Animal embryos ) of the wrinkled frog (
Rana rugosa) and
snake embryos In animals, fertilization begins the process of embryonic development with the creation of a zygote, a single cell resulting from the fusion of gametes (e.g. egg and sperm). The development of a zygote into a multicellular embryo proceeds through a series of recognizable stages, often divided into cleavage, blastula, gastrulation, and organogenesis. Cleavage is the period of rapid mitotic cell divisions that occur after fertilization. During cleavage, the overall size of the embryo does not change, but the size of individual cells decrease rapidly, as they divide to increase the total number of cells. Cleavage results in a blastula. The embryo's cells continue to divide and increase in number, while molecules within the cells such as RNAs and proteins actively promote key developmental processes such as gene expression, cell fate specification, and polarity. Before
implanting into the uterine wall the embryo is sometimes known as the
pre-implantation embryo or
pre-implantation conceptus. Sometimes this is called the
pre-embryo, a term employed to differentiate from an embryo proper in relation to embryonic stem cell discourses. Gastrulation is the next phase of embryonic development, and involves the development of two or more layers of cells (germinal layers). Animals that form two layers (such as
Cnidaria) are called diploblastic, and those that form three (most other animals, from
flatworms to humans) are called triploblastic. During gastrulation of triploblastic animals, the three germinal layers that form are called the
ectoderm,
mesoderm, and
endoderm. For example, the ectoderm will give rise to the skin epidermis and the nervous system, the mesoderm will give rise to the vascular system, muscles, bone, and connective tissues, and the endoderm will give rise to organs of the digestive system and
epithelium of the digestive system and respiratory system. Many visible changes in embryonic structure happen throughout gastrulation as the cells that make up the different germ layers migrate and cause the previously round embryo to fold or invaginate into a cup-like appearance. For example, in neurogenesis, a subpopulation of cells from the ectoderm segregate from other cells and further specialize to become the brain, spinal cord, or peripheral nerves. The embryonic period varies from species to species. In human development, the term fetus is used instead of embryo after the ninth week after conception, whereas in
zebrafish, embryonic development is considered finished when a bone called the
cleithrum becomes visible. In animals that hatch from an egg, such as birds, a young animal is typically no longer referred to as an embryo once it has hatched. In
viviparous animals (animals whose offspring spend at least some time developing within a parent's body), the offspring is typically referred to as an embryo while inside of the parent, and is no longer considered an embryo after birth or exit from the parent. However, the extent of development and growth accomplished while inside of an egg or parent varies significantly from species to species, so much so that the processes that take place after hatching or birth in one species may take place well before those events in another. Therefore, according to one textbook, it is common for scientists to interpret the scope of
embryology broadly as the study of the development of animals. The zygote, which will divide multiple times as it progresses throughout embryonic development, is one part of a
seed. Other seed components include the
endosperm, which is tissue rich in nutrients that will help support the growing plant embryo, and the seed coat, which is a protective outer covering. The first cell division of a zygote is
asymmetric, resulting in an embryo with one small cell (the apical cell) and one large cell (the basal cell). The small, apical cell will eventually give rise to most of the structures of the mature plant, such as the stem, leaves, and roots. The larger basal cell will give rise to the suspensor, which connects the embryo to the endosperm so that nutrients can pass between them. ground tissue will give rise to inner plant material that functions in
photosynthesis, resource storage, and physical support, and vascular tissue will give rise to connective tissue like the
xylem and
phloem that transport fluid, nutrients, and minerals throughout the plant. In heart stage, one or two
cotyledons (embryonic leaves) will form.
Meristems (centers of
stem cell activity) develop during the torpedo stage, and will eventually produce many of the mature tissues of the adult plant throughout its life. Once the embryo begins to
germinate (grow out from the seed) and forms its first true leaf, it is called a
seedling or plantlet. Plants that produce
spores instead of seeds, like
bryophytes and
ferns, also produce embryos. In these plants, the embryo begins its existence attached to the inside of the
archegonium on a parental
gametophyte from which the egg cell was generated. The inner wall of the archegonium lies in close contact with the "foot" of the developing embryo; this "foot" consists of a bulbous mass of cells at the base of the embryo which may receive nutrition from its parent gametophyte. The structure and development of the rest of the embryo varies by group of plants. Since all land plants create embryos, they are collectively referred to as
embryophytes (or by their scientific name, Embryophyta). This, along with other characteristics, distinguishes land plants from other types of plants, such as
algae, which do not produce embryos. ==Research and technology==