Early Development in Animals

Fertilization

The fusion of sperm (n) and ovum (n) that results in a zygote (2n).

Zygote

A single cell formed by the fertilization of sperm and ovum, marking the start of embryonic development.

Cleavage

Rapid mitotic divisions of the zygote that do not increase overall size and produce smaller cells called blastomeres.

Morula

A solid, compact mass of 16-32 cells formed during the cleavage stage.

Blastulation

The process in which the morula develops a cavity, becoming a hollow structure of cells called the blastula.

Blastocyst

The stage of development in mammals where the blastula contains an inner cell mass that will form the embryo and a trophoblast that will become the placenta.

Implantation

The process that occurs 7-10 days after fertilization, where the blastocyst embeds in the endometrium (uterine lining).

hCG (Human Chorionic Gonadotropin)

A hormone secreted by the trophoblast, maintaining the corpus luteum and progesterone production, detected in blood and urine for pregnancy tests.

Gastrulation

A critical phase occurring approximately 2-3 weeks after conception, forming three germ layers: ectoderm, mesoderm, and endoderm.

Ectoderm

The outer germ layer that develops into skin and the nervous system.

Mesoderm

The middle germ layer that forms muscles, bones, and the circulatory system.

Endoderm

The innermost germ layer that develops into the lining of the digestive tract and respiratory system.

Gastrula

The embryo at the stage of gastrulation, which consists of three germ layers.

Neurulation

The process of forming the nervous system, where the notochord signals the ectoderm to form the neural plate, which folds into the neural tube.

Organogenesis

The phase where organs are formed from the germ layers, starting after gastrulation and neurulation.

Embryo

The early developmental stage of the organism during the first approximately 8 weeks.

Fetus

The stage after about 2 months when organs begin functioning.

Gestation

The duration of pregnancy in humans, approximately 9 months, including all developmental stages.

Oviparous animals

Animals, such as birds and fish, that develop in eggs outside the body.

Yolk distribution

The distribution of yolk in eggs that affects cleavage patterns and blastula structures.

High-Yield Summary

Key points summarizing the stages of embryonic development: rapid mitosis (cleavage), solid ball formation (morula), hollow structure (blastocyst), three germ layers (gastrulation), nervous system formation (neurulation), pregnancy signal (hCG), and organ structure formation (organogenesis).