Human Ovum (Egg) and Sperm © Dennis Kunkel

In sexual reproduction, two parents donate genes to the young, resulting in offspring with a mix of inherited genes. These genes are donated through a process called fertilization. There are two mechanisms by which fertilization can take place. The first is external (the eggs are fertilized outside of the body), and the second is internal (the eggs are fertilized within the female reproductive tract).

External fertilization occurs mostly in wet environments and requires both the male and the female to release their gametes into their surroundings (usually water). An advantage of external fertilization is that it results in the production of a large number of offspring. Animals that use internal fertilization specialize in the protection of the developing egg. For example, reptiles and birds secrete eggs that are covered by a protective shell that is resistant to water loss and damage. Mammals, with the exception of monotremes, take this idea of protection a step further by allowing the embryo to develop within the mother.

An artery is an elastic blood vessel that transports blood away from the heart. There are two main types of arteries: pulmonary arteries and systemic arteries. Pulmonary arteries carry blood from the heart to the lungs where the blood picks up oxygen. The oxygen rich blood is then returned to the heart via the pulmonary veins. Systemic arteries deliver blood to the rest of the body. The aorta is the main systemic artery and the largest artery of the body. It originates from the heart and branches out into smaller arteries which supply blood to the head region, the heart itself, and the lower regions of the body.

Think you know all about arteries and the heart? Test your knowledge of blood vessels and heart anatomy, take the Heart Anatomy Quiz!

Photo courtesy FreeDigitalPhotos.net

Scientists have identified the location of the neurons in the brain that are responsible for fear conditioning. The neurons are found in the region of the brain called the amygdala in mammals. The amygdala is a limbic system structure that is involved in many of our emotions and motivations, particularly those that are related to survival.

Fear conditioning is an associative learning process by which we learn through repeated experiences to fear something. Our experiences can cause brain circuits to change and form new memories. This particular study was centered on how and where fear memories are formed in the brain. The information gained may be useful for understanding phobias and anxiety disorders.

For additional information, see:

• Neuroscientists Locate Where Fear Is Stored In The Brain (Science Daily)

Can you identify the stage of meiosis that the following cell is in?

Lily Anther © Jon C. Glase

• Prophase II
  
• Telophase II
  
• Metaphase II
  
• Anaphase II

Test your knowledge of meiosis by taking the Meiosis Quiz!

Don't know much about meiosis? It is a two-part cell division process in organisms that sexually reproduce. To learn more about meiosis before taking the quiz, visit the Meiosis Study Guide.