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20.7 Twinning in Humans

Because humans have regulative development, remaining cells can compensate when cells are lost in early cleavages. Monozygotic (identical) twins can result when cells in the early blastula become physically separated and each group of cells goes on to produce a separate embryo.

Division of blastomeres during early blastula formation.

2-cell embryo

.produces monozygotic twins with separate placentas. Two ch°ri°ns

Division of blastomeres during early blastula formation.

2-cell embryo

Trophoblasts

Trophoblasts

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Uterus

Embryos

Two amnions

Two amnions

Uterus

Embryos

Blastomeres become determined—committed to specific fates—at different times in different species. In some species, such as roundworms and clams, blastomeres are determined by the 8-cell stage. If one of these blastomeres is experimentally removed, a particular portion of the embryo will not form. This type of development has been called mosaic development because each blastomere seems to contribute a specific set of "tiles" to the final "mosaic" that is the adult animal. In contrast, other species, such as sea urchins and vertebrates, have regulative development: The loss of some cells during cleavage does not affect the developing embryo because the remaining cells compensate for the loss.

If some blastomeres can change their fate to compensate for the loss of other cells during cleavage and blastula formation, are those cells capable of forming an entire embryo? To a certain extent, they are. During cleavage or early blastula formation in mammals, for example, if the blastomeres are physically separated into two groups, both groups can produce complete embryos (Figure 20.7). Since the two embryos come from the same zygote, they will be monozygotic twins—genetically identical. Non-identical twins occur when two separate eggs are fertilized by two separate sperm. Thus, while identical twins are always of the same sex, non-identical twins have a 50 percent chance of being the same sex.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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