(8) image description from CK-12 Biology textbook
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Cleavage

Blastulation

The zygote spends the next few days traveling down the Fallopian tube. As it travels, it divides by mitosis several times to form a ball of cells called a morula . The cell divisions, which are called cleavage , increase the number of cells but not their overall size. More cell divisions occur, and soon a fluid-filled cavity forms inside the ball of cells. At this stage, the ball of cells is called a blastocyst . The process of blastocyst formation is shown in Figure below.

Blastulation
The morula (1) continues to undergo cell divisions. As it does, cells start to migrate into separate layers, and a cavity starts to develop inside the ball of cells. When cells have migrated into distinct layers, the organism is called a blastocyst (2).
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Blastocyst

The cells of the blastocyst form an inner and an outer cell layer. This is apparent in Figure below. The inner layer of cells is called the embryoblast. This layer of cells will soon develop into an embryo. The outer layer of cells is called the trophoblast. This layer will develop into other structures, including the placenta, which you will read more about below.

Blastocyst
Figure shows the two layers of the ball shaped blastocyst. The outer layer is the trophoblast. Inside the blastocyst cavity is the inner cell mass (embryoblast).
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Implantation

The blastocyst continues the trip down the Fallopian tube and reaches the uterus about four or five days after fertilization. When the outer cells of the blastocyst contact cells lining the uterus, the blastocyst embeds in the lining. The process of embedding is called implantation . It generally occurs about a week after fertilization. Once implantation occurs, the blastocyst is called an embryo.

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Units of Measurement

The measurements that scientists use are based on the International System of Units (SI), which is a form of the metric system. The term SI is shortened from the French term Le Système international d'unités . It is the world's most widely used system of units, both in science and business. It is useful to scientists because it is based on multiples of 10. The SI was developed in 1960 from an older metric system and is used in almost every country.

SI Base Units - Description

Name

Symbol

Quantity

meter

m

length

kilogram

kg

mass

second

s

time

ampere

A

electric current

kelvin

K

thermal energy (temperature)

mole

mol

amount of substance

candela

cd

luminous intensity
Description

A prefix may be added to SI units to make a multiple of the original unit.

SI Prefixes - Description

Name

Symbol

Factor of 10

Name

tera-

T

1,000,000,000,000 (10 12 )

trillion (thousand billion)

giga-

G

1,000,000,000 (10 9 )

billion (thousand million)

mega-

M

1,000,000 (10 6 )

million

kilo-

k

1000 (10 3 )

thousand

hecto-

h

100 (10 2 )

hundred

deca-

da

10 (10 1 )

ten

deci-

d

1 (10 -1 )

tenth

centi-

c

0.1 (10 -2 )

hundredth

milli-

m

0.01 (10 -3 )

thousandth

micro-

µ

0.00001 (10 -6 )

millionth

nano-

n

0.00000001 (10 -9 )

billionth

pico-

p

0.00000000001 (10 -12 )

trillionth
Description

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Appendix: extended descriptions for images and tables

"Blastulation" (click to expand) Further information about this "Blastulation" image is available at Wikipedia (Public Domain).

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"Blastocyst" (click to expand) The blastocyst consists of an outer layer of cells called the trophoblast, a fluid-filled cavity, and an inner cell mass called the embryoblast.
Further information about this "Blastocyst" image is available at Wikipedia (GNU Free Documentation License).

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"SI Base Units" (click to expand) The SI is not static, as the technology of measurement progresses, units are created and definitions are changed through international agreement among many nations. The international system of units is made up of a seven base units. From these seven base units several other units are derived.

Navigate back to the "SI Base Units" table.

"SI Prefixes" (click to expand) An SI prefix is a name or symbol that is put before a unit of measure (or its symbol) to form a decimal or a multiple of the unit. For example, kilo - is a multiple of a thousand and milli - is a multiple of a thousandth, so there are one thousand millimeters in a meter, and one thousand meters in a kilometer . All prefixes are multiples of 10. The prefixes are never combined; a millionth of a kilogram is a milligram not a microkilogram .

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