13 topics with concise reviews: definitions, key facts, and class questions.
Ontogenesis is the entire life history of an organism, from the moment of gamete fusion to natural death. In this section, we will sequentially examine all stages of individual development. We will trace the path from egg cell classification, insemination, and cleavage to the complex processes of gastrulation, histogenesis, and organogenesis. You will learn how provisional organs are formed and how the postnatal stage proceeds.
Special emphasis is placed on clinically significant topics: human embryo development, twin formation, and modern reproductive technologies such as IVF.
During classes and exams, professors most strictly test the mechanisms of gastrulation, derivatives of the three germ layers, and the causes of embryonic development disorders. Be sure to pay attention to the impact of teratogenic factors and the pathogenesis of TORCH infections—these are classic exam questions that bridge fundamental biology with actual clinical practice.
From the moment of fertilization and the formation of the zygote (a diploid single-celled embryo).
What is the main difference between complete and incomplete metamorphosis?
The key difference lies in the presence of a resting stage—the pupa. In complete metamorphosis, the larva reorganizes inside the pupa, whereas in incomplete metamorphosis, it transforms into the adult form gradually, bypassing this stage.
Why do mammalian egg cells contain so little yolk?
Although mammals evolved from reptiles, in the course of evolution they secondarily lost yolk reserves. This occurred due to the emergence of intrauterine development, in which the embryo receives nutrients from the maternal organism.
What is the difference between androgamones and gynogamones?
Androgamones are produced by spermatozoa, while gynogamones are produced by egg cells. Upon meeting, they ensure the chemical interaction of gametes at a distance.
Does the embryo increase in size during cleavage?
No, the overall size of the embryo by the end of cleavage does not exceed the size of the initial zygote. This is due to the fact that the cell cycle of blastomeres lacks growth periods ($G_1$ and $G_2$).
What is the difference between the gastrocoel and the blastocoel?
The blastocoel is the cavity inside the blastula, into which cells migrate during immigration. The gastrocoel is the cavity of the primitive gut, formed by invagination and communicating with the external environment through the blastopore.
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