Embryo

The organism in the early stages of growth and differentiation from fertilization to, in humans, the beginning of the third month of pregnancy. After that point in time, it is termed a fetus.

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1. An organism in the early stages of development. 2. In humans, the developing organism from conception until approximately the end of the second month; developmental stages from this time to birth are commonly designated as fetal. 3. A primordial plant within a seed. [G. embryon, fr. en, in, + bryo, to be full, swell]

- heterogametic e. a male e. with XY karyotype.

- hexacanth e. the e. of tapeworms of the subclass Cestoda, such as Taenia saginata, characterized by three pairs of hooks used for penetration through the gut of an intermediate host. SYN: oncosphere e..

- homogametic e. a female e. with XX karyotype.

- oncosphere e. SYN: hexacanth e..

- presomite e. an e. before the appearance of the first pair of somites, which are notable about 20–21 days after fertilization in humans.

- previllous e. the e. of a placental mammal prior to the formation of chorionic villi.

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em·bryo 'em-brē-.ō n, pl em·bry·os

1) archaic a vertebrate at any stage of development prior to birth or hatching

2) an animal in the early stages of growth and differentiation that are characterized by cleavage, the laying down of fundamental tissues, and the formation of primitive organs and organ systems esp the developing human individual from the time of implantation to the end of the eighth week after conception compare FETUS

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n.

an animal at an early stage of development, before birth. In humans the term refers to the products of conception within the uterus up to the eighth week of development, during which time all the main organs are formed. Compare fetus.

• embryonic adj.

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em·bryo (emґbre-o) [Gr. embryon] 1. in animals, those derivatives of the zygote that will eventually become the offspring, during their period of most rapid development, i.e., from the time the long axis appears until all major structures are represented. 2. in humans, the developing organism from fertilization to the end of the eighth week. Cf. fetus. 3. in plants, the element of the seed that develops into a new individual. embryonal, embryonic adj

Human embryo at various stages of development. The relative size has been distorted to emphasize correspondence of parts, and the ages are approximate.