The whole surface of a frog blastula can be divided into three main regions. In side each region thee can be recognized areas corresponding to the future organs of the animal.
(1) The animal region:
It is a large dark, gray or black part of the blastula around the animal pole. It co9nsists of two main regions.
(a) Region of prospective central nervous system which contains material for brain spinal cord and the area of sense organs like eyes, ears and nose.
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(b) Region of prospective central nervous system which contains material for brain spinal cord and the area of sense organs like eyes, ear and nose.
(2) The Intermediate or marginal zone:
It is the grey crescent area that extends all-around the equater of blastula and contains following regions.
a) Notochordal Region: It is the region of notochord present in the dorsal side of blastula.
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b) Pre-chordal Plate: It lies next top the notochordal region towards the vegetal pole and develops into the pre-chordal connective tissue of the embryo
c) The area for anterior part of alimentary canal lies below the prechodal plate and gives rise to endodermal lining of mouth, gill region and pharynx.
d) Region of segmental muscle: That lies on both the sides of notochordal area and develops into trunk muscles of somites.
e) Ventrolateral mesoderm consists of lateral and ventral parts of marginal zone and gives raise the mesodermal lining of the body cavity, kidney and reproductive organs.
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(3) Vegetal region:
It occupies the vegetal half of the embryo and is formed of unpigmented large macromeres. This region develops into the midgut and hind gut.