Reproductive System

There are two different fundamental methods that organisms use to reproduce: asexual reproduction, which is a form of reproduction from a single parent that makes a clone of itself through mitosis, and sexual reproduction, which is when two parents produce offspring with unique combinations of genes from both parents. Some multicellular animals, like corals, use both asexual and sexual reproduction, while others, like mammals, only reproduce sexually.

The sequence of events in sexual reproduction starts the same way in all sexually reproducing animals - with gametogenesis, which is the formation of gametes. You may remember that gametes are specialized sex cells produced by each parent for sexual reproduction. There are two types of gametes: sperm and ova (ovum).

Sperm, which are the male gametes, are created during the process of spermatogenesis. Ova are the female gametes and are created during the process of oogenesis.

After gametogenesis, the process of sexual reproduction takes many forms. Somehow, the male and female gametes need to combine to form a zygote, which is basically a fertilized ovum. In some animals, like many fish species, fertilization occurs externally outside the bodies of both parents, while in others, like mammals, fertilization occurs internally inside the female.

Likewise, the early development of the offspring can also occur either internally or externally. Some animals, like common garden snails, are hermaphrodites, or organisms that have both male and female sexual organs.

Hermaphrodites still reproduce sexually by cross-fertilizing each other's ova. In any case, the variety of reproductive methods and developmental processes that exist in the animal world are too numerous to fully cover here, so we're going to focus on vertebrate development with a special emphasis on human development. However, you should be aware that most of the experiments in the field of developmental biology are not done in humans but rather in a variety of other organisms.