Chapter 20: Reproduction and Pregnancy | Reading and Writing Practice

Read the passage below and then answer the questions.

The single-celled zygote undergoes cell division through a process called cleavage, which is a type of mitosis. The roughly spherical zygote develops a furrow, which deepens and appears to cleave, or split the zygote into two new cells. These two cells then cleave, producing four cells, and so on.

Cleavage produces smaller and smaller cells, so the resulting mass of cells remains small enough to get to the uterus through the tiny passageway of a fallopian tube. Five days after fertilization, the zygote has cleaved seven times, forming a ball of 128 cells. This ball of cells, called a blastocyst, has traveled to the uterus. After eight to ten days, the blastocyst implants in the endometrium. This implanted mass of cells is now called an embryo.


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