From the time of conception until life’s end, the human being continually develops, ever changing, ever growing. The miracle of life begins at fertilization, when two cells, an egg and a sperm, unite, each carrying half of the genetic complement of a human. That moment and every subsequent moment in the following eight weeks comprise the embryonic period, at the end of which is a fetus with unmistakably human characteristics.
Zygote
A zygote is the term used for a fertilized egg and is the beginning of a human being. The egg contains 23 chromosomes within which are hereditary elements called genes. The normal number of chromosomes in a human is 46; half come from the mother’s egg and half from the father’s sperm. Their commingling results in a new human being, with inherited traits from each parent but unique unto itself. Fertilization also initiates a cleavage process that transforms the single-cell zygote into the next multicellular stage of human development, the blastomere.
Blastomere
Division of the zygote occurs by mitosis, during which the 46 chromosomes duplicate, resulting in one cell with two complete sets of chromosomes. The original cell next divides into two cells called blastomeres, each cell taking one set of 46 chromosomes. The cleavage process continues, and subsequently four blastomeres develop from the initial two blastomeres, then eight and so on. By the third day of life, a solid ball or cluster of 12 to 32 blastomeres is present, resembling a blackberry or mulberry, known as a morula, from the Latin "morum" for mulberry.
Morula
While the blastomeres are dividing, the cellular mass moves into the uterine cavity, accumulating fluid between the cells as it travels. The fluid between the cells of the morula aggregates in the center of the cell mass, forming a sac-like structure, or cyst. The fluid separates the cells eccentrically, with some cells clustering at one end and the rest radially spread out around the fluid. The cluster of cells will become the embryo. For the meantime, though, this collection of cells with a cystic space inside of it comprises the blastocyst, present at day four of life.
Blastocyst
The blastocyst is unattached to the uterus, floating freely within the uterine cavity until day six of life, when it attaches to and invades the uterine wall. By the end of the first week of life, the blastocyst lies embedded within the innermost layer of the uterus, the endometrium.
Embryo
Around day eight of development, the embryonic disc forms, after which the fertilized egg becomes an embryo. What follows is the most critical period of human development. During the third week of life, the primary germ layers give rise to all the tissues and organs in the embryo form, along with the primitive skeleton, nervous system and cardiovascular system. By the end of the fifth week, the forehead, jaw and primitive limbs emerge. The end of week eight finds the embryo accommodating the formation of the brain and head, the eyes, nose, ears, heart, liver, primitive skeletal and neural tissues, limbs, hands, feet and even toes.
