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. 2020 Apr 15;6:5. doi: 10.1186/s40851-020-00157-5

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© The Author(s) 2020

Open AccessThis article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.

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Fig. 3 — Scanning and transmission electron micrographs of immature oocytes and mature eggs before and after fertilization. a A fractured immature oocyte showing microvilli (M) beneath the vitelline layer (VL) and organelles in the cytoplasm. b Microvilli project from the oocyte’s surface into the VL. Cortical granules (CG). c A mature egg treated with 1-MA for 50 min. At this optimal period of fertilizability, microvilli (M) beneath the VL are shorter. d The ultrastructure of a mature egg stimulated with 1-MA for 50 min shows shortened microvilli (M) and cortical granules (CG) positioned beneath the egg surface. e Fertilization envelope (FE) elevation in a mature egg 3 min after insemination showing the spherical bodies (arrow) in the perivitelline space following CG exocytosis. Hyaline layer (HL). f The electron-dense spherical bodies of the CG in the egg surface before fertilization in D are now seen in the perivitelline space (PS) upon insemination. Same magnification in all figures