1. Descriptive Embryology
We are all related!!!

3. BCR
Vertebrate embryos, for example, reptiles, birds, & mammals, have many similarities at the embryonic stage. They all have a notochord that will eventually become the backbone. A dorsal nerve cord will become the spinal cord, brain, and nerves. They all have gill pouches that permit breathing. Every vertebrate embryo has a heart with left and right sides that pump blood. All have a tail, which remains in some but not in others.
This evidence demonstrates that they have evolved from a common ancestor and share the same kinds of genes. It also shows that natural selection builds on what has come before, rather than starting from scratch.

4. I. Steps of Fertilization
A. Sperm lysins (think Drano) of millions of sperm, more than 80 million, dissolve the gel coat of an egg.
B. Receptor-mediated endocytosis occurs with one sperm and the outer egg membrane called the vitelline layer.
C. Egg Activation:

5. C. Egg Activation
1. The cell membrane ensures that only one sperm enters the egg. This is important because multiple fertilizations would result in genetic imbalances and a nonviable embryo. After the sperm enters the egg, a protective envelope called the fertilization membrane forms around the egg.
2. Syngamy – The fusion of the sperm nucleus and the egg nucleus. This results in formation of a single cell called a zygote.
3. Rotational and sliding movements of the egg’s cytoplasm begin. This results in a region where there is less yolk, more mitochondria, & more ribosomes called the animal pole. The cells of the animal pole sometimes multiply faster than the cells of the opposite, vegetal pole of the egg.

6. II. Embryonic Development
Once the zygote begins mitosis, it is called an embryo.
Cleavage – refers to how the cell divides during embryonic development
Blastomeres – the cells produced by cleavage; some of the blastomeres produce the yolk
Yolk – mixture of proteins, lipids, and glycogen that is the food for the developing embryo

7. Embryonic Development Cont’d
A. Egg sizes, cleavage patterns, and the length of embryonic periods of animal species are related to differences in the quantity and distribution of yolk in an egg.
1. Animals with relatively small amounts of yolk (amphibians and echinoderms) have larval stages that begin to feed after a short embryonic stage, and the yolk is entirely absorbed.
2. Animals with longer embryonic stages (birds and reptiles) have larger yolks.
3. Animals with extremely long embryo stages (mammals) have huge yolks that are continually supplemented by the mother’s blood supply (placenta).

8. Embryonic Development Cont’d
B. Within just a few hours of fertilization, the cells have divided enough to form a solid ball of cells called the morula.
C. The next step results in a hollow ball of cells called the blastula.
D. Now, if the animal has a larval stage, it occurs now. If not, then gastrulation occurs.

9. Embryonic Development Cont’d
E. Gastrulation
1. Next, the cells come together at the vegetal end and start to fold in on themselves creating a hollow cavity, the archenteron, and an opening called the blastopore.
2. The archenteron is what becomes the gut and the blastopore becomes….the other end.
3. With time, the archenteron will lengthen and become the coelom (what you know as your stomach cavity) and the second opening becomes the mouth.

10. III. Primary Germ Layers
A. Ectoderm – becomes the outer body wall and nervous system.
B. Endoderm – becomes the inner lining of the digestive cavity
C. Mesoderm – forms the internal structures
Differentiation of these layers forms specific, function-oriented cells, i.e., cardiac cells, respiratory cells, ocular cells, etc.

11. Pregnancy in Humans Pregnancy hormones
Human chorionic gonadotropin (hCG) is a hormone present in the blood within about a week of conception. It is the hormone detected in a blood or urine pregnancy test, but it usually takes three to four weeks for levels of hCG to be high enough to be detected by pregnancy tests. It is secreted by cells that develop into the placenta.

12. IV. Vertebrate Embryology
Vertebrates, or chordates, are animals with a back bone and spinal chord.
A. The chordate body plan consists of a notochord (what becomes the backbone), neural tube (what becomes the spinal chord), pharyngeal slits (gills to breathe), and a tail.
B. The amnion develops which encloses the embryo in a fluid-filled sac to protect against shock and drying out.
C. The chorion develops which aids in gas exchange (breathing).
D. The allantois develops which stores the baby’s waste.

13. V. Amphibian Embryology
A. Unlike mammals, amphibians lay unfertilized eggs in a watery environment. The eggs are fertilized by sperm AFTER they leave the female’s body.
B. Amphibian embryos develop into a larval stage (tadpole) before metamorphosizing into an adult amphibian.

14. VI. Avian Embryology A. Birds lay eggs.
B. Yolk is the yellow part. Albumen is the white part.
C. As hatching approaches, the chick pecks into the air pocket with its beak, its lungs then inflate, and it starts to breathe air.

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