The human egg is huge - 250 times bigger than a normal red blood cell, and it weighs 100-fold more. Its nucleus, reminiscent of the great swirling spot of Jupiter, dominates the inner reaches of its cytoplasm¹. Those sperm that reach the egg's outer surface look like explorers on a vast, curving landscape. (The sheer number of male gametes² make up for their small size. On average, over 100 million sperm attempt to reach the egg.) Once fertilized, the egg becomes the ancestor of all cells, including stem cells. Within it lays a nascent³ power that science has only begun to understand.

Fertilization occurs when one sperm penetrates the egg's outer membrane, as illustrated in the figure to the right. The chromosomes from each parent merge in one nucleus, and development begins. Little is known about the delicate process of early human development - much has to be inferred from studies of other animals. But animal research shows that both the male and female genomes are required to start the process. After fertilization is finished, the egg is called a zygote (from the Greek zygous, meaning yoked⁴). At this stage the zygote - the earliest form of the embryo - is just a single cell, capable of becoming a complete organism. Survival isn't a sure bet - nearly half the zygotes die at this stage. But if it thrives, it will embark on a remarkable journey. It will become trillions of cells; its infinitesimal mass will grow to well over a hundred pounds, and its individual character will change into dozens of complex organs, such as the liver, the heart, and the brain.

About one day after fertilization, the formation of the embryo begins. The zygote begins a series of mitotic⁵ divisions, collectively called cleavage⁶, that result in smaller and smaller cells. Imagine dividing a perfectly plastic soap bubble (one that won't pop when you cut it) with a knife. Where there was one bubble, now there are two smaller bubbles. Cut it again, and four bubbles appear. In the zygote, a similar process repeats for several more cycles, resulting in eight cells, sixteen cells, thirty-two cells, and so on, surrounded by a transparent membrane. Just like our soap bubble, the total volume changes very little as the zygote divides. The progression of drawings on the right shows that each new cell is smaller than the one that preceded it. The interior becomes quite crowded with densely packed cells.

At three days the embryo resembles a mulberry, hence its Latin name, the morula. On the fourth day, fluid passing into the morula forms a cavity⁷. The cells rearrange somewhat into two regions, an outer layer called the trophoblast (from the Greek trophe, or nutrition) and a few dozen internal cells, called the inner cell mass or ICM, the source of embryonic stem cells. At this stage, between four and six days old, the developing embryo is called a blastocyst. Embryonic stem cells is a term used for laboratory cultures of cells made from the ICM of blastocysts. The genes involved in development have begun an important initial task: instructing the cells of the trophoblast to change into the placental₈ and amniotic tissue that will eventually support the embryo. These genes also send cues to the ICM that eventually cause it to develop into the embryo proper.

The term embryo is easily misunderstood when taken out of the context of early development. The zygote, the morula, and the blastocyst are the earliest kinds of embryos, and there are nineteen other recognized stages before fetal development begins. Embryologists also call the blastocyst the early or pre-implantation embryo because it has not yet attached to the uterus. The four-day-old embryo - the blastocyst - is a corpuscle¹⁰ about 0.1 millimeter across, smaller than the period at the end of this sentence.
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