Reading Comprehension

PT121 · S3 · P4 · Q27 Embryo Polarity

A free, expert breakdown of this official LSAT Reading Comprehension question.

Save & drill this skill — build targeted practice sets from questions like this one
Video walkthroughs — watch every question solved step by step
81 official LSATs — as questions, timed sections & full-length tests

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.

Topic

How embryos across different species establish polarity (body orientation), and how this phase differs from later development

Framework

Old / New (There is also a strong "Highlight Noteworthy" element as the passage spotlights a paradox, but the main structure is an Old/New contrast between early vs. later mechanisms.)

Main Point

The earliest step in embryonic development—establishing polarity (front/back, top/bottom)—uses very different mechanisms across species, but for later developmental stages, like forming limbs and organs, animals rely on remarkably similar genetic processes. (The Most Valuable Sentence is in the last paragraph: )

P1: Introducing Embryonic Polarity and Its Diversity

All embryos must first establish their basic body orientation (polarity), but the ways in which different organisms create this early spatial framework vary a lot—the signals flies use, for example, are very different from those used by nematodes or mammals.

P2: How Different Species Establish Polarity

The paragraph details how specific animals get polarity started: fruit flies prepare the "instructions" in the egg before fertilization; nematodes use the site of sperm entry as a guide, moving certain proteins to help create body orientation; some simple vertebrates use a sperm-driven cue, but mammals don't seem to. In humans and other mammals, polarity happens much later and the process remains mysterious.

P3: Paradox—Similarity in Later Development

Once body orientation is set up, all animals rely on a surprisingly similar set of genes to develop complex structures like eyes and limbs. This creates a paradox where embryos use very different strategies early on, but later use almost identical ones to elaborate their body plans.

27.

The author's primary purpose in the passage is to

articulate a theory of how
Out of Scope: articulate a theory
There's no unitary theory of how early polarity is established. The passage is emphasizing that there are lots of different ways that early polarity is established.
4% picked this
Correct
describe a phase in the
Why this is right
"Establishing polarity" is a phase in the development of organisms, so, yes, the passage describes the phase where organisms establish polarity. Does the passage say that the genetic mechanisms are disparate? Yes, this is half of the paradox we were looking for. As the final paragraph says, "the mechanisms of development used [to establish polarity] are vastly different". Does the author discuss why this disparity is surprising? Yes, she calls it a "seeming paradox". Paradoxes are inherently surprising. It's surprising that organisms vary so much in terms of how they establish polarity, because once they've established polarity, "there is an astonishing conservation of mechanism --- the developmental mechanisms used are remarkably similar" when it comes to building eyes and ears and spinal cords and such.
Skill tested: Primary Purpose · how this choice captures the passage's function is the move to repeat next time.
71% picked this
provide a classification of the
Out of Scope: a classification
The author definitely discusses several mechanisms by which different life forms establish early polarity. But does she provide a classification? Does she say, "birds use p-granules, amphibians use a pre-polarized egg, mammals use X, etc.?" No, there are just a handful of one-off examples. We know how the fruit fly establishes polarity and how certain nematodes establish polarity. She then says that some simple vertebrates might do something similar, but no one really knows what more complex vertebrates like mammals do. So it's hard to say that there's any classification system provided. This answer also would do a worse job than the correct answer does of describing what happens in the final paragraph.
14% picked this
argue that a certain genetic
Too Strong: must occur in all
The author never says anything super-extreme like "there is a genetic process that must occur in all life forms". We could potentially say that the first sentence identifies "establishing polarity" as something that must occur in all life forms for an embryo to develop". But the author isn't arguing that must occur. She just states that it does occur and then starts talking about different ways that it occurs.
7% picked this
explain why an embryo must
Out of Scope: explain why
The author never explains why an embryo must establish early polarity. It's just sort of common sense that you can't build a building if you don't know which direction is up and which direction is down. Similarly, DNA can't build an organism if it doesn't have an initial sense of top vs. bottom. The author never explains this, and it's certainly not what the bulk of the passage is doing.
3% picked this

Continue the review in LSAT Lab

Save this question, watch the video walkthrough, and drill similar questions in your LSAT Lab account.

Save and practice this skill

LSAT Lab

Turn this review into a targeted study plan.

Save this question, drill more like it, watch the video walkthrough, and track your progress in your LSAT Lab account.

Start practicing free