Types of Eggs of Animals in Zoology (Embryology ) | Biology Blog

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                                             Types of Egg


Hello guys!

We'll examine a crucial area of zoology in this blog: different varieties of eggs. So let's define reproduction first before moving on to the subject.

Reproduction is the biological process in which a parent is responsible to produce their offsprings which may be of their own kind. Reproduction is a must process for every individual for the continuing existence of the species.

As we all know, there are many different species on the planet, and each of them has a unique method of egg production and inside the egg they contain yolk . The yolk is composed of proteins, fats, vitamins, and minerals, which are vital for the growth and development of the embryo. 
Eggs can be classified into different types based on the amount of yolk they contain.

l. On the basis of the amount of Yolk - Depending on the quantity of yolks, eggs can be 
  • Alecithal / Microlecithal / oligolecithal - The amount of yolk present in an organism's egg is of a very small amount or absent.   
      Example -  Sea urchin,  Amphioxus, Tunicates, Eutherian egg

  • Mesolecithal -  In this type of egg, the yolk amount is medium or moderate & concentrated towards one pole of the egg.
        Example -   Amphibian, Dipnoi fishes, lungfishes, Petromyzon

  • Polylecithal / macrolecithal / megalecithal -  The amount of yolk content is very large.         Example -    prototherian mammals,  insects, annelids, reptiles                
Types Of Eggs - Zoology


2. On the basis of the distribution of yolk - Depending on how they are distributed, eggs fall into one of the following categories:
  • Isolecithal / homolecithal Egg -   The yolk is equally distributed in the whole egg.Since the amount of yolk is relatively small, these eggs undergo holoblastic cleavage, resulting in the formation of cells of similar size during early embryonic development.
       Example -  Alecithal , micro, oligolecithal egg.

  • Teloleithal  Egg -  Here the yolk is found on the one pole of the egg. Usually, the egg has two opposite poles, one is the animal pole and the other one is the vegetal pole. Telolecithal types of the egg are those in which yolk is present on the vegetal pole while the other pole is free.
      Example -  amphibian egg.

  • Centrolecithal Egg -  

  • Centrolecithal eggs have a centrally located yolk, surrounded by a thin layer of cytoplasm and a large central nucleus. This type of egg is found in insects, such as flies and beetles. The yolk acts as a nutrient reserve for the developing embryo. Centrolecithal eggs undergo superficial cleavage, where cell division occurs only in the outermost layer of the cytoplasm, while the yolk remains intact.

           
                               
Isolecithal , Telolecithal Eggs

3. On the Basis of Shell - on the basis of a shell, there are two types of eggs -

  • Cleidoic Eggs -  The egg is surrounded by a hard covering or shell. These types of eggs are generally seen in terrestrial animals. 
          Example - reptile, bird
                                       
  • Non - Cleidoic Eggs -  The egg which is not surrounded by any hard covering.
           Example -  All viviparous animals.
                                                   
                                                 
4. On the basis of the development of egg - There are two different sorts of eggs based on how they develop:

Types Of Cleavage In Egg
  • Determinate /mosaic type - Every component of some eggs has been preset in terms of its capacity for future development. A damaged embryo will develop if any part of the embryo is removed. This is so that no organs from the egg are permanently lost when certain parts are removed. The remaining section cannot possibly come from the missing piece. Therefore, the determinate type of egg is one that has the potential for development in the future that is predefined and takes the form of a mosaic.
          Example - annelids, molluscs.
  • Indeterminate type - An indeterminate egg is a type of egg found in certain animals, particularly reptiles and birds. It is characterized by having a high degree of developmental flexibility, meaning that the fate of the embryo is not predetermined at the time of fertilization. In indeterminate eggs, the blastomeres—the cells resulting from cleavage—do not have a fixed developmental fate, and each cell has the potential to develop into a complete and functional organism on its own.
          Example - amphibians.                 

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