, that mating must have occurred near the time of ovulation. Sperm can be stored in the female tract and remain viable there for as long as _______, though viability is reduced after the ________.

Answer 40

40 minutes; 35 days; third week

Question 41

The mouth of the oviduct is not attached to the ovary. It is not a given that every egg that is ovulated successfully enters the oviduct. In fact, as many as ____ of eggs ovulated normally miss the opening and fall into the body cavity, where they are usually absorbed.

Answer 41

5%

Question 42

After passing through the infundibulum, albumen begins to be secreted around the yolk. Most of the albumen is secreted by cells in the region of the oviduct called the ________.

Answer 42

magnum

Question 43

The egg passes from the magnum through the isthmus, where the _______ and the _________ are added

Answer 43

chalazae; inner and outer shell membranes

Question 44

In the ______, also called the shell gland, _________ traverse the shell membranes and are added to the albumen, plumping out the shell membranes.

Answer 44

uterus; water and salts

Question 45

Having traveled with its small end first, its orientation is now reversed, and the egg is laid with the large blunt end first.

Question 46

The entire process from ovulation to laying takes about _______, the major portion of which (about 19 hours) is spent in the _______.

Answer 46

24 hours; shell gland

Question 47

If the egg was fertilized, development of course has been taking place all this time, and the embryo is already undergoing gastrulation by the time it is laid.

Question 48

Cleavage stages cannot be observed in the living egg under normal conditions, since they occur while the egg is still in the oviduct.

Question 49

________ divides this single cell into a lot of smaller building blocks that then can be rearranged and molded into the multicellular organism.

Answer 49

Cleavage

Question 50

What type of cleavage does chicken egg undergo

Answer 50

Meroblastic cleavage

Question 51

The cleavage pattern in the chick is also often called _________, since it is restricted to the circular disc of cytoplasm, the blastodisc.

Answer 51

discoidal

Question 52

In avian eggs, a blastodisc is the small, white, embryo-forming disc on the yolk's surface, and when fertilized, it becomes a _______, a single-layered embryonic tissue that will develop into the embryo.

Answer 52

blastoderm

Question 53

The cells produced during cleavage are called _______, and together they make up what is called the _________

Answer 53

blastomeres; blastoderm

Question 54

In the center of the blastoderm, the cells become separated from the underlying yolk by a space called the __________

Answer 54

subgerminal cavity

Question 55

The space makes the central area of blastoderm look lighter and more translucent than the surrounding area. This central region is called the __________.

Answer 55

area pellucida

Question 56

The surrounding area of blastoderm, still connected to the underlying yolk, looks darker and opaque; it is called the _________.

Answer 56

area opaca

Question 57

Late cleavage stages in the chick occurs within the oviduct, 3-4 hours prior to laying.

Question 58

By the time the egg is laid, the embryo is undergoing gastrulation.

Question 59

maps have been constructed showing the location of each germ layer prior to gastrulation and its route of travel during gastrulation.

Answer 59

Fate maps

Question 60

What does gastrulation achieve?

Answer 60

It separates the germ layers so that they are in appropriate positions for the organ formation that will follow.

Question 61

Since _______ forms the epithelium of the gut and gut derivatives, it must become a centrally located tube. The _______, which forms the epidermis and nervous system, must cover the outside. And in between must come all the packing material provided by the _________—muscle, skeleton, circulatory system, kidney, and dermis

Answer 61

endoderm; ectoderm; mesoderm

Question 62

It is only the central region of the gastrula sandwich that will form the body of the embryo. The outer regions will form extraembryonic membranes: the ______, _______, _______, and ________.

Answer 62

amnion; chorion; allantois; yolk sac

Question 63

Function of amnion and chorion

Answer 63

Protect the embryo

Question 64

Function of allantois

Answer 64

Stores nitrogenous waste and where it is attached to the chorion provides a vast respiratory surface that also resorbs calcium from the eggshell for the growing embryo

Question 65

Function of yolk sac

Answer 65

absorbs yolk and transports it back to the embryo in its vitelline blood vessels

Question 66

The first step in chick gastrulation is the splitting off of the major bulk of endoderm from the underside of the blastoderm to form a layer of endoderm below the rest of the cells. This splitting is called _________.

Answer 66

delamination

Question 67

Delamination produces an upper epiblast, consisting of the ________, ________, and some ___________; and a lower hypoblast, consisting of the ___________.

Answer 67

presumptive ectoderm; mesoderm; endoderm; extraembryonic endoderm

Question 68

_________, will form the endodermal derivatives of the embryo itself, but it must first join the hypoblast before doing so.

Answer 68

embryonic endoderm

Question 69

The line along which cells are moving inward from the epiblast, or ingressing, is called the ________.

Answer 69

primitive streak

Question 70

The prospective mesoderm cells are the next to enter the primitive streak and leave the epiblast. However, they do not move down to join the hypoblast but instead make a U-turn and migrate outward between the epiblast and hypoblast to form a separate layer between the two.

Question 71

In the chick embryo at the top of the primitive streak called _________ (or ________). It is here that the notochordal cells converge, ingress, and migrate forward, rather than laterally, to form a streak of notochordal cells down the midline.

Answer 71

Hensen’s node; primitive node

Question 72

By 24 hours of incubation, the anterior half of the embryo is undergoing ________ while the posterior half is still gastrulating.

Answer 72

neurulation

Question 73

24-Hour Chick Whole Mount Neurulation Area opaca has greatly expanded

Question 74

_______ are masses of blood-forming cells that will later anastomose to form a capillary network that will bring yolk nutrients back to the embryo proper.

Answer 74

Blood islands

Question 74

2 subdivisions of area opaca:

Answer 74

1. Outer area vitellina (vitellina means yolk), where cells contain many yolk granules 2. Inner area vasculosa (referring to vascularization), where blood islands are forming.

Question 75

______, the clear area around the embryo; it looks almost like a footprint.

Answer 75

area pellucida

Question 76

Just in front of the embryo’s head is a region of the area pellucida that is particularly clear. This is called the ___________

Answer 76

proamnion

Question 77

The reason the proamnion is so translucent is that it ______________

Answer 77

consists only of ectoderm and endoderm

Question 78

This is where an anterior fold has undercut the developing head and raised it above the level of the blastoderm.

Answer 78

head fold

Question 79

The head fold has caused the endoderm to fold into a closed tube to form the ________.

Answer 79

foregut

Question 80

Posteriorly, where the endoderm is still flat, is the open midgut, and the transition between the closed foregut and open midgut is the _________

Answer 80

anterior intestinal portal

Question 81

Previously it was an open neural plate, but by 24 hours, the anterior region has folded upward to form a closed neural tube. A major part of this folding is caused simply by a change in shape of the neural plate cells from cuboidal to _________

Answer 81

truncated-pyrimidal

Question 82

Notice that at the anterior tip of the embryo, the neural plate does not close completely but leaves an open channel called the ___________. This will be closed by 36 hours.

Answer 82

anterior neuropore

Question 83

As the embryo develops, additional somites will form from the more posterior ____________.

Answer 83

unsegmented somite mesoderm

Question 84

dye for staining blastoderm

Answer 84

Neutral red solution

Question 85

Area pellucida forms the circulatory system of the chick. At the top of the embryo is a flap of tissue called the neural crest, this is the beginning of the chick's brain. Running the length of the embryo is a dark line called the neural groove, this is the beginning of the chick's spinal cord. In the center of the neural groove is a light line called the primitive streak. This is a row of cells that form the digestive organs, nervous system, and muscles of the chick. About halfway down the neural groove, we see 2 rows of round structures. 1 row on each side of the neural groove, these are called somites and they are clusters of cells that will develop into the bones, ligaments, and tendons of the chick.

Question 86

In mounting, _______ is used for permanent slide and __________ for temporary slide preparation

Answer 86

DPX; Glycerol