Vitellogenesis and yolk composition.

Yolk is the stored food. It is stored for the use of future embryo. Yolk is     stored in this phase. Small amount of yolk is produced by the egg but most of the yolk is produced in liver.
The yolk which is formed by liver is soluble and liquid. The mitochondria of egg, converts it into insoluble and viscous form by the help of kinase enzymes (mitochondria group of egg are known as yolk nuclei). After growth stage primary oocytes are formed

TWO TYPES OF YOLK IS FOUND IN ANIMALS

       (a) Granular yolk - Formed of microgranules. Eg. most of the protostomia (Invertebrates)         animals.
      (b) Yolk platelets - In most of the chordates. The yolk is found in the form of large granlues and disc like granules. e.g. Deuterostomia animals (Vertebrates)

VARIOUS STORED SUBSTANCES IN YOLK

       (1) Phospholipid - It is present in large amount (mainly lecithin) - 32-33%
       (2) Proteins - Different types of proteins are found in yolk.

       Eg.          (I) Simple proteins -Albumins 16-17%, Globin, Globulin (Albumin maximum)
                     (ii) Phospho proteins - Phosvitin, Ovovitellin
                     (iii) Lipoprotein - Lipovitellin
       (3) Carbohydrate - It prsent in least amount in yolk (1%).
       The most common carbohydrate which is present in yolk is Mannose.
       The percentage of substances is variable in different animals
       Eg. In yolk of frog- Phospholipid - 32-33%, Protein - 16-17%, Carbohydrate - 1%,
       Water- 48-    49%