Overview of Animal Reproduction and Development

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  • 0:07 Sexual Reproduction
  • 2:30 Early Vertebrate Development
  • 2:56 Cleavage
  • 3:27 Primary Germ Layers
  • 4:29 Lesson Summary
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Lesson Transcript
Instructor: Joshua Anderson
At this stage in life, you likely know where babies come from. However, did you know that some animals can make clones of themselves and reproduce sexually too? In this lesson, you'll learn more about sexual reproduction and early vertebrate development.

Sexual Reproduction

As you might recall, 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.

Definition of gametes
Gametes Definition

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.

Definition of hermaphrodite
Hermaphrodites Definition

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.

Early Vertebrate Development

After fertilization, the zygote must establish the three major body axes. The anteroposterior axis organizes the body from head to tail. The dorsoventral axis organizes the body from back to belly. The left-right axis organizes the body from left to right.

Illustration of early vertebrate development highlighting the dorsoventral axis
Dorsoventral Axis


Next, the zygote must undergo cleavage, which is the process by which the zygote rapidly divides without growing to become multicellular. This is an obvious step in the development of a multicellular organism.

Definition of cleavage
Cleavage Definition

However, it's important that cleavage occurs early in development because a single cell is very limited in the shapes it can form, and a single-celled zygote also cannot begin the process of differentiating into different tissue types.

Primary Germ Layers

This brings us to the third fundamental task that must be completed during early development, the formation of the three primary germ layers. These three layers are called the ectoderm, the mesoderm and the endoderm.

The three primary germ layers
Primary Germ Layers

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