Molecules and events on the egg surface are critical in all stages of fertilization. To reach the plasma membrane, sperm must first dissolve or enter any protective layer around the egg.

Following that, chemicals on the sperm surface attach to receptors on the egg surface, assisting in the fertilization of a sperm and egg of the same species.

Finally, modifications on the egg's surface inhibit polyspermy, which is the entrance of several sperm nuclei into the egg. If polyspermy occurs, the embryo will die as a result of the improper amount of chromosomes.

The cell-surface processes that occur during fertilization have been investigated most thoroughly in sea urchins, which are members of the phylum.

The attached image shows the developmental events in the life of a frog.

Development happens at several stages of the life cycle, regardless of the species being examined (as shown in the attached image).

A key developmental stage of a frog, for example, is metamorphosis, during which the larva (tadpole) experiences dramatic changes in anatomy before becoming an adult.

Adult animals also develop, such as when stem cells in the gonads create sperm and eggs (gametes). However, in this chapter, we will concentrate on embryonic development.

Many animal species' embryonic development contains similar phases that occur in a predetermined order.

The first is fertilization, which is the joining of sperm and egg. The cleavage-stage follows, which is characterized by a sequence of cell divisions that split, or cleave, the embryo into two halves.

Morphogenesis is the process through which cells change the form, location, and survival in animals.

Gastrulation transforms the blastula into a gastrula, which contains a rudimentary digestive cavity and three germ layers: ectoderm (blue), which creates the embryo's outer layer, mesoderm (red), which develops the middle layer, and endoderm (yellow), which forms the embryo's interior tissues.

The cortical response alters the zona pellucida in mammalian fertilization as a gradual impediment to polyspermy.

Following fertilization, there occurs cleavage, a phase of fast cell division without expansion that results in a high number of cells known as blastomeres.

The volume and distribution of yolk have a significant impact on the pattern of cleavage. In many species, the completion of the cleavage stage results in the formation of a blastula with a fluid-filled chamber, known as the blastocoel.

The mechanisms of gastrulation and organogenesis in mammals are similar to those in birds and other reptiles. The blastocyst implants in the uterus after fertilization and early cleavage in the oviduct.

The trophoblast starts the creation of the fetal placenta, while the embryo properly develops from a cell layer within the blastocyst called the epiblast.

Bird, reptile, and mammalian embryos grow within a fluid-filled sac enclosed within a shell or the uterus. The three germ layers in these creatures create four extraembryonic membranes: the amnion, chorion, yolk sac, and allantois.

Organs develop from particular parts of the three embryonic germ layers. Neurulation is an early process in vertebrate organogenesis that involves the creation of the notochord by cells of the dorsal mesoderm and the development of the neural tube from infolding of the ectodermal neural plate.

Cytoskeletal rearrangements alter the morphology of cells, which underpin cell motions in gastrulation and organogenesis, such as invaginations and convergent extension.

The cytoskeleton is also involved in cell migration, which is aided by cell adhesion molecules and the extracellular matrix. Migratory cells develop from both the neural crest and somites.

Some animal growth processes need apoptosis or programmed cell death.

Cell fate is controlled by cytoplasmic determinants and inductive cues. Embryo destiny maps developed experimentally indicate that certain areas of the zygote or blastula grow into specific portions of older embryos.

The full cell lineage of C. elegans has been determined, demonstrating that controlled cell death contributes to animal development. As embryonic development progresses, the developmental capacity of cells in all animals becomes progressively more restricted.

Positional information changes with location and cells in a growing embryo receive and respond to it. This data is frequently in the form of signaling molecules released by cells in specific areas of the embryo, such as the dorsal horn.