Embryology

Study of embryos and their formation. Historical embryology combined dissection, optical microscopy, comparison of developmental stages and, increasingly, experimental intervention.

A cloaca is a common chamber receiving outputs from more than one of the digestive, urinary and reproductive systems. It occurs in many vertebrates, while the early human embryo has a cloacal region that becomes partitioned during development.

The developmental chamber and the adult arrangement must be distinguished: a shared embryonic space does not mean the mature human rectum, urinary tract and reproductive tract all empty through one opening.

The connecting stalk is an early extraembryonic attachment between the developing embryo and the chorion. As the body folds and extraembryonic structures are reorganized, this attachment contributes to the umbilical cord.

Its position helps explain the arrangement of the chorionic cavity: most early embryonic and associated structures can be surrounded by that cavity while remaining attached at the connecting stalk.^[ref]

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.

The extraembryonic coelomic space surrounding the early amniotic and yolk-sac structures except where the connecting stalk provides attachment.

The allantois is an extraembryonic outgrowth associated with the embryonic hindgut region. Its roles differ among amniote lineages: in an egg it can contribute to waste storage and respiratory exchange, while in placental mammals its associated tissues participate in umbilical and placental development.

In the human developmental account, the connection toward the developing urinary bladder is related to the urachus. The same inherited structure can therefore have different physiological prominence in different reproductive systems.

Embryonic endodermal outpocketing between pharyngeal arches. The first contributes to the tubotympanic recess.

Embryonic region containing the developing dorsal aorta and associated tissues; a site of emergence of definitive haematopoietic stem cells.

The yolk sac is an extraembryonic structure associated with the embryonic gut. In yolk-rich eggs it mediates access to stored nutrients; the human yolk sac also has early developmental roles despite not containing a large yolk store.

Those roles include early blood formation and contributions associated with the developing gut and germ-cell lineage. The vitelline duct connects it with the embryonic intestine during a stage of development, so its name alone does not describe every function or the persistence of the connection.^[ref]

The vitelline duct connects the embryonic intestine to the yolk sac. In normal human development this connection narrows and disappears as the gut and surrounding structures mature.

Persistence of all or part of the connection can produce different anatomical remnants. Following the original connection explains why a remnant can occur at the intestinal side, the umbilical side or along the intervening route.^[ref]

An embryologist studies embryos and their development. Work can compare stages of cell differentiation and morphogenesis, or test how an intervention changes later developmental outcomes. The scientific role therefore connects observation of structure with experimental tests of developmental organization.

Embryologist whose separated sea-urchin blastomeres demonstrated regulatory development in the tested conditions.

Embryologist who used interventions on frog embryos to investigate developmental organization.

Embryologist who investigated fertilization and nuclear participation in the development of embryos.

Embryologist whose 1827 work describes the mammalian ovum.

Progressive formation and differentiation of an organism during biological development, historically contrasted with preformationism. It must not be confused with the modern concept of epigenetics.

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