Video Transcript
The figure shows the stages of
development that occur after fertilization. Approximately, how long does this
process take? (A) 12 hours, (B) one day, (C) six
days, (D) two weeks, or (E) one month.
In this question, we are being
asked about the stages of development that follow fertilization, which is shown to
us in a diagram consisting of five stages.
Fertilization is the process by
which a male sperm cell and a female egg cell combine to produce a zygote. The first image in the diagram
provided by the question shows the sperm cell and egg cell about to combine, and we
can consider fertilization as having occurred on day one. Initially, a zygote consists of
only one cell, which then divides by mitosis, splitting into two cells, shown in the
second image in the question. These two cells divide again,
forming the four-cell stage, and so on.
Each of these divisions is called a
cleavage, and each cleavage occurs over the span of about a day. So the four-cell stage, shown in
the third image in the diagram, will occur around day three. As these cleavage stages are
occurring, we refer to the zygote as an embryo. And each cell within the embryo is
called a blastomere. When the embryo contains between
eight and 32 blastomeres, as we can see in the fourth image in the diagram in the
question, it is called a morula, which forms around day four.
The final image we can see in the
diagram shows that the morula has developed further to form a small mass of cells
called the inner cell mass, surrounded by a layer of cells called the
trophoblast. There is a space within this
structure called the blastocoel. And at this stage, which occurs
around day six following fertilization, the embryo is called a blastocyst. The blastocyst is the stage at
which the embryo is ready to implant into a female’s uterus, where it can grow and
develop into a fetus.
Now that we have reviewed the
timeline following fertilization, we know the answer to our question. The duration of the stages of
development from fertilization to the formation of a blastocyst, and the correct
answer to our question, is (C) six days.
