BLASTULA

• Blastula is a hollow sphere of blastomeressurrounding a cavity.

• The cavity of blastula is called blastocoel.

• The process of formation of blastula isknownas blastulation.

• Blastula -32-64 cell stage

DIFFERENT TYPES OF BLASTULA

• The blastula of various groups of animalsdiffers in form and structure depending upona variety of factors such as the size of theamount and distribution of yolk etc. Thefollowing categories of blastulae have beenrecognized in different groups of animals

Coeloblastula

• It is a hollow blastula containing a largespacious blastocoel.

• The blastocoel is filled with a fluid containingmucopolysaccharides.

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• The blastula resulting from holoblastic equalcleavage, as in the case of echinoderms andamphioxus, is called equal coeloblastula .

• The blastoderm is single layered.

• Holoblastic unequal cleavage, as in frog,results in unequal coeloblastula.

• lt has a blastocoel displaced towards theanimal pole and a multilayered blastoderm.

Stereoblastula• This type of blastula is composed of an

aggregate of larger sized and relativelylesser number of cells without or withextremely small blastocoelic space inthe centre.

• Stereoblastula occurs in a variety ofanimals such as insects, some wormslike Nereis, mollusks like Cripidula,gymnophionan amphibians and certainfishes.

Discoblastula• Discoblastula consists of a disc

- shaped mass of blastomeresoverlying a large yolk mass.

• This blastula is the result ofmeroblastic discoidal cleavageas in most fishes, reptiles andbirds.

• There is no blastocoel, insteada slit like cavity calledsubgerminal cavity appears inbetween the blastoderm andthe yolk mass.

Blastocyst• It is the blastula stage of

mammals;• It consists of a hollow spherical

vesicular blastula, containingan inner cell mass at the animalpole.

• The embryo proper developsfrom the inner cell mass.

• The outer single layer of cellswhich encloses the blastocoelis called the trophoblast.

• The trophoblast establishesrelations with uterine wall andhelps in nutrition of thedeveloping embryo

Periblastula

• A stage in the embryonic development ofmost arthropods having centrolecithal eggs.

• The periblastula is a vesicle whose wallconsists of one layer of cells and whose cavityis filled with unbroken yolk.

• It forms as a result of the superficialsegmentation of the egg.

Fate Map

• Fate map is t