Embryonic development. This is the period from the moment of fertilization until the baby leaves the egg (birth). It includes the stages of crushing, gastrulation, organogenesis.
Crushing is a series of zygote divisions by mitosis. The interval between divisions is very short, only DNA doubling occurs in it, and cell growth does not occur (the egg was already very large). In the process of crushing, the cells gradually decrease until they reach normal sizes. After crushing, a blastula is formed – a hollow ball from one layer of cells (blastomeres).
Then the blastula turns into gastrula – a two-layer ball. The outer layer of the gastrula cells is called the ectoderm, the inner layer is the endoderm, the hole in the gastrula is called the primary mouth, it leads into the intestinal cavity.
Organogenesis (organ formation) begins with the formation of a neural plate in the ectoderm on the dorsal side of the embryo. Further:
from the ectoderm, the skin and nervous system are formed;
from the endoderm – the digestive and respiratory system;
from the mesoderm – everything else (skeleton, muscles, circulatory, excretory, reproductive systems).
POSTEMBRIONAL DEVELOPMENT. It lasts from birth (hatching) to the beginning of reproduction (puberty). There are two types:
Direct – when the child looks like a parent, only smaller in size and some organs (mammals, birds) are underdeveloped.
Indirect (with transformation, with metamorphosis) – when the child (larva) is very different from the parent (frogs, insects). The advantage of indirect development is that parents and children do not compete with each other for food and territory.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.