ONTOGENY: STAGES OF ANIMAL DEVELOPMENT AND THEIR CHARACTERISTICS - BIOLOGY - 2025

The ontogeny is the process by which happens the development of an individual. The phenomenon begins with fertilization, and extends to the aging of organic beings. The field of biology responsible for the study of ontogeny is developmental biology.

In this process, the "translation" of the genotype - all the genetic information of a biological entity - into the phenotype that we can observe occurs. The most dramatic transformation occurs in the early stages of development, with the transformation of a cell to a complete individual.

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Today, the fusion of developmental biology and evolutionary theory, known as evo-devo, is a very popular body of knowledge that is growing by leaps and bounds. This novel field aims to explain the evolution of the immense diversity of morphologies exhibited by living organisms.

"Ontogeny recapitulates phylogeny"

Historical perspective

The relationship between ontogeny and phylogeny was a predominant view throughout the 21st century. It is widely known that different species of organisms are much more similar to each other in their embryonic stages than in adult forms. In the year 1828, Karl Ernst von Baer noticed this pattern in the Vertebrata sybphylum.

Baer noted that in different species of tetrapods there are certain similarities in the embryo, such as gills, notochord, segmentation and fin-shaped limbs.

These are formed before the typical characteristics that allow the group in question to be diagnosed in a more specific hierarchical classification order.

This idea was reformulated by the famous - and one of Charles Darwin's most passionate followers - German-born biologist Ernst Haeckel.

Haeckel is credited with the famous phrase "ontogeny recapitulates phylogeny." In other words, recapitulation proposes that the development of an organism repeats its evolutionary history from the adult forms of its ancestors.

Current vision

Although the phrase is well known today, by the middle of the 21st century it was clear that Haeckel's proposal is rarely fulfilled.

SJ Gould, the famous paleontologist and evolutionary biologist, laid out his ideas regarding recapitulation in what he called the "terminal addition principle." For Gould, recapitulation can occur as long as evolutionary change occurs by the successive addition of stages at the end of an ancestral ontogeny.

In the same way, it should also be true that the temporal duration of ancestral ontogeny had to be shortened as the lineage evolved.

Today, modern methodologies have managed to refute the concept of addition proposed by the biogenetic law.

For Haeckel, this addition occurred because of the continuous use that was given to the organs. However, the evolutionary implications of the use and disuse of organs have been discounted.

It is currently known that the branchial arches in the embryonic stages of mammals and reptiles never have the shape corresponding to adult fish.

In addition, there are variations in the timing or timing of certain stages in development. In evolutionary biology, this change is called heterochrony.

Stages of animal development

Ontogeny encompasses all the processes of development of organic beings, beginning with fertilization and ending with aging.

Logically, the most dramatic transformations occur in the first stages, where a single cell is capable of forming an entire individual. Next we will describe the ontogeny process, emphasizing the embryonic stages.

Oocyte maturation

During the process of oogenesis, an ovum (the female gamete, also called an egg) prepares for fertilization and the early stages of development. This occurs by accumulating reserve material for the future.

The cytoplasm of the ovum is an environment rich in different biomolecules, mainly messenger RNA, ribosomes, transfer RNA and other machinery necessary for the synthesis of proteins. The cell nucleus also undergoes significant growth.

Spermatozoa do not require this process; their strategy is to eliminate as much cytoplasm as possible and condense the nucleus to preserve small dimensions.

Fertilization

The event that marks the beginning of ontogeny is fertilization, which involves the union of a male and a female gamete, generally during the act of sexual reproduction.

In the case of external fertilization, as occurs in many marine organisms, both gametes are expelled into the water and are found randomly.

In fertilization, the diploid number of the individual is reintegrated and allows the combination processes between the paternal and maternal genes.

In certain cases, sperm is not necessary to activate development. But in most individuals, the embryo does not develop in the correct way. Similarly, some species can reproduce by parthenogenesis, where normal embryo development occurs without the need for a sperm.

In contrast, some eggs require activation of the sperm, but do not incorporate the genetic material of this male gamete into the embryo.

The sperm and ovum must be recognized correctly so that all post-fertilization events can take place. This recognition is mediated by a series of proteins specific to each species. There are also barriers that prevent an egg, once fertilized, from being reached by a second sperm.

Embryogenesis

After fertilization and activation of the egg, the first stages of development occur. In segmentation, the embryo repeatedly divides to become a group of cells called blastomeres.

During this last period, no cell growth occurs, only the subdivision of the mass takes place. In the end you have hundreds or thousands of cells, giving way to the blastula state.

As the embryo develops it acquires a polarity. Therefore, one can distinguish between the plant pole, located at one end, and the animal pole, rich in cytoplasm. This axis provides a reference point for development.

Types of eggs

Depending on the amount of yolk that the egg has, and the distribution of said substance, the egg can be classified as oligolecytes, heterolecytes, telolecytes, and centrolecytes.

The former have, as their name indicates, a small amount of yolk and it is distributed more or less uniformly throughout the egg. Generally its size is small. Heterolecytes have more yolk than oligolecytes, and yolk is concentrated at the vegetative pole.

The telolecitos present an abundant amount of yolk, occupying almost the entire egg. Finally, the centrolecitos have all the yolk concentrated in the central region of the egg.

Blastulation

The blastula is a mass of cells. In mammals, this cell grouping is called a blastocyst, while in most animals the cells are arranged around a central fluid cavity, called a blastocele.

In the blastula state, it has been possible to show a great increase in terms of the amount of DNA. However, the size of the entire embryo is not much larger than the original zygote.

Grastrulation

Gastrulation converts the spherical and simple blastula into a much more complex structure with two germ layers. This process is heterogeneous if we compare the different lineages of animals. In some cases, a second layer is formed without making an internal cavity.

The opening to the intestine is called the blastopore. The fate of the blastopore is a very important characteristic for the division of two great lineages: the protostomates and the deuterostomes. In the first group, the blastopore gives rise to the mouth, while in the second, the blastopore gives rise to the anus.

Thus, the gastrula has two layers: an outer layer that surrounds the blastocele, called the ectoderm, and an inner layer called the endoderm.

Most animals have a third germ layer, the mesoderm, located between the two layers mentioned above. The mesoderm can be formed in two ways: the cells arise from a ventral region of the lip of the blastopore and from there they proliferate or arise from the central region of the walls of the archnteron.

At the end of gastrulation, the ectoderm covers the embryo and the mesoderm and endoderm are located in the inner portion. In other words, the cells have a different end position than the one they started with.

Coelom formation

The coelom is a body cavity that is surrounded by mesoderm. This occurs because during the gastrulation process, the blastocele is almost completely filled with mesoderm.

This coelomatic cavity can appear in two ways: schizocelic or enterocelic. However, both coeloms are functionally equivalent.

Organogenesis

Organogenesis comprises a series of processes where each of the organs are formed.

The most relevant events include the migration of particular cells to the place where they are necessary to form said organ.

Gene expression during ontogeny

In development, it has been determined that epigenesis proceeds in three stages: pattern formation, determination of body position, and induction of the correct position for the extremities and various organs.

To generate a response, there are certain gene products, called morphogens (the definition of these entities is theoretical, not chemical). These work thanks to the formation of a differential gradient, supplying spatial information.

Regarding the genes involved, homeotic genes play a fundamental role in the development of individuals, since they define the identity of the segments.

References

1. Alberch, P., Gould, SJ, Oster, GF, & Wake, DB (1979). Size and shape in ontogeny and phylogeny. Paleobiology, 5 (3), 296-317.
2. Curtis, H., & Barnes, NS (1994). Invitation to biology. Macmillan.
3. Gould, SJ (1977). Ontogeny and phylogeny. Harvard University Press.
4. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology. McGraw – Hill.
5. Kardong, KV (2006). Vertebrates: comparative anatomy, function, evolution. McGraw-Hill.
6. McKinney, ML, & McNamara, KJ (2013). Heterochrony: the evolution of ontogeny. Springer Science & Business Media.