blastocyst
a fluid-filled ball of cells descended from a fertilized ovum
bastomere
a cell of a clastocyst
cleavage
a series of rapid mitotic cell divisions after fertilization
critical period
the time during prenatal development when a structure is sensitive to damage from a mutation or an environmental intervention
crossing over
an event during prophase I when homologs exchange parts
dizygotic twins
twins that originate as two fertilized ova; fraternal twins
ectoderm
the outermost primary germ layer of the primordial embryo
embryo
in humans, the prenatal organism from fertilization until the end of the 8th week, when all basic structures are present
endoderm
the innermost germ layer of the primordial embryo
epigenetic
any effect that modifies gene expression without changing the DNA sequence, such as methylation
equational division
the second meiotic division, yielding four cells from two
fetus
the prenatal human after the 8th week of development, when structures grow and specialize
gamete
a sex cell
gastrula
a three-layered embryo
gonads
paired structures in the reproductive system where sperm or oocytes are manufactured
homologous pair
two chromosomes with the same gene sequence
independent assortment
inheritance of a gene on one chromosome does not influence inheritance of a gene on a different chromosome (mendels second law) because of the random arrangement of homologous chromosome pairs, in terms of maternal and paternal origin, down the center of a cell in metaphase 1
inner cell mass
a clump of cells on the inside of the blastocyst that will continue developing into an embryo
mesoderm
the middle primary germ layer
monozygotic twins
twins that originate as a single fertilized ovum; identical twins
morula
the very early prenatal stage that resembles a mulberry
neural tube
a structure in the embryo that develops into the brain and spinal cord
oocyte
the female gamete, an egg
oogenesis
oocyte formation
