Development Leading to Birth The Cute Little Sweller Gets Ready to Stick His Head

Thanks to internal fertilization of an ovum, we all start our lives as a single cell, the zygote. But the zygote doesn't remain a single cell for very long! As it moves through the oviduct and towards the uterus, it undergoes a series of mitoses (my-TOH-seez). The zygote just keeps dividing into two cells, four cells, eight cells, on-and-on. As the name, embryo, literally means a ''sweller,'' the zygote (beginning embryo) keeps dividing and ''swelling'' in size, in greater Biological Order, and in the growing pattern of its complexity.

Figure 15.6 outlines the major developments leading from the zygote to the fetus (FEE-tus) or final, human-like ''offspring.''

The single-celled zygote becomes a solid mass of cells called a morula (mor-OO-lah). The word, morula, translates into Common English to mean ''little mulberry.'' The morula, then, looks somewhat like a real mulberry hanging on a bush, which has many little bumps or ''cells'' on its fruit surface.

The morula eventually passes out of the oviduct and enters the body (main hollow cavity) of the uterus. Here it becomes a blastula (BLAS-chew-lah), also called a blastocyst (BLAS-toh-sist). The blastula is a ''little sprouter'' (blastul) or ''hollow sprouting bladder'' (blastocyst). To be sure, the blastula, being called a blastocyst, with cyst meaning hollow ''bladder,'' looks quite a bit like a hollow raspberry inside, instead of a solid mulberry!

We get the idea of ''sprouter'' from what the blastula does. It implants itself into the endometrium (en-doh-ME-tree-um), the ''inner'' (endo-) epithelial lining of the ''uterus'' (metr). After it implants, the blastula sends out spreading roots of cells (like a sprouting plant), thereby firmly anchoring it into the endometrium.

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