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Biology - Implantation

The fertilized egg travels to the uterus where it grows into the side of the uterine wall. This is called implantation. At this site, it will eventually form a structure called the placenta. 

Placenta is made of both fetal and maternal cells. It allows the transfer of the maternal nutrients to the fetus and prevents the mixing of maternal and fetal blood. It is basically a barrier that lets food get in and waste get out. Another structure, the umbilical cord connects the fetus to the placenta and conducts all the stuff between mother and fetus. 

At fertilization, a haploid ovum and a haploid sperm get together and together they produce a diploid zygote. The sperm has 23 chromosomes, the ovum has 23 chromosomes and the zygote then has 46 chromosomes. The zygote undergoes mitosis over and over and over again and all of the cells of the developing embryo are diploid. 

You should know a few things and a few words that pertain to the development of a new organism. The sperm and ovum are gametes, they are haploid, the two get together to form the beginnings of the new organism. When they get together, they form the first cell of the new organism and that first cell is called a zygote. 

The zygote is diploid and it starts dividing. It goes through lots of stages and its name changes many times and ultimately it becomes a new organism. 

A newly forming organism naturally changes shape many times in many ways before it completes the developmental process that is called morphogenesis. The first thing a zygote does is it undergoes mitosis. Then, each daughter cell also undergoes mitosis. The resultant daughter cells continue to undergo mitosis several more times. This series of mitotic divisions is called cleavage. 

Cleavage is the first thing to happen after fertilization. After the zygote has undergone several mitotic divisions, it is not called the zygote anymore. It is a morula. As the cells divide and pressed against to each other, it forms a hollow sphere of cells and it is called a blastula. 

The space in the sphere is called a blastocoele. After the blastula is formed, it quickly invaginates and it is called a gastrula. The gastrula gives rise to three different cell layers called germ layers. 

These three layers are named ectoderm, mesoderm and endoderm. Every cell, tissue and organ in the body derives from one of these three cell layers. The digestive tract and all the structures attached to it like the esophagus, stomach; small intestine, large intestine; pancreas, gall bladder and the liver arise from endoderm. The inner lining of respiratory tract, bronchi, bronchioles, alveoli arise from endoderm. Ectoderm gives rise to the epidermis, the nervous system and the eye. Everything else arises from mesoderm. 

After gastrulation, some mesodermal cells move under the ectoderm and shape themselves into something that looks like a long rod called the notochord. The ectoderm above the notochord starts to thicken and it is called the neural plate. 

The neural plate rolls up into a tube by itself and it is called the neuronal tube. This tube sits just above the notochord and it becomes the brain and spinal chord. This process is called neurulation. After the embryo undergoes neurulation, it is called a neurula. 

Differentiation is when one cell of a developing organism develops a long one line of specialization and another cell develops along another line of specialization. Differentiation is controlled by induction. 

Induction causes a nearby tissue to develop along some line of specialization. The name induction pertains to the fact that one tissue is inducing another tissue to develop in a certain way.