EMBRYOLOGY OF FROG-Structure of chick egg

Chick egg is megalecithal or polylecithal, discoidal and clidoic Yolk is present  in large amount and cytoplasm is found  in the form of a disc. This disc is known  as Germinal disc. Beneath the germinal disc yolk is present. On the basis of colour and conent yolk is of 2 types Yellow yolk-It have more phospholipids White yolk – It is rich in protein.  Phospholipids are in less amount.  Both types of yolks are arranged in alternative and concentric layers.  Innermost and  Outermost layer is of white yolkA stalk like structure of white yolk is present just beneath the germinal disc. This is known as latebra and upper part of latebra is known as Isthmus of pander. Primary envelop vitelline membrane is found around the egg.Remaining all envelops of chick egg are secreted by oviduct after fertilization. These are tertiary envelops. [Birds do not have copulatory organs but still they perform internal fertilization]. Egg of chick is filled with liquid albumin which is called as "White of egg". It is produced by lsthumus part of oviduct. Dense albumin on the sides of egg upto shell  membrane are coiled chord like structures.These are called as Chalaza. It keeps the ovum upward.
               Two shell membranes are present just beneath the shell. These are made up of keratin. A air pocket or air space is found between shell membranes in broad (wider) part of egg. The outermost part of egg is a shell made up by calcium carbonate [CaCO3). This shell is porous for exchange of gases.

                The oviduct of birds is differentiated into three parts —
                (i)Anterior part -- Magnum part—Fertilization of egg occurs in this part. After fertilization this part secretes liquid albumin arround the egg.
                (ii) Middle part — Isthmus — Keratin shell membranes are secreted around the egg by this part. (iii) Posterior part — Nidamental part — This part secretes a shell of CaCO3 around the egg.