PT121.S3.P4.Q21 > Analytics

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The first thing any embryo must do before it can develop into an organism is establish early polarity—that is, it must set up a way to distinguish its top from its bottom and its back from its front. ███

Intro topic · Embryo polarity

How an embryo develops different “sides”

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Contrast species · Different species' embryos use different processes to establish polarity

Example of polarity process · Fruit fly

Fruit fly eggs already have all the information needed for polarity even before fertilization; “fertilize-and-go” approach

Consequences · of fruit fly polarity process

Tradeoff: eggs take a long time to “set up” ahead of time, but embryo grows very quickly

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Contrasting example · Nematode

Unlike fruit flies, nematode eggs don’t have all the polarity information they need; sperm also provides information

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Elaborate on example · Details of process in nematode egg

When sperm enters egg, proteins collect on one side of egg, signaling front/back

Similarity/contrast · Nematode-like process used in some vertebrates (frogs) but not others (humans)

Continue contrast · Mammal embryos establish polarity much later

But underlying process is unknown

New contrast · between establishing polarity and growing specific body parts

Species differ widely in how they signal polarity in the embryo, yet they’re very similar in how they signal specific body parts to grow

Main point · Developmental mechanisms differ between species early on, but are similar later in development

Passage Style

Single position

Spotlight

21.

The passage suggests that the ██████ █████ ██ ████ ██████ ██ █████ ████ █████ ███ ██ ███ █████████ ███████████

The simpler the █████████ ███ ███████ ███ █████ ██ █████ ██ ████████ ████ ██████████ ███ ██ ███████

Not supported, because the author doesn’t tell us anything about the general relationship between simplicity and speed of development. Even if you think we have one or two examples of organisms where the simpler one develops faster from egg to embryo, that doesn’t support a claim that applies across the whole spectrum of simplicity.

14%

Scientists have determined ███ ████████ ██ ███████████ ██ ████ ██████ ████████████

“Most” isn’t supported. We don’t know that scientists have determined how polarity is established in over half of simple vertebrates.

12%

Scientists will try ██ █████████ ███ ████████ ██ ███████████ ██ ███████

Supported by the author’s claim that how polarity is established in mammals (such as humans) is “currently a tempting mystery.”

64%

Very few observations ██ █████████ ███████████ █████ ████████ ██ ███████████ ███ █████████████ ██ ████ ████ █ ██████ ████████

Not supported, because development in the later stages is actually more similar between different species than development in the early stages. So the author might agree that more than just “very few” observations of development after polarity are generalizable between different species.

Simpler organisms take ██████ ██ █████████ ████████ ████ ██ ████ ███████ ██████████

Not supported, because the author doesn’t tell us anything about the general relationship between simplicity and speed of polarity establishment. Even if you think we have one or two examples of organisms where the simpler one develops polarity more slowly, that doesn’t support a claim that applies across the whole spectrum of simplicity.

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PrepTest 121 - Section 3 - Passage 4 - Question 21

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