K12 Comparative Embryology

Embryology - 21 Oct 2026        Expand to Translate
Google Translate - select your language from the list shown below (this will open a new external page)
العربية | català | 中文 | 中國傳統的 | français | Deutsche | עִברִית | हिंदी | bahasa Indonesia | italiano | 日本語 | 한국어 | မြန်မာ | Pilipino | Polskie | português | ਪੰਜਾਬੀ ਦੇ | Română | русский | Español | Swahili | Svensk | ไทย | Türkçe | اردو | ייִדיש | Tiếng Việt    These external translations are automated and may not be accurate. (More? About Translations)

Introduction

All human and animal embryos go through very similar stages of early development. See also Humans and Animal Embryology. What are the key things in development that we share? This page introduces a few of the concepts of comparative development shared with all animals. Meiosis | Mitosis | Gastrulation | Body Plan K12 Links: Start Here | Week 1 | Week 2 | Week 3 | Week 4 | Week 5 to 8 | Arms and Legs | Heart | Fetus | Brain Growth | Eyes and Ears | Animal Development Times | Humans and Animal Embryology | Comparative Embryology | Thalidomide Teacher Note  This is currently only a draft designed to help K12 students understand comparative embryology. I am currently looking to simplify concepts and include images on this page. I am happy to receive feedback as too what you may like to be included here. I have also begun to add some simple exercises that can be used in class to help understand concepts in embryonic development and comparison. Note some of the links on this page leave the K12 notes section and may be beyond the level of your students, bookmark this page to easily return here. This page can be printed using the menu from the list at the bottom of the page "Printable version" or Printable version. Also look at the K12 Human and Other Animal Development that simply compares development times and embryo structures. K12 Professional Development 2016 | K12 Professional Development 2014 Meiosis In the male and female in all animals (and plants) that reproduce sexually to form an embryo, these very first cells form by meiosis. Meiosis a reductive form of cell division that only occurs in the egg (oocyte) and sperm (spermatozoa) and allows new genetic combinations of offspring to be generated. Meiosis has 2 key components: Genetic reorganisation - the genetic material (chromosomes) that you have from your mother and father are recombined. Genetic reductive - the chromosome number is halved and only fertilisation will allow the paired chromosomes that we all contain in all our cells. <html5media height="200" width="300">File:Oocyte_Meiosis_01.mp4</html5media> This movie shows the egg (oocyte) completing the first part of meiosis (meiosis I). The chromosomes are coloured blue, the cell membrane is green. The small structure on the left are chromosomes that will not be used in the embryo. Mitosis Mitosis is the type of normal cell division that allows growth and development of all animal embryos. After the first cell has been formed by the egg (oocyte) and sperm (spermatozoa) fusing, every cell division in the embryo forms 2 genetically identical daughter cells. Every mitosis in all animal cells has the same features. Mitosis has 2 key components: Chromosome duplication - has to occur before cell division can occur. Mitosis - a set of 5 standard phases dividing the nucleus (and the chromosomes it contains) before the cytoplasm divides. <html5media height="300" width="300">File:Mitosis 11.mp4</html5media> This movie shows a cell that has already begun mitosis separating the chromosomes in the nucleus then the cell cytoplasm. The chromosomes are white. Gastrulation Development of any animal requires the differentiation of different cell types and tissues from essentially the same initial cells. One of the first shared steps is the process of "gastrulation" (means gut formation). This forms the 3 cell layers that will form all the embryo, they are often described as the "germ layers" and this stage of development as the "gastrula". The same layers form the same structures in all these animals. 3 Germ Layers: Ectoderm - outer layer of skin (epidermis) and nervous system (central nervous system and peripheral nervous system). Mesoderm - bone, muscle, cartilage, blood (connective tissues) Endoderm - lining of the gut, lungs, endocrine organs. Body Plan The next step in development of any animal requires the embryo body plan (axes) by a patterning process. Body plan (axes) head and body left and right side front and back Many signals used to establish this patterning have been identified and are shared (similar or the same) between different animals. Note that some of the signals used to establish the overall body plan can be reused later at other stages in embryo development and within the body organs and tissues. Different animal body plan axes. Below is an example of what happens in a fly if these patterning signals get disrupted, putting legs from the body on the head where antenna should be located. Normal fly head with antenna Abnormal fly head with legs for antenna This fly mutation identified a common patterning gene family called Hox that establishes the head to tail axes in the embryo. This complicated picture shows how different Hox genes are expressed at different embryo levels in different species. Cite this page: Hill, M.A. (2026, October 21) Embryology K12 Comparative Embryology. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/K12_Comparative_Embryology What Links Here? © Dr Mark Hill 2026, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G