Online College Courses for Credit

2 Tutorials that teach Fertilization and Early Development
Take your pick:
Fertilization and Early Development

Fertilization and Early Development

Author: Amanda Soderlind

Understand the process of fertilization and the earliest human developments.

See More
Fast, Free College Credit

Developing Effective Teams

Let's Ride
*No strings attached. This college course is 100% free and is worth 1 semester credit.

37 Sophia partners guarantee credit transfer.

299 Institutions have accepted or given pre-approval for credit transfer.

* The American Council on Education's College Credit Recommendation Service (ACE Credit®) has evaluated and recommended college credit for 32 of Sophia’s online courses. Many different colleges and universities consider ACE CREDIT recommendations in determining the applicability to their course and degree programs.



Source: Video and Images Created by Amanda Soderlind

Video Transcription

Download PDF

Welcome to this lesson on fertilization.

Today we are going to be discussing the process of fertilization, and how it can lead to the development of an embryo.

Fertilization is the process in which male sperm and female egg combine genetic information, generally as a result of sexual intercourse. In this process, semen is ejaculated from the penis into the vagina, and travels through the vagina, cervix, and uterus, and eventually into the ova duct. An ova duct is the area where an oocyte is then fertilized by a sperm cell.

I'm going to zoom into this diagram right here, and this is going to illustrate a little bit more about what is happening during fertilization. OK. Basically, what happens during fertilization, once the sperm reaches the egg, is that the acrosome, which is that enzyme cap on the head of the sperm, will allow the sperm to penetrate the zona pellucida. The zona pellucida is this thick layer that surrounds the egg.

So the acrosome cap contains these enzymes which allow the sperm to penetrate this zona pellucida.

Once inside, fertilization has happened. So the egg matures once fertilization happens, and is then called an ovum. So the oocyte, once it's been fertilized, matures and is called an ovum.

Once a sperm has entered the egg, the nuclei of the sperm will fuse with the nuclei of the egg. Now the nuclei of the sperm-- we're going to label that right here-- so this is the sperm, has a total of 23 chromosomes. So it is haploid, meaning that it has half the number of chromosomes as a regular body cell. And this is going to be the nuclei of our egg cell. It also has 23 chromosomes. So that what happens when the sperm and the egg cell combine, when they fuse nuclei, the 23 chromosomes from the sperm combine with the 23 from the egg to give a total of 46 chromosomes, which is the proper number in a human body cell.

So from there, once the sperm and egg nuclei have combined, we now have a cell that has a total of 46 chromosomes, forming a zygote, which is the first cell of an individual. And then the ovum, at that point, will then start to divide by mitosis in order to begin to form an embryo.

This lesson has been an overview on the process of fertilization.

Early Development

Source: Video and Images Created by Amanda Soderlind

Video Transcription

Download PDF

Welcome to this lesson today on early development. Today, we are going to be discussing the development of the three germ layers, which eventually give rise to organs and tissues of an embryo. So the very first step, the first thing that needs to happen before an embryo can even develop, is fertilization has to occur.

So fertilization is the beginning of life, when an egg and a sperm combine nuclei. And this forms a single celled zygote. And so a zygote, as I mentioned, is composed of one cell. It's an individual's first cell. And in order to develop into an embryo, that one cell has to undergo divisions. And those divisions will turn this one cell into a ball of many cells, which will then later specialize and develop into an embryo.

So cleavage are these rounds of cell division. So, as I mentioned, that one cell has to divide into many cells, and those rounds of cell division are called cleavage. So it's turning this one cell into two cells, and then each of those cells will divide, and so on and so forth. So it's turning this one cell into balls of more cells.

So this cleavage, this process of cleavage, happens while the zygote is moving towards the uterus. So fertilization occurs here in the Fallopian tubes, and then that's where the zygote is formed. And then that zygote will move towards the uterus and then eventually implant in the uterus.

So each time a new cell is added to this ball of cells through the process of cleavage, that new cell is called a blastomere. And that blastomere cell mirror will then take on a part of that cell's cytoplasm. And the part of the cytoplasm that it gets will eventually determine the structure and function of that cell, what type of cell it's going to become. So, eventually, through this process of cleavage, we'll end up with a morula. And this is a cluster of 16 cells, so it's about 16 cells around the time that it reaches the uterus.

So gastrulation is this process that occurs after cleavage. And what it does is it rearranges the cells of the morula into three tissues. And we call these three tissues germ layers. So our three germ layers in a developing embryo are the ectoderm, mesoderm, and endoderm.

Now, the ectoderm is the outer layer, the mesoderm is the middle layer, and the endoderm is the inner layer. So this ball of 16 cells gets rearranged into these different germ layers, and these different germ layers will develop, then, into body tissues and organs. So each layer is going to differentiate in a process called cell differentiation. So this is when the cells become specialized.

So this process of cell differentiation, or this process of the cells becoming specialized, becoming a specific type of cell, it's called morphogenesis. So morphogenesis is when the tissues and organs form as the cells began to become specialized. So one cell might develop into a nerve cell, which composes the nervous system. One type of cell might develop into a cell that becomes a part of your skin.

So these cells begin to specialize into tissues and organs. So, generally, the ectoderm-- cells of the ectoderm will develop into the nervous system, sense organs, pituitary gland, epidermis, and hair. All of those will develop from cells of the ectoderm. The mesoderm cells will specialize into cartilage, bone, muscle, connective tissue, cardiovascular system, the lymphatic system, the urinary system, the respiratory system, and then the outer layer of the digestive tube.

And then the endoderm, those cells will specialize into becoming lining of the digestive tube and also lining up the airways. So these different layers produce different cells, which differentiate to become different structures throughout the body. So this lesson has been an overview on early development and the three germ layers.

Terms to Know

Roughly 5 days after implantation, a fluid-filled cavity will open up in the morula (ball of cells). This ball - with a thin layer of cells (the trophoblast) surrounding the cavity in which lies the inner cell mass - is called the blastocyst.


A cell that results from the cleavage of a zygote.

Cell Differentiation

The process of cells taking on specific forms and functions; also called specialization of cells.


A general term to describe the splitting or breaking apart of an object or molecule; in embryology, this is when the zygote starts to divide into multiple cells.


The outer germ layer that forms the nervous system, skin, melanocytes and lenses of the eyes.


A stage of development of an individual, formed from the inner cell mass of a blastocyst after it has implanted in the uterus.


The innermost germ layer that forms the digestive tract, respiratory tract, endocrine glands, auditory systems, and urinary bladder.


The process by which the a male gamete (sperm) combines with a female gamete (egg/oocyte).

Germ Layers

The first three layers of tissue that develops in the embryo as it starts to form (ectoderm, mesoderm, and endoderm).

Inner Cell Mass

A layer of the blastocyst which will develop into an embryo.


The middle germ layer that forms cartilage, bone, muscle, connective tissue, cardiovascular system, lymphatic system, urinary system, respiratory system, and the outer layer of the digestive tract.


When an organism develops a specific shape.


A mature egg cell.

Sexual Intercourse

The process in which semen is ejaculated from the male penis into the female vagina which can result in the fertilization of an egg cell.


The first cell of an individual formed when a sperm cell fertilizes an egg cell.