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The chorion is an outer extraembryonic membrane surrounding the early conceptus in amniotes. In humans, it includes trophoblastic tissue and extraembryonic mesoderm, and its villous region contributes to the fetal part of the placenta.

The chorion and the amnion are distinct structures. Their changing relationship during embryonic development is important for understanding the early chorionic cavity and the organization of maternal-fetal exchange.

Embryonic development is the organized sequence through which an embryo forms and differentiates before the later juvenile or fetal stages defined for its species. Cell division, movement, changing gene expression and interactions among neighbouring cells jointly build tissues and body organization. Developmental stage therefore describes more than elapsed time or total cell number.

Light-sheet imaging of living zebrafish embryos enabled repeated three-dimensional observation and tracking of nuclei across early development. ^[ref] Following one developing specimen distinguishes movement and ancestry from an ordering inferred solely from fixed specimens of different ages. A lineage record still does not by itself identify the molecular signals determining every descendant cell fate.

Juliana G. Roscito is a biologist whose research connects embryonic development, evolution and genomics. She earned her doctorate in developmental and evolutionary biology at the University of São Paulo in 2010 and subsequently conducted evolutionary-genomics research in Dresden. Her later work includes scientific management.^[ref]

In Digit evolution in gymnophthalmid lizards by Juliana G. Roscito and others, she and collaborators compared the skeletal patterns of tetrapod limbs across Gymnophthalmidae. They distinguished mild reductions, such as loss of a few bones within limb digits, from more extreme reductions that leave no externally distinct adult limb. The study also examined Sonic hedgehog gene expression in strongly reduced developing limbs.^[ref]

The comparison joins an adult phenotype to an embryonic process. Two animals with superficially similar limb reduction need not have changed the same developmental step. Examining which skeletal elements remain, alongside spatial gene expression, makes that question more precise than simply counting visible toes. Those observations identify patterns to explain; they do not by themselves prove that every case of limb loss has one molecular cause.

A live birth is delivery of offspring showing signs of life after separation from the mother. In human vital statistics, breathing or a beating heart can provide such evidence; being born alive and surviving the subsequent neonatal period are distinct outcomes.^[ref]

In comparative reproduction, delivery of living young is associated with viviparity, whereas oviposition releases eggs whose embryonic development continues outside the maternal body. The evolutionary question concerns where and how the developing offspring is supported, rather than whether the mother produces ova.

A group of New World Primates used in comparative studies of embryonic development; conclusions from a particular species or experiment should not be assigned to all Primates.

The historical view that embryonic development enlarges an already formed organism rather than generating its organization progressively.