Domain: Natural Science — Cell & Developmental Biology
Target concept: The fertilization process of sea urchin eggs under the microscope (during the formation of the fertilization membrane)

Video generation prompt: Under a light microscope, a sea urchin egg was placed in seawater and exposed to motile sea urchin sperm, and their interactions were observed.

Evaluation guide:
(i) Target concept: Cortical reaction-induced fertilization membrane formation as a rapid, extracellular, species-specific block to polyspermy in sea urchins.
Minimum mechanism: Sperm contact must be the necessary and sufficient trigger; membrane must originate from egg surface modification (not secretion into medium), must be continuous and circumferential, and must remain strictly extracellular.
(ii)Storyline: Frame 1–2: Static sea urchin egg centered in seawater medium under microscope view. Frame 3–4: Motile sperm enter field, swimming directionally. Frame 5–6: At least one sperm makes direct, sustained contact with egg surface (visible touch point). Frame 7–8: Fertilization membrane initiates as a localized elevation at contact site. Frame 9–10: Membrane propagates radially to fully encircle egg as a smooth, unbroken, translucent ring — completing before video ends. Perspective: Fixed microscope viewpoint; no panning/zooming; entire egg and immediate periphery must remain fully visible throughout; membrane must be optically resolvable as distinct from egg and sperm.

Manual:
Experimental Procedure
1. Place a live sea urchin egg in a drop of natural seawater on a microscope slide.
2. Add a small volume of motile sea urchin sperm suspension to the same drop.
3. Observe under high-power light microscope (400×) with phase contrast.
Observation of Experimental Phenomena
- Sperm swim actively toward and contact the egg surface.
- Within seconds of first sperm contact, a raised, transparent fertilization membrane lifts uniformly off the egg surface, forming a complete circumferential barrier.
- The egg remains intact, single-celled, and motionless; sperm remain outside the membrane.
Summary of Experimental Principles
- Fertilization triggers cortical granule exocytosis in the egg, releasing enzymes that modify the vitelline layer into an impenetrable fertilization membrane — a key block to polyspermy in echinoderms.

Prompt Grounding rubric:
level_1: No sea urchin egg is shown, no sperm are shown, no seawater medium appears, there is no microscope-style view, or no interaction between egg and sperm is shown at all.
level_2: At least one of the egg, motile sperm, or a seawater-like liquid appears, but they never appear together in the same scene, or there is no shared spatial context between them, for example the egg and sperm each appear in isolation with no proximity or contact shown.
level_3: The egg and motile sperm both appear together in the same seawater-filled field of view and come close to each other at some point in the sequence; direct contact is not required at this level.
level_4: The 3-point requirements are all met, and further, the sperm makes visible physical contact with the egg's surface, and the egg stays in place while the sperm show motile movement, whether visible or clearly implied swimming or flagellar motion.
level_5: The 4-point requirements are all met, and further, a new ring-like structure, the fertilization membrane, is visible around the egg following the contact, appearing as a distinct halo separate from the egg's surface and from the sperm, rather than the egg looking unchanged or the sperm appearing to merge into it.

Scientific and Causal Correctness rubric:
level_1: The fertilization membrane appears before the sperm makes contact, forms around something other than the egg, is missing even though a sperm appears to fully enter the egg, or the egg divides internally, any of which is inconsistent with the sea-urchin cortical reaction.
level_2: The sperm contacts the egg, but no membrane forms at all; or a membrane forms but is uneven, patchy, appearing on only one side, collapsed, or fused with the sperm body; or the egg turns opaque white or yellow instead of staying translucent with a newly visible membrane outline.
level_3: The fertilization membrane appears as a complete ring fully surrounding the egg only after the sperm makes contact; the membrane is clearly distinct from both the egg's interior and the sperm; the egg stays a single cell with no division visible; the sperm stay outside the egg with none appearing to enter it.
level_4: The 3-point requirements are all met, and further, the membrane's first appearance follows the sperm's first touch rather than preceding or coinciding with the sperm's approach, and the membrane stays evenly translucent and smoothly continuous all the way around, with no unexplained breaks or bulges away from the contact point.
level_5: The 4-point requirements are all met, and further, the membrane forms only as a result of the sperm's contact with no membrane appearing on its own, no sperm are shown embedded in or crossing through the membrane, the membrane's thickness and height look consistent all the way around the egg, and nothing else in the scene, such as bubbles, debris, or an optical halo, is mistakable for the membrane.

Spatiotemporal Consistency rubric:
level_1: Severe break in continuity: the egg or sperm teleport between frames, the zoom or scale suddenly jumps, the lighting flips from bright to fully dark in a single frame, the egg appears to split apart and reassemble, or all motion freezes for more than a couple of frames with no cause.
level_2: Noticeable break: the egg or sperm flicker in and out of view across frames, the membrane appears in disconnected segments rather than spreading outward, or a highlight moves in a direction inconsistent with where the light is coming from.
level_3: The egg and sperm keep a consistent position, size, and orientation relative to each other across frames; neither one jumps to a new spot without visible motion, and no sperm passes behind the egg and reappears in front without a visible path; the overall lighting stays physically coherent from frame to frame.
level_4: The 3-point requirements are all met, and further, the motion is continuous with no strobing or sudden jump-cuts; the membrane grows outward from the contact point in a steady, evenly expanding ring until it fully encircles the egg; there are no dropped frames or pauses that break up the sequence.
level_5: The 4-point requirements are all met, and further, the sperm's flagella show a genuine undulating swimming motion rather than rigid spinning or sliding; the egg's surface stays smooth except right at the forming membrane; the membrane's rise looks like one smooth expansion with no pulsing or vibrating; and neither the egg nor the field of view shows any visible shaking or drifting during the sequence.