--- title: "Fertilisation and the first cleavages" description: "Compare sperm-entry barriers in sea urchins and mammals, including exceptions to one-sperm entry. Explain how early cleavage makes more, smaller cells, how early cell cycles vary between animal groups" canonical: https://lightmysky.com/learn/science/fertilisation-and-the-first-cleavages-mt_9BXQsdP8fI source: https://lightmysky.com/learn/science/fertilisation-and-the-first-cleavages-mt_9BXQsdP8fI.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Fertilisation and the first cleavages Compare sperm-entry barriers in sea urchins and mammals, including exceptions to one-sperm entry. Explain how early cleavage makes more, smaller cells, how early cell cycles vary between animal groups, and how yolk helps shape cleavage. Subject: Science · Area: Organisms & Life Processes · Ages 19 to 20 Page: https://lightmysky.com/learn/science/fertilisation-and-the-first-cleavages-mt_9BXQsdP8fI ## Ready when they can - Describe the electrical fast block and cortical slow block in sea urchins, and distinguish these from mammalian sperm-entry barriers. - Explain how cleavage increases cell number with little overall growth while retaining DNA replication, and identify limits of the rapid S/M-cycle model. - Compare whole-egg cleavage in sea urchins with disc cleavage in chickens, relating the difference to yolk amount and distribution. ## Lesson: Sperm-entry barriers and early cleavage You have met haploid gametes, each with one chromosome set. In normal human fertilisation, one sperm fuses with one egg, restoring two sets in the zygote. Extra sperm entry is called polyspermy and usually disrupts human development. This is not an animal-wide rule: bird eggs normally admit several sperm, although only one sperm nucleus combines with the egg nucleus. In sea urchins, sperm-egg fusion rapidly depolarises the egg membrane, changing its voltage. This gives a brief fast block to further sperm fusion. A rise in calcium inside the egg triggers cortical granules to release their contents. These lift and modify the vitelline envelope around the egg, forming a fertilisation envelope. This slow block provides a longer-lasting barrier to extra sperm. *(drawing: Follow the slow-block pathway. The boxes show events in order, not the egg's anatomy.)* Mammals have no established fast electrical block like the sea urchin's. Cortical-granule release alters the zona pellucida, a coat around the egg. Changes in this coat and at the egg membrane help limit further sperm binding, penetration or fusion. The details differ between species, so a mechanism described in one animal should not be assumed for every animal. Cleavage consists of mitotic divisions that produce cells called blastomeres. Early cleavage increases cell number with little overall growth, so average cell size falls. In a model where every cell divides once per round, each complete round doubles the cell count. If the divisions are equal and there is no overall growth, divide the starting volume by the final cell count to find each cell's volume. *(drawing: In the equal-division, no-growth model, two complete rounds make four cells, each with one quarter of the starting volume.)* Little overall growth does not mean no DNA copying. S phase copies DNA before mitosis, allowing daughter cells to retain the chromosome set. In many early non-mammalian embryos, rapid cycles alternate mainly between S phase and M phase, with G1 and G2 absent or short. Mammalian cleavage is generally slower. Early cell-cycle structure varies with species and stage, so the rapid S/M model is not universal. Yolk amount and distribution help shape cleavage. Holoblastic cleavage divides the whole egg, as in sea urchin eggs with little yolk. Meroblastic cleavage divides only part of the egg. In a chicken's yolk-rich egg, a small disc of cytoplasm above the yolk divides while the large yolk mass remains undivided. Other developmental controls also affect cleavage; yolk is not its only influence. **Recap.** Sperm-entry barriers differ between animals. Cleavage makes more, smaller cells while retaining DNA copying. Yolk helps shape whether the whole egg or only part divides. *Original lesson, questions and flashcards written for this dataset.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Meiosis and Why Gametes Are Haploid](https://lightmysky.com/learn/science/meiosis-and-why-gametes-are-haploid-mt_hvu0bczcfX) ## Optional connections - [Cell Cycle Checkpoints, Control and Cancer](https://lightmysky.com/learn/science/cell-cycle-checkpoints-control-and-cancer-mt_nwNmCVnZ7O) - [Human Reproduction](https://lightmysky.com/learn/science/human-reproduction-mt_tQkCzRcWG7) ## Opens up - [Gastrulation and Laying Down the Body Axes](https://lightmysky.com/learn/science/gastrulation-and-laying-down-the-body-axes-mt_PABfw_4RPs)