In developmental biology, an embryo is divided into two hemispheres: the animal pole and the vegetal pole within a blastula. The animal pole consists of small cells that divide rapidly, in contrast with the vegetal pole below it. In some cases, the animal pole is thought to differentiate into the later embryo itself, forming the three primary germ layers and participating in gastrulation. The vegetal pole contains large yolky cells that divide very slowly, in contrast with the animal pole above it. In some cases, the vegetal pole is thought to differentiate into the extraembryonic membranes that protect and nourish the developing embryo, such as the placentain mammals and the chorion in birds. The development of the animal-vegetal axis occurs prior to fertilisation. Sperm entry can occur anywhere in the animal hemisphere. The point of sperm entry defines the dorso-ventral axis - cells opposite the region of sperm entry will eventually form the dorsal portion of the body a. Males release so many sperm that the egg is covered by them. b. The egg has a plasma membrane, a vitelline envelope, and a jelly coat. c. Acrosome enzymes digest away the zona pellucida around the egg as it extrudes a filament that attaches to a receptor on the vitelline jelly layer envelope. d. This interaction between filament and receptor is a lock-and-key reaction that is species- specific. e. The egg plasma membrane and the sperm nuclear membrane fuse, allowing the nucleus to enter. f. Fusion takes place and the zygote begins development. g. As soon as the plasma membranes of sperm and egg fuse, the plasma membrane and the vitelline envelope undergo changes that prevent entrance of any other sperm. h. The vitelline envelope now becomes the fertilization envelope. Early Developmental Stages 1. Development includes events and processes that occur as a single cell becomes a complex organism. 2. All chordate embryos go through same early developmental stages: zygote, morula, blastula, early and late gastrula. 3. The presence of yolk, dense nutrient material, affects how the embryonic cells complete the first three stages. 4. Following fertilization, a zygote undergoes cleavage, cell division without growth. a. DNA replication and mitosis occur repeatedly; the cells get smaller each division. b. As deuterostomes, lancelets have a radial and indeterminate pattern of cleavage. 1) In radial cleavage, any plane passing through will divide the embryo into symmetrical halves. 2) In indeterminate cleavage, cells have not differentiated; if separated, each one develops a complete organism. 5. Because the lancelet has little yolk, the cell divisions are equal in the resulting morula. 6. A cavity called the blastocoel develops forming the hollow ball called the blastula. 7. Gastrulation is invagination of some cells of the blastocyst into blastocoel to form three primary germ layers. a. The outer layer of cells becomes the ectoderm. b. The inner layer of cells becomes the endoderm. c.

1. In developmental biology, an embryo is divided into two hemispheres: the animal pole and the
vegetal pole within a blastula.
The animal pole consists of small cells that divide rapidly, in contrast with the vegetal pole
below it. In some cases, the animal pole is thought to differentiate into the later embryo itself,
forming the three primary germ layers and participating in gastrulation.
The vegetal pole contains large yolky cells that divide very slowly, in contrast with the animal
pole above it. In some cases, the vegetal pole is thought to differentiate into the extraembryonic
membranes that protect and nourish the developing embryo, such as the placentain mammals and
the chorion in birds.
The development of the animal-vegetal axis occurs prior to fertilisation. Sperm entry can occur
anywhere in the animal hemisphere. The point of sperm entry defines the dorso-ventral axis -
cells opposite the region of sperm entry will eventually form the dorsal portion of the body
a. Males release so many sperm that the egg is covered by them.
b. The egg has a plasma membrane, a vitelline envelope, and a jelly coat.
c. Acrosome enzymes digest away the zona pellucida around the egg as it extrudes a filament
that attaches to a receptor on the vitelline jelly layer envelope.
d. This interaction between filament and receptor is a lock-and-key reaction that is species-
specific.
e. The egg plasma membrane and the sperm nuclear membrane fuse, allowing the nucleus to
enter.
f. Fusion takes place and the zygote begins development.
g. As soon as the plasma membranes of sperm and egg fuse, the plasma membrane and the
vitelline envelope undergo changes that prevent entrance of any other sperm.
h. The vitelline envelope now becomes the fertilization envelope.
Early Developmental Stages
1. Development includes events and processes that occur as a single cell becomes a complex
organism.
2. All chordate embryos go through same early developmental stages: zygote, morula, blastula,
early and late gastrula.
3. The presence of yolk, dense nutrient material, affects how the embryonic cells complete the
first three stages.
4. Following fertilization, a zygote undergoes cleavage, cell division without growth.
a. DNA replication and mitosis occur repeatedly; the cells get smaller each division.
b. As deuterostomes, lancelets have a radial and indeterminate pattern of cleavage.
1) In radial cleavage, any plane passing through will divide the embryo into symmetrical halves.

2. 2) In indeterminate cleavage, cells have not differentiated; if separated, each one develops a
complete organism.
5. Because the lancelet has little yolk, the cell divisions are equal in the resulting morula.
6. A cavity called the blastocoel develops forming the hollow ball called the blastula.
7. Gastrulation is invagination of some cells of the blastocyst into blastocoel to form three
primary germ layers.
a. The outer layer of cells becomes the ectoderm.
b. The inner layer of cells becomes the endoderm.
c. The cavity created by the invagination becomes the primitive gut or archenteron.
d. The pore created by the invagination is the blastopore; in the lancelet, this becomes the anus.
e. The outer ectoderm gives rise to epidermis of skin, epithelial lining of mouth and rectum, and
nervous system.
f. The inner endoderm gives rise to the epithelial lining of the digestive tract and respiratory
tract, the associated glands of the digestive system and the respiratory system, and the lining of
the urinary bladder.
g. Middle mesoderm gives rise to skeleton, dermis of skin, skeletal system, muscular system,
excretory system, reproductive system (including most epithelial linings), and outer layers of
respiratory and digestive systems.
8. The germ layers develop into future organs,
Solution
In developmental biology, an embryo is divided into two hemispheres: the animal pole and the
vegetal pole within a blastula.
The animal pole consists of small cells that divide rapidly, in contrast with the vegetal pole
below it. In some cases, the animal pole is thought to differentiate into the later embryo itself,
forming the three primary germ layers and participating in gastrulation.
The vegetal pole contains large yolky cells that divide very slowly, in contrast with the animal
pole above it. In some cases, the vegetal pole is thought to differentiate into the extraembryonic
membranes that protect and nourish the developing embryo, such as the placentain mammals and
the chorion in birds.
The development of the animal-vegetal axis occurs prior to fertilisation. Sperm entry can occur
anywhere in the animal hemisphere. The point of sperm entry defines the dorso-ventral axis -
cells opposite the region of sperm entry will eventually form the dorsal portion of the body
a. Males release so many sperm that the egg is covered by them.
b. The egg has a plasma membrane, a vitelline envelope, and a jelly coat.

3. c. Acrosome enzymes digest away the zona pellucida around the egg as it extrudes a filament
that attaches to a receptor on the vitelline jelly layer envelope.
d. This interaction between filament and receptor is a lock-and-key reaction that is species-
specific.
e. The egg plasma membrane and the sperm nuclear membrane fuse, allowing the nucleus to
enter.
f. Fusion takes place and the zygote begins development.
g. As soon as the plasma membranes of sperm and egg fuse, the plasma membrane and the
vitelline envelope undergo changes that prevent entrance of any other sperm.
h. The vitelline envelope now becomes the fertilization envelope.
Early Developmental Stages
1. Development includes events and processes that occur as a single cell becomes a complex
organism.
2. All chordate embryos go through same early developmental stages: zygote, morula, blastula,
early and late gastrula.
3. The presence of yolk, dense nutrient material, affects how the embryonic cells complete the
first three stages.
4. Following fertilization, a zygote undergoes cleavage, cell division without growth.
a. DNA replication and mitosis occur repeatedly; the cells get smaller each division.
b. As deuterostomes, lancelets have a radial and indeterminate pattern of cleavage.
1) In radial cleavage, any plane passing through will divide the embryo into symmetrical halves.
2) In indeterminate cleavage, cells have not differentiated; if separated, each one develops a
complete organism.
5. Because the lancelet has little yolk, the cell divisions are equal in the resulting morula.
6. A cavity called the blastocoel develops forming the hollow ball called the blastula.
7. Gastrulation is invagination of some cells of the blastocyst into blastocoel to form three
primary germ layers.
a. The outer layer of cells becomes the ectoderm.
b. The inner layer of cells becomes the endoderm.
c. The cavity created by the invagination becomes the primitive gut or archenteron.
d. The pore created by the invagination is the blastopore; in the lancelet, this becomes the anus.
e. The outer ectoderm gives rise to epidermis of skin, epithelial lining of mouth and rectum, and
nervous system.
f. The inner endoderm gives rise to the epithelial lining of the digestive tract and respiratory
tract, the associated glands of the digestive system and the respiratory system, and the lining of
the urinary bladder.

4. g. Middle mesoderm gives rise to skeleton, dermis of skin, skeletal system, muscular system,
excretory system, reproductive system (including most epithelial linings), and outer layers of
respiratory and digestive systems.
8. The germ layers develop into future organs,