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What is Blastula?

The embryo, blastula and morula are formed after the process of fertilization. When copulation or coitus takes place, the semen is secreted or released by the penis into the vagina of the female. This process is known as insemination. The blastula is formed from the morula. The morula is an 8-16 celled structure and when this morula receives the nutrition it is then converted into a blastula. This blastula then gets implanted into the uterus and further development takes place. This is the simple blastula definition. 

Blastula Formation

After understanding what blastula is, we will learn about its formation. After the process of insemination, the human sperms can survive for 48-72 hours in the body of the females. These sperms move in a liquid medium. This liquid is secreted by the female genital tract. There is an organ in the sperm of a male that is known as prostaglandin. This prostaglandin helps in the movement of the sperms in the liquid medium. The sperms can move at a speed of 2-3mm/minute. When these sperms enter the female genital tract then there is a process known as capacitation of sperms. In this process, the cholesterol from the acrosome membrane of the sperm is removed. The decapacitation factors start to dilute and the membrane cover also starts weakening. The sperms show whip-lash movement with the help of their tails. They swim to the ampulla-isthmus junction in the females. The egg of the female is also brought towards this junction so that both the egg and sperm can meet. The meeting of the egg and sperm and then fusing them to form a zygote is known as fertilization. When the sperm enters the ovum, then the ovum completes its meiotic division and both these haploid structures fuse to form a zygote that is diploid. The ts of the blastula stands for the transverse section of the blastula. 

Embryonic Development

The development of the embryo after the process of fertilization is known as embryonic development. Cleavage, blastulation, implantation, gastrulation and organogenesis are the processes that take place for the development of gametes. We will learn about all of them one by one. 


It is known as the division of cells when the zygote is formed. It is also called an internal zygote division. After 30 hours of fertilization, the first cleavage is completed. There is a furrow formed that is known as cleavage furrow. It passes from the animal-vegetal axis and also from the center of the zygote. Two blastomeres are formed after this first division. This type of cleavage is known as holoblastic cleavage. In 60 hours, the second cleavage is completed. This cleavage is at a right angle to the first one. This cleavage is also meridional in nature. This forms a 3-celled stage. 8 blastomeres are formed in the third cleavage. This cleavage is horizontal in nature. This division is slightly unequal in nature. And then thereafter the rate and pattern of the cleavage are non-specific in nature. The humans show the slowest cleavage division. There is an asynchronous type of division in humans. When the cleavage divides, the blastomeres are increased in an arithmetic division. The cleavages show mitotic division and the daughter cells that are formed are known as blastomeres. When cleavage takes place then at that time no growth is seen in the blastomeres. Here, the total size and volume of the embryo remain the same. This is because there is no growth phase in the interphase stage. There is a decrease in the size of blastomeres at the time of cleavage. This is because there is no growth in the blastomeres. At the time of cleavage divisions, the zona pellucida remains intact. At the time of cleavage, there is no increase in the mass of the cytoplasm. But, the DNA content and the chromosomal amount keep on increasing. On the amount and distribution of yolk, the rate of cleavage depends. 


A solid ball of cells is formed as a result of cleavage. This is known as Morula. It is an 8-16 celled structure. The outer cover is formed by the zona pellucida. There is a process of compaction that takes place in the morula. The outer cells of the morula are smaller in size. They are also flat. They are present with tight junctions with the inner cell mass. The inner cell mass has slightly large cells. They are also round in nature with the presence of gap junctions. As to progress for the process of implantation, the morula starts descending towards the uterus. When this process happens, then the corona radiata is detached from the structure. 

What is Blastulation?

We will understand what blastula is and the process of blastulation. This is the process of formation of the blastocyst. A nutrient fluid is secreted by the endometrium layer. Due to this, its mucosal cells are enlarged and they store nutrients. This is because when the morula starts entering the uterus, then it receives a rich supply of nutrients. There is also an enlargement in the outer peripheral cells and they start to become flatter. The trophoblast is formed from them. It is also known as trophectoderm. The cavity inside it is known as blastocoel. Fluid is secreted by the trophoblast cells into this blastocoel cavity. 

Structure of Blastula

After studying the blastula slide and understanding the blastula under the microscope. On one side of the embryonal knob, the inner cell mass starts to lie. When the blastocoel is formed then, the morula is converted to the blastula. In mammals, it is called a blastocyst. This is because it has a different nature of the surface layer and the inner cell mass is eccentric in nature. As the blastocyst grows, there is an increase in pressure and due to this a small hole is produced in the zona pellucida. Through this hole, the blastocyst squeezes out. So while coming out sometimes this blastocyst can be broken down into two pieces. When such an event happens then, there are two identical blastocysts present and this results in the formation of identical twins. These identical twins are also known as maternal twins or monozygotic twins. The trophoblast cells that are in direct contact with the embryonal knob are known as cells of Rauber. The animal pole is the area of an embryonal knob. Just opposite the animal pole is the abembryonic pole. The embryonic disc is formed when the embryonal knob starts to show rearrangement. Periclinal division takes place in the cells of the trophoblast layer. Syncytiotrophoblast and cytotrophoblast are the two layers that are formed. The syncytiotrophoblast in the outer layer and cytotrophoblast in the inner layer. Further, these two layers give rise to the chorion, amnion and the fetal part of the placenta. The blastula diagram or the ts of the blastula is shown below. 

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This is the process when the blastocyst is embedded into the uterus. It is embedded in the endometrium layer of the uterus. In the region of the embryonal knob, the blastocyst comes in contact with the endometrium. The corrosion of the endometrium lining is caused by the surface cells of the trophoblast. These surface cells secrete lytic enzymes. They also help in giving rise to finger-like projections that are known as villi. The function of villi is to help in fixation and also absorption of the nutrients. When the process of implantation occurs then there is nutrient enrichment and cells are also enlarged. This results in the formation of the uterine part of the placenta and this part is known as decidua. The decidua is divided into three layers. They are:

  • Decidua Basalis: This layer lies below the chorionic villi and above the myometrium layer. 

  • Decidua Capsularis: This layer lies between the embryo and the lumen of the uterus.

  • Decidua Parietalis: This layer helps to inline the uterus. 

The covering layer of the trophoblast secretes the human chorionic gonadotropin hormone. This hormone can be detected in the urine. This is the basis of pregnancy tests. Pregnancy tests are also known as Gravindex tests. This HCG hormone helps in maintaining the corpus luteum and this corpus luteum secretes progesterone. This progesterone hormone prevents the process of menstruation. Another name of progesterone hormone is pregnancy hormone as this hormone is essential for the maintenance of pregnancy. 

About Blastula Provided by Vedantu

Blastula, its formation and development is an extremely complex concept that requires a critical understanding of various terminologies that are discussed in biology, certain concepts related to blastulation are discussed in chapter 3 human reproduction of the NCERT book of the biology of class 12th. The chapter - human reproduction is prescribed by the Central Board of Secondary Education, according to the curriculum set by the CBSE, it carries significant weightage in the board examination and is, therefore, extremely important in the study of biology.

The study notes provided by Vedantu on the topic of blastula are extremely important as they are curated in a way that simplifies the complex concept of blastula, it is mainly for students who may find it difficult to grasp the concept of blastula that is not critically explained in the NCERT book, therefor, Vedantu has done extensive research and analyzed previous year question papers to give you the best experience at learning biology and the various concepts associated with it.

The formation of a blastula is extremely important In understanding the study of the anatomy and physiology of a female reproductive system. During the development of an embryo, blastula or blastomeres or hollow spheres of cells are produced by repeated cleavage of a fertilized egg. The blastocoel also known as a fluid-filled cavity is enclosed by a blastoderm which is an epithelial covering or layer formed by the cells of the blastula. The next step after the development of a blastula is called gastrulation in which the blastula goes under a transition to the gastrula. A blastula should not be confused by a blastocyst. A blastocyst is developed in organisms such as mammals -the earlier morula which is a berry-like cluster of cells is developed into a different form of blastula, known as a blastocyst.

Key Concepts Important in the Understanding of the Blastula are as Follows-

  • The Female Reproductive System

  • Gametogenesis

  • Menstrual Cycle

  • Fertilization and Implantation

  • Pregnancy and Embryonic Development

  • Parturition and Lactation

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FAQs on Blastula

1. What is a blastula?

A blastula is formed after a zygote undergoes cell division, it is a hollow structure that consists of about 6-32 cells. The development of a blastula takes place during the development of an embryo which is done by repeated cleavage of a fertilized egg. The blastocoel also known as a fluid-filled cavity is enclosed by a blastoderm which is an epithelial covering or layer formed by the cells of the blastula. The next step after the development of a blastula is called gastrulation in which the blastula goes under a transition to the gastrula.

2. What is embryonic development?

Embryo development is the stage of the development of an embryo after the process of fertilization. These stages are as follows- cleavage, blastulation, implantation, gastrulation and organogenesis, these take place for the development of gametes. The stages of embryonic development can be further understood by going through the study notes provided by Vedantu, these notes are written in an extremely simplified language that helps students to get a comprehensive understanding of the topic that is being discussed. The notes can be downloaded for free in a pdf format.

3. What is cleavage?

A single-celled zygote responsible for the development of multicellular organisms, it undergoes rapid cell division and forms the blastula. These rapid multiple rounds are termed cleavage. The blastula is formed after the cleavage has produced a hundred cells. At this stage mammals form a blastocyst which is an inner cell mass that is different from the surroundings of a blastula. There are two types of cleavage: holoblastic and meroblastic, holoblastic is a total cleavage while meroblastic is a partial cleavage. The type of cleavage is your pendant on the amount of yolk in the eggs.

4. What is a morula?

A morula is a stage of an embryo that consists of 8-16 blastomeres. Morula divides mitotically and transforms into a blastocyst. It is formed after cleavage as a solid ball. The outer cover is formed by zona pellucida. Compaction is a process that takes place inside a morula. The outer cells are smaller in size and are flat of the morula, which is present with tight junctions connected to the inner cell mass which has slightly large cells. They have gap junctions which makes them round in nature.

5. What is implantation in relation to blastulation?

Implantation is the process when the blastocyst is embedded onto the uterus. The layer in which it is embedded is called the endometrium. The blastocyst is in contact with the endometrium in the embryonal knob. The surface cells of the trophoblast are responsible for the corrosion of the endometrium lining which then gives rise to certain finger-like projections called villi. Villi helps in the absorption and fixation of some nutrients. Nutrient enrichment and cell enlargement occur during the process of implantation. This results in the formation of the uterine part of the placenta and this part is known as decidua. The decidua is divided into three layers. They are:

Decidua Basalis, Decidua Capsularis and Decidua Parietalis.

6. Explain the Process of Spermatogenesis.

In testis, the immature male germ cells, spermatogonia produce sperms by process of spermatogenesis that begins at puberty. It occurs in four stages:

  • Spermatocytogenesis: The spermatogonia present on the wall of the seminiferous tubules multiply by mitotic division and increase in numbers. Each spermatogonium is a diploid containing 46 chromosomes.

  • Meiosis-I: A primary spermatocyte is diploid with 44+ XY chromosomes. It completes the primary meiotic division resulting in the formation of two equal, haploid cells called secondary spermatocytes which have 23 chromosomes.

  • Meiosis-II: The secondary spermatocytes undergo the second meiotic division to supply four equal spermatids. The amount of chromosomes in each spermatid is 23.

  • Spermiogenesis: The transformation of spermatids into sperms is understood as spermiogenesis. From one primary spermatocyte, four haploid sperms are formed.

7. How Does Gamete Formation Take Place?

Gametes in most of the animals have a diploid body hence they form gametes by meiotic division. a couple of animal species example ants, wasps show an unusual sort of sex differentiation. The males are haploid and make haploid sperms by mitosis. The females are diploid and make haploid eggs by meiosis. If an egg is fertilized, it develops into a female while unfertilized eggs become males. Meiosis is an important feature of the sexual cycle. It leads to the formation of daughter cells, each with half the amount of chromosomes of the parent cell. During fertilization, the nuclei of two gamete cells fuse and therefore the zygote thus features a fixed number of chromosomes for every species. altogether organisms, this number of chromosomes represents the diploid condition. If meiosis doesn't occur, a fusion of gametes would end in the doubling of the chromosomes for every successive sexually reproduced generation. This example is prevented by a discount within the diploid number of chromosomes to the haploid number during gametogenesis.