Anatomy & Physiology
Animal Development
Presence Of Germ (tissue) Layers Within The Embryo
In sponges there are no tissues, so the embryo does not form cell layers during its body development. In all
animals except sponges, however, there are two or more germ layers – rings of cells within the embryo
from which specialized tissues and organs are produced. The germ layers are created in several stages
during the maturing of a zygote after fertilization (Figure 10.3).
Blastula
After several cleavages (successive cell divisions by mitosis), an eight-cell stage is followed by a blastula (
BLAS -tyoo-lah), a hollow ball of cells. The central cavity within the middle of the blastula is called the
blastocele ( BLAS -toh- seal).
Gastrula and Archenteron
Following the blastula is a gastrula ( GAS -true-lah). The gastrula is literally a “little stomach” ( gastrul )
or hollow ball of several layers of germ cells . It is these germ cells (or germ layers) that eventually give
rise to the specialized tissues in the later embryo and, finally, the adult stages of life. The gastrula is created
by an infolding of the layer of surface cells around the blastula. This infolding creates another cavity, called
the archenteron ( ark - EN -ter-ahn) – the “beginning” ( arch ) form of the “intestine” ( enteron). The
archenteron (like the mature intestine) is connected to the surface by an opening.
Endoderm, Ectoderm, and Mesoderm
Around the archenteron is the endoderm ( EN -doh- derm ). The endoderm is an “inner” ( endo -) “skin” (
derm ) of germ cells from which the lining of the intestine and interior of other major body cavities,
eventually develops. An ectoderm ( EK -toh- derm ) – “outer” ( ecto -) “skin” – covers the surface of the
embryo. The ectoderm ultimately gives rise to the skin and the central nervous systems of many types of
animals. Finally, most eumetazoans have a third germ layer, the mesoderm ( ME -soh- derm ) or “middle
skin,” sandwiched in between the endoderm and the ectoderm. The mesoderm forms the muscles and most
other organs.
Various radiolarians besides the jellyfish – such as the hydras ( HIGH -drahs), sea anemones (ah- NEM oh- nees ), and coral animals – lack a mesoderm. But they still have the other two germ layers, the
endoderm and ectoderm, from which all of their adult tissues eventually develop.
Fig. 10.3 The three germ layers in the embryo.