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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.

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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
Illustration of early vertebrate development highlighting the dorsoventral axis
Dorsoventral Axis

Cleavage

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

The ectoderm is the outermost layer of cells in the early embryo. The mesoderm is the middle layer of cells in the early embryo. Finally, the endoderm is the innermost layer of cells in the early embryo.

Each primary germ layer will eventually become a specific set of tissue and cell types of the organism, so the formation of the different layers is one of the critical early events in animal development. After the three primary germ layers are formed, organogenesis, or the formation of organs, can begin in the embryo, and the different body structures can begin to take form.

Lesson Summary

Sexual reproduction in multicellular animals is a complicated process with a lot of steps. It begins with gametogenesis, which is the formation of gametes.

Gametes are specialized sex cells produced by each parent for sexual reproduction, and there are two types of gametes: sperm and ova. 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.

During fertilization, the male and female gametes combine to form a zygote, which is basically a fertilized ovum.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. Next, the zygote undergoes cleavage, which is the process by which the zygote rapidly divides without growing to become multicellular.

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 the ectoderm, which is the outermost layer of cells in the early embryo, the mesoderm, which is the middle layer of cells in the early embryo, and the endoderm, which is the innermost layer of cells in the early embryo. After the three primary germ layers are formed, organogenesis, or the formation of organs, can begin in the embryo and the different body structures can begin to take shape.

Learning Outcomes

After watching this lesson, you should be able to:

  • Differentiate between asexual and sexual reproduction
  • Define gametogenesis, zygote, hermaphrodite and cleavage
  • Identify the three body axes and the three primary germ layers

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