Book - The Frog Its Reproduction and Development 15

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Rugh R. Book - The Frog Its Reproduction and Development. (1951) The Blakiston Company.

Historic Disclaimer - information about historic embryology pages

Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Glossary of Frog Embryological Terms

A supplementary list of some 350 specialized terms may be found in the author's "Experimental Embryology, a Manual of Techniques and Procedures," Minneapolis, Minn., Burgess Publishing Co., 1948.

A

Acidophil — oxyphil: cell constituents which stain with acid dyes, often used to designate an entire cell type. (See Basophil.)

Acrosome — apical organ at tip of mature spermatozoon, derived from spermatosphere (idiosome or centrosome) and presumably functional in aiding penetration of egg cortex by spermatozoon during fertilization. (Syn., perforatorium.)

Activation — process of initiating development in egg, normally achieved by spermatozoon of same species but also accomplished artificially (parthenogenesis); term also used to refer to stimulation of spermatozoon to accelerated activity by chemical (fertilizin) means.

Adnexa — extra-embryonic structures (e.g., yolk sac) discarded before adult condition is attained.

Aestivation — reduced activity of some animals during heat of summer. Opposed to hibernation.

Agglutination — cluster formation; a spontaneously reversible reaction of spermatozoa to certain chemical situations (e.g., egg water).

Aggregation — coming together of cells, such as spermatozoa, without sticking; a non-reversible response comparable to chemotropism.

Albuginea of Testis — stroma of primitive testis which forms a layer between germinal epithelium and seminiferous tubules.

Albumen — protein substance secreted by walls of oviducts around egg of reptiles and birds.

Albumen Sac — 2-layered ectodermal sac enclosing albumen of chick egg during early development of embryo, separated for a time from yolk by vitelline membrane; later to release some of its contents into amniotic cavity through ruptured sero-amniotic connection.

Amitosis — direct nuclear division without chromosomal rearrangements; generally thought to be a sign of decadence or of high specialization, if it occurs at all.

Amphiblastula — double-structured blastula as in Porifera (sponges).

Amphimixis — mixing of germinal substances accomplished during fertilization.

Amphitene — one end of chromosome is thick, one end is thin, moving toward full pachytene during maturation.

Amplexus — sexual embrace of female amphibian by male, a process which may (frogs and toads) or may not (urodeles) occur at time of oviposition.

Anal Plate — thickening and invagination of mid-ventral ectoderm which meets evaginating endoderm of hindgut, later to be perforated as proctodeum (anus). (Syn., cloacal membrane.)

Analogy — similarity of parts in respect to function rather than to structure.

Anamniota — forms which never develop an amnion, e.g., cyclostomes, fishes, amphibia.

Anaphase — phase of mitosis when paired chromosomes are separating at equatorial plate and begin to move toward ends of spindle.

Anastomosis — joining together, as of blood vessels and nerves, generally forming a network.

Androgen — hormonal secretion of interstitial tissue of testis.

Androgenesis — development of an egg with paternal (sperm) chromosomes only, accomplished by removing or destroying egg nucleus before syngamy.

Angioblast — migratory mesenchyme cell associated with formation of vascular endothelium.

Animal Pole — region of egg where polar bodies are formed; region of telolecithal egg containing nucleus and bulk of cytoplasm; gives rise largely to ectodermal derivatives. (Syn., apical pole or hemispheres.)

Anlage — rudiment; group of cells which indicate a prospective development into a part or an organ. (Syn., ebauche or primordium.)

Anterior — toward head; head end. (Syn., cephalic, cranial, rostral.)

Anura — tailless amphibia (e.g., frogs and toads). (Syn., Salientia.)

Aortic Arch — blood vessel which connects dorsal and ventral aortae by way of visceral arch.

Aqueduct of Sylvius — ventricle of mesencephalon (mesocoel) becomes aqueduct of Sylvius, connecting with cavities of optic lobes. (Syn., iter.)

Aqueous Humor — fluid which fills anterior and posterior chambers of eye between lens, probably derived from mesoderm.

Archencephalon — pre-chordal brain, e.g., forebrain. Brain anterior to anterior end of notochord.

Archenteric Pouch — See Enterocoel.

Archenteron — primitive gut found in gastrula and communicating with outside by blastopore; precursor of embryonic gut. (Syn., gastrocoel, enteron.)

Arcualia — small blocks of sclerotomal connective tissue involved in formation of vertebrae.

Asexual Reproduction — reproduction without union of gametes; generally with no maturation divisions.

Aster — "star-shaped structure" surrounding centrosome (Fol, 1877); lines radiating in all directions from centrosome during mitosis.

Astral Rays — lines which make up aster.

Astrocytes — stellate-shaped cells arising from spongioblasts of mantle layer, classified under the more general term of neuroglia.

Atrium — two upper chambers of frog's embryonic heart, later to be known as auricles.

Attachment Point — point of chromosome to which spindle fiber is attached and therefore portion of chromosome nearest centrosome in anaphase. (Syn., centrosome, chromocenter, kinetochore.)

Attraction Sphere — See Centrosphere.

Auricles — two upper chambers of adult frog's heart, derived from embryonic atria.

Autogamy — self-fertilization.

Autosome — any chromosome except so-called sex (X or Y) chromosomes.

Auxocyte — pre-meiotic germ cell. (Syn., primary cyte, meiocyte.)

Axial Filament — central fiber in tail of a spermatozoon.

Axial Mesoderm — that portion of epimeric mesoderm nearest notochord. (Syn., vertebral plate.)

Axis — imaginary central or median line, generally correlated with a gradient.

Axis of the Cell — imaginary line passing through centrosome and nucleus of a cell, generally also through geometrical center of cell. In an egg such an axis generally is also gradient axis of materials such as cytoplasm, yolk, pigment, etc.

Axis of the Embryo — imaginary line representing antero-posterior axis of the future embryo.

B

Balancers — cylindrical and paired projections of ectoderm with mesenchymatous cores, used as adhesive organs in place of (anuran) suckers by many urodele amphibia.

Balfour's Law — "The velocity of segmentation in any part of the ovum is, roughly speaking, proportional to the concentration of the protoplasm there; and the size of the segments is inversely proportional to the concentration of the protoplasm." The intervals between cleavages increase in proportion to the amount of yolk which a cell contains in its protoplasm.

Basal Plate — ventro-lateral wall of myelencephalon, separated from dorsolateral alar plate by sulcus limitans.

Basophil — cell constituents having an affinity for basic dyes, often used as an adjective for an entire cell. (See Acidophil.)

Bidder's Organ — anterior portion of anuran pro-gonad, somewhat ovarian in character, developing from part of gonad rudiment consisting wholly of cortex; its development indicates failure of medullary substance to diffuse to anterior extremity of gonad rudiment.

Biogenetic Law — embryos of higher species tend to resemble embryos of lower species in certain respects but are never like adults of lower species. Embryonic development is a gradual deviation from the more general (phylogenetic) to the more specific characters of the individual species. Not to be confused with recapitulation theory.

Blastema — indifferent group of cells about to be organized into definite tissue; nev/ly formed cells covering a cut surface, functional in regeneration of tissues.

Blastocoel — cavity of blastula. (Syn., segmentation or subgerminal cavity.)

Blastoderm — living portion of egg from which both embryo and all of its membranes are derived. The cellular blastodiscs. "Because the embryo chooses this as its seat and its domicile, contributing much to its configuration out of its own substance, therefore, in the future we shall call it blastoderm" (Pander, 1817).

Blastomere — cellular unit of developing egg or early embryo, prior to time of gastrulation. Smaller blastomeres are micromeres; intermediate ones are mesomeres; larger ones are macromeres, where there is great disparity in size.

Blastopore — opening of archenteron (gastrocoel) to exterior, occluded by yoliv plug in amphibian embryos; consisting of a slit-like space between elevated margin of blastoderm and underlying yolk of chick egg; represented in amniota as primitive streak.

Blastopore, Dorsal Lip of — region of first involution of cells in amphibian gastrula; general area of the "organizer"; original gray crescent area; cells which turn in beneath potential central nervous system (Amphioxus) and form roof of archenteron. (Syn., germ ring or marginal zone.)

Blastopore, Ventral Lip of — region of blastopore opposite dorsal lip; region which gives rise to peristomial mesoderm of frog. (Syn., germ ring.)

Blastula — stage in embryonic development between appearance of distinct blastomeres and end of cleavage (i.e., beginning of gastrulation); a stage generally possessing a primary embryonic cavity or blastocoel; invariably monodermic. (See specific types under specific names.)

Blood Islands — pre-vascular groups of mesodermal cells found in splanchnopleure, from which will arise blood vessels and corpuscles.

Bowman's Capsule — double-walled glomerular cup associated with uriniferous tubule.

Branchial — having to do with respiration. (Syn., gill.)

Branchial Arch — visceral arches, beginning with third pair, which contain blood vessels which (phylogenetically) have respiratory function during embryonic development. Mesodermal components which support those blood vessels are branchial arches. (Syn,, gill arch.) (See Visceral Arches.)

Branchial Artery — blood vessel which actually passes through gills (external or internal) of frog embryo. (Syn., gill artery.)

Branchial Chamber — closed chamber (except for a single spiracular opening on left side) which encloses internal gills of frog embryo. (Syn., opercular or gill chamber.)

Branchial Cleft — opening between branchial arches formed by invaginating head ectoderm and evaginating pharyngeal endoderm (pouch) through which water passes from pharynx to outside of frog. (Syn., gill cleft or slit, some visceral clefts.)

Branchial Groove — ectodermal invagination anterior or posterior to visceral arch, which joins branchial pouch to form branchial cleft, in most instances.

Branchiomery — type of serial metamerism involving respiratory structures exemplified by visceral arches.

Bud — undeveloped branch, generally an anlage of an appendage (e.g., limb or wing bud).

Budding — reproductive process by which a small secondary part is produced from parent organism, and which gradually grows to independence.

Bulbus Arteriosus — most anterior division of early, tubular, embryonic heart which leads from ventricle to truncus arteriosus.

C

Cardinal Veins — anterior, posterior, and sub-cardinal veins; anterior veins receive blood from head, including first three segmental veins; posterior veins receive blood from all pairs of trunk segmental veins and from veins of Wolffian bodies; paired cardinals enlarge and fuse, left half degenerates, and balance fuses with developing inferior (posterior) vena cava.

Cell — protoplasmic territory under control of a single nucleus, whether or not territory is bounded by a discrete membrane. By this definition a syncytium is made up of many cells with physiological rather than morphological boundaries.

Cell Lineage — study of origin and fate of specific blastomeres in embryonic development. (Syn., cytogeny.)

Cell Theory — body of any living organism is composed of structural and functional units, the primary agents of organization called cells. Each cell consists of a nucleus and its sphere of influence, including the cytoplasm, generally circumscribed by a membrane. "Omnis cellula e cellula" (Virchow).

Central Canal — See Neurocoel.

Centriole — granular core of centrosome.

Centrosome — granule (centriole) and surrounding sphere of rays (centrosphere) which function as kinetic centers in mitosis. Center of aster which does not disappear when astral rays disappear. Dynamic center of mitosis.

Centrosphere — rayed portion of centrosome; structure in spermatid which gives rise to acrosome. (Syn., spermatosphere, idiosome, attraction sphere. )

Cephalic Flexure — ventral bending of embryonic head at level of midbrain and hindbrain.

Chimera — compound embryo generally derived by grafting major portions of two embryos, usually of different species; may be derived by abnormal chromosome distribution in cleavage after normal fertilization.

Choana — openings of olfactory organ into pharynx, internal nares. Sometimes also used in connection with external olfactory opening.

Chondrification — process of forming cartilage, by secretion of a homogeneous matrix between the more primitive cells.

Chondrin — chemical substance in cartilage which makes it increasingly susceptible to basic stains.

Chondrocranium — that portion of skull which is originally cartilaginous.

Chorda Dorsalis — Syn., notochord.

Chorda Mesoderm — region of the late (amphibian) blastula, arising from gray crescent area, which will give rise to notochord and mesoderm and will, if transplanted, induce formation of secondary medullary folds.

Choroid Coat — mesenchymatous and sometimes pigmented coat within sclerotic coat but surrounding pigmented layer of eye in vertebrate embryos.

Choroid Fissure — inverted groove in optic stalk whose lips later close around blood vessels and nerves that enter eyeball.

Choroid Knot — thickened region of fused lips of choroid fissure, near pupil, from which arise cells of iris.

Chromatid — one of the parts of a tetrad (McClung, 1900); really a longitudinal half of a chromosome.

Chromatin — deeply staining substance of nuclear network and chromosomes, consisting of nuclein; gives Feulgen reaction and stains with basic dyes.

Chromatophore — pigment-bearing cell frequently capable of changing size, shape, and color; cells responsible for superficial color changes in animals; behavior under control of sympathetic nervous system or neurohumors.

Chromidia — granules within cytoplasm which stain like chromatin and which may actually be extruded chromatin granules.

Chromomere — unit of chromosome recognized as a chromatin granule.

Chromonema — slender thread of chromatin which is core of chromosome during mitosis.

Chromophil — cells which have an affinity for dyes.

Chromophobe — cells whose constituents are non-stainable; have no affinity for dyes.

Chromosome — chromatic or deeply staining bodies derived from nuclear network and containing a matrix and one or more chromonemata during process of mitosis; bodies found in all somatic cells of normal organism in a number characteristic of the species; bearers of gene.

Cleavage — mitotic division of egg resulting in blastomeres. (Syn., segmentation. )

Cleavage, Accessory — cleavage in peripheral or deeper portions of (chick) germinal disc caused by supernumerary sperm nuclei following (normal) polyspermy, sometimes occurring in urodeles.

Cleavage, Asymmetrical — extremely unequal divisions of egg as in Ctenophore.

Cleavage, Bilateral — cleavage in which egg substances are distributed symmetrically with respect to median plane of future embryo.

Cleavage, Determinate — cleavage in which certain parts of future embryo may be circumscribed in certain specific (early) blastomeres; cleavage which produces blastomeres that are not qualitatively equipotential, i.e., when such blastomeres are isolated they will not give rise to entire embryos. (Syn., mosaic development.)

Cleavage, Dexiotropic — cleavage resulting in a right-handed production of daughter blastomere(s), as in spiral cleavage.

Cleavage, Discoidal — See Cleavage, Meroblastic.

Cleavage, Equatorial — cleavage at right angles to egg axis, opposed to vertical or meridional; often the typical third cleavage plane. (Syn., latitudinal or horizontal cleavage.)

Cleavage, Holoblastic — complete division of egg into blastomeres, generally equal in size although not necessarily so (e.g., Amphioxus). (Syn., total cleavage.)

Cleavage, Horizontal — See Cleavage, Equatorial.

Cleavage, Indeterminate — cleavage resulting in qualitatively equipotential blastomeres in early stages of development. When such blastomeres are isolated from each other they give rise to complete embryos. Opposed to mosaic development. (Syn., regulatory development.)

Cleavage, Latitudinal — See Cleavage, Equatorial.

Cleavage Laws — See specific laws under names of Balfour, Hertwig, and Sachs.

Cleavage, Levotropic — cleavage resulting in left-handed or counterclockwise production of daughter blastomere(s) as in some cases of spiral cleavage.

Cleavage, Meridional — cleavage along egg axis, opposed to equatorial; generally the first two cleavages on any egg. (Syn., vertical cleavage.)

Cleavage, Meroblastic — cleavage restricted to peripherally located protoplasm, as in chick egg. (Syn., discoidal cleavage.)

Cleavage Nucleus — nucleus which controls cleavage. This may be syngamic nucleus of normal fertilization, egg nucleus of parthenogenetic or gynogenetic eggs, or sperm nucleus of androgenetic eggs.

Cleavage Path — path taken by syngamic nuclei to position awaiting first division.

Cleavage, Radial — holoblastic cleavage which results in tiers of cells.

Cleavage, Spiral — cleavage at an oblique angle with respect to egg axis so that resulting blastomeres (generally micromeres) lie in an interlocking fashion within furrows of original blastomeres, due to intrinsic genetic factors (e.g., Mollusca).

Cleavage, Superficial — cleavage around periphery of centrolecithal eggs. (Syn., peripheral cleavage.)

Cochlea — portion of original otic vesicle associated with sense of hearing; supplied by vestibular ganglion of eighth cranial nerve, having to do with equilibration.

Coeloblastula — spherical ball of blastomeres with a central cavity (e.g., Echinoderms).

Coelom — mesodermal cavity from walls of which gonads develop; cavity subdivided in higher forms into pericardial, pleural, and peritoneal cavities. (Syn., extra-embryonic body cavity and exocoel.)

Coitus — copulation of male and female, term generally used in connection with mammals. Comparable situation in amphibia is called amplexus.

Collecting Tubule — portion of nephric tubule system leading to nephric duct (Wolffian, etc.); term also used to refer to tubules which conduct spermatozoa from seminiferous tubule to vasa efferentia, within testis.

Colloid — dispersed substance whose particles are not smaller than 1 ^ and not larger than 100 jx, approximately. Physical state of protoplasm.

Columella — bone in tubo-tympanic cavity of frog which aids in auditory sensations. (Syn., plectrum, malleus.)

Competence — ability of embryonic area to react to stimulus (e.g., evocator).

Concrescence — coming together of previously separate parts (cell areas) of embryo, generally resulting in a piling up of parts. One of the corollaries of gastrulation where a bottle-neck of cell movements occurs at lips of blastopore. Original meaning (His, 1874) referred to presumed preformed parts of fish germ ring. (See Confluence.)

Cone, Fertilization — conical projection of cytoplasm from surface of egg to meet spermatozoon which is to invade egg cortex. Cone makes contact and then draws sperm into egg. Not universally demonstrated or seen in frog, but seen in starfish (Chambers). (Syn., exudation cone.)

Cones of Growth — enlarged outgrowth of neuroblast forms axis cylinder or axon of nerve fiber and is termed cone of growth because growth processes by which axon increases in length are supposed to be located there.

Confluence — similar to concrescence except that this term refers specifically to "flow" of cells (or areas) together, whether or not they are piled up.

Constriction — gradual closure of blastopore (diametrical reduction of germ ring) over yolk toward vegetal pole. May be due to stretching of marginal zone, to pull or tension of dorsal lip, or even to narrowing of marginal zone. (Syn., convergence [Jordan] or Konzentrisches Urmundschluss [Vogt].)

Convergence, Dorsal — material of marginal zone moves toward dorsal mid-line as it involutes during gastrulation, resulting in a compensatory ventral divergence. (Syn., confluence [Smith] or dorsal Reffung [Vogt].)

Copulation Path — second portion of sperm migration path through egg toward egg nucleus, when there is any deviation from entrance or penetration path; path of spermatozoon which results in syngamy.

Cords, Medullary — structures which give rise to urogenital connections and take part in formation of seminiferous tubules, and are derived from blastema of mesonephric cords.

Cords, Sex — strands of somatic cells and primordial germ cells growing from cortex toward medulla of gonad primordium. Best seen in early stages of testes development.

Cornea — transparent head ectoderm plus underlying mesenchyme form a layer directly over eye of vertebrates, known as cornea.

Corticin — sex-differentiating substances which spread in some amphibia by blood stream and in other forms by diffusion and act as a hormone. (See Medullarin.)

Cranial — relative to head; "craniad" means toward head. (Syn., rostral, cephalad.)

Cranial Flexure — bending of forebrain forward with angle of bend occurring transversely at level of midbrain. (See Cephalic Flexure.)

Crescent, Gray — crescentic area between original animal and vegetal pole regions on surface of frog's egg, gray in color because of migration of pigment away from area and toward sperm entrance point (Roux, 1888); region of presumptive chorda-mesoderm, future blastopore, and anus.

Crest, Neural — paired cell masses derived from ectoderm cells along edge of former neural plate, and wedged into space between dorso-lateral wall of closed neural tube and integument. Gives rise to spinal ganglia, sympathetic ganglia, and chromatophores.

Crest Segment — original neural crest becomes divided into segments from which develop spinal and possibly cranial ganglia.

Cross-Fertilization — union of gametes produced by different individuals which, if they are of different species, may produce hybrids.

Crossing Over — mutual exchange of portions of allelomorphic pairs of chromosomes during process of synapsis in maturation.

Cyclopia — failure of eyes to separate; median fusion of eyes which may be due to suppression of rostral block of tissue which ordinarily separates eyes; exaggeration of vegetativization tendencies.

Cyst — tubular portions of testis within which aggregations of germ cells mature, often (e.g., Rhomaleum) containing cells all in same stage of maturation.

Cystic Duct — narrow, proximal portion of embryonic bile duct leading from gallbladder to common bile duct.

Cytasters — asters arising apart from nucleus in cytoplasm.

Cyte — suffix meaning cell (e.g., osteocyte for bone cell, oocyte for egg cell). (See specific definitions.)

Cytology — study of cells.

Cytolysis — breakdown of cell indicated by dispersal of formed components.

Cytoplasm — material of cell exclusive of nucleus; protoplasm apart from nucleoplasm.

D

Delamination — separation (of cell layers) by splitting, a process in mesoderm formation.

Dermal Bones — bony plates which originate in dermis and cover cartilaginous skull.

Dermatome — outer unthickened wall of somite which gives rise to dermis. (Syn., cutis plate.)

Dermis — deeper layers of skin entirely derived from mesoderm (dermatome).

Dermocranium — portion of skull which does not go through an intermediate cartilaginous stage in development. (Syn., membranocranium.)

Determination — process of development indicated when a tissue, whether treated as an isolated unit or as a transplant, still develops in the originally predicted manner.

Determination of Sex — mechanism by which realization of sex differences is achieved, generally thought to be associated with chromosomal relations.

Deutencephalon — caudal region of brain which later forms mesencephalon and rhombencephalon.

Deutoplasm — yolk or secondary food substances of egg; non-living.

Development — gradual transformation of dependent differentiation into self-differentiation; transformation of invisible multiplicity into a visible mosaic elaboration of components in successive spatial hierarchies.

Development, Mosaic — "all the single primordia stand side by side, separate from each other like the stones of a mosaic work, and develop independently although in perfect harmony with each other, into the finished organism" (Spemann, 1938). Some believe there is prelocalization of embryonic potencies within egg, test for which would be self-differentiation.

Development, Regulative — type of development requiring organizer or inductor influences since each of the early blastomeres could develop into whole embryos. Structures are progressively determined through action of evocators.

Diencephalon — portion of forcbrain posterior to telencephalon, including second and third neuromeres.

Differentiation — acquisition of specialized features which distinguish areas from each other; progressive increase in complexity and organization, visible and invisible; elaboration of diversity through determination leading to histogenesis; production of morphogenetic heterogeneity; process of change from a simple to a complex organism. (Syn., diff erenzierung. )

Differentiation, Axial — variations in density of chemical and often indefinable inclusion in direction of one diameter of the egg, called egg axis.

Differentiation, Dependent — all difi[erentiation that is not self-diflferentiation; development of parts of organism under mutual influences, such influences being activating, limiting, or inhibiting. Inability of parts of organism to develop independently of other parts.

Differentiation, Self perseverance in a definite course of development of a part of an embryo, regardless of its altered surroundings (Roux, 1912).

Diocoel — cavity of diencephalon, ultimate third ventricle.

Diploid — normal complement of chromosomes in somatic and primordial germ cells, twice the haploid number characteristic of mature gametes.

Diplotene — stage in maturation following pachytene when chromosomes again appear double and do not converge toward centrosome. Sometimes refers to split individual chromosomes.

Discoblastula — disc-shaped blastula found in cases of discoidal (meroblastic) cleavage (e.g.. Cephalopoda and chick).

Distal — farther from any point of reference, away from main body mass.

Divergence, Ventral — divergence of material from mid-ventral line, compensatory to process of dorsal convergence in gastrulation (Vogt).

Diverticulum — blind outpocketing of a tubular structure (e.g., liver or thyroid anlage).

Dominance — parts of a system which have greater growth momentum and also which gather strength from the rest, such as dorsal lip of blastopore.

Dorsal Mesentery — membrane formed by doubling of peritoneum from mid-dorsal line of body cavity, which supports intestine.

Dorsal Root Ganglion — aggregation of neuroblasts which are derived from neural crests and which send their processes into dorsal horns of spinal cord.

Dorsal Thickening — roof of mesencephalon which gives rise to optic lobes.

'Duct — See ducts under specific names.

Ductus Arteriosus — See Ductus Botalli.

Ductus Botalli — dorsal portion of sixth pair of aortic arches which normally becomes occluded after birth, remainder of arch giving rise to pulmonary arteries. (Syn:, ductus arteriosus.)

Ductus Cuvieri — union of all somatic veins which empty directly into heart, specifically the vein which unites common cardinals and sinus venosus. Sometimes regarded as synonymous with common cardinal.

Ductus Endolymphaticus — dorsal portion of original otic vesicle which has lost all connections with epidermis, and which is partially constricted from region which will form semi-circular canals.

Duodenum — portion of embryonic gut associated with outgrowths of pancreas and liver (bile) ducts.

Dyads — aggregations of chromosomes consisting of two rather than four (tetrad) parts, term used to describe condition during maturation process.

E

Ecdysis — process of molting a cuticular layer, shedding of epithelium.

Ectoblast — See Epiblast.

Ectoderm — outermost layer of didermic gastrula. (Syn., epiblast.)

Ectoplasm — external layer of protoplasm of egg cell; layer immediately beneath cell membrane. (Syn., egg cortex.)

Edema — condition in which tissues hold an excess of water, common in parthenogenetic tadpoles. (Older spelling: oedema.)

Egg, Alecithal — eggs with little or no yolk. Literally means "without yolk."

Egg, Cleidoic — eggs, such as those of reptiles, birds, and oviparous mammals, which are covered by a protective shell.

Egg, Ectolecithal — egg having yolk around formative protoplasm. Opposed to centrolecithal.

Egg Envelope — material enveloping egg but not necessarily a part of the egg, such as vitelline membrane, chorion, jelly, albumen.

Egg, Giant — abnormal polyploid condition where chromosome complexes are multiplied, resulting in giant cells and embryos.

Egg, Homolecithal — egg (e.g., mammal) in which but little yolk is scattered throughout cytoplasm.

Egg, Isolecithal — eggs with homogeneous distribution of yolk; may be isolecithal, alecithal, or homolecithal.

Egg Jelly — mucin covering deposited on amphibian egg as it passes through oviduct.

Egg, Macrolecithal — egg with large amount of yolk, generally telolecithal.

Egg Membranes — include all egg coverings such as vitelline membrane, chorion, and tertiary membranes.

Egg, Microlecithal — egg with small amount of yolk. (Syn., meiolecithal egg, oligolecithal egg.)

Egg Receptor — part of Lillie's scheme picturing parts that go into the fertilization reaction involving fertilizin. Egg receptor plus amboceptor plus sperm receptor gives fertilization.

Egg, Telolecithal — egg with large amount of yolk concentrated at one pole.

Egg Water — watery extract of materials diffusing from living eggs, presumably the "fertilizin" of Lillie. (Syn., egg water extract.)

Ejaculation — forcible emission of mature spermatozoa from body of male.

Ejaculatory Duct — short portion of mesonephric duct (mammal) between seminal vesicles and urethra.

Emboitement — preformationist theory of Bonnet and others based on idea that ovary of first female (Eve?) contained the miniatures of all subsequently existing human beings. (Syn., encasement theory.)

Embryo — any stage in ontogeny of fertilized egg, generally limited to period prior to independent food-getting. Stage between second week and second month of human embryo.

Endocardium — delicate endothelial tissue forming lining of heart.

Endochondral Bone — bone preformed in cartilage. (Syn., cartilage bone.)

Endoderm — innermost layer of didermic gastmla. (Syn., entoderm.)

Endolymphatic Duct — See Ductus Endolymphaticus.

Endolymphatic Sac — See Saccus Endolymphaticus.

Endoplasm — inner medullary substance of (egg) cell which is generally granular, soft, watery, and less refractive than ectoplasm.

Entelchy — Driesch's theory of an (intangible) agent controlling development. (Syn., elan vital.)

Enterocoel — cavity or pouch within mesoderm just formed by evagination of gut (enteron) endoderm as in Amphioxus. (Syn., gut pouch, coelomic pouch, archenteric pouch.)

Enteron — definitive gut of embryo, always lined with endoderm.

Ento-mesoderm — refers to portion of invaginating blastoporal lips which will induce formation of medullary fields in amphibian embryo.

Entrance Cone — temporary depression on surface of egg following entrance of spermatozoon.

Entrance Path — See Path, penetration.

Ependymal Cells — narrow zone of non-nervous and ciliated cells which surround central canal (neurocoel), from outer ends of which branching processes extend to periphery, such processes forming a framework for other cellular elements in spinal cord and brain.

Epiblast — outermost layer of early embryo from which the various germ layers may be derived.

Epiboly — growing, spreading, or flowing over; process by which rapidly dividing animal pole cells or micromeres grow over and enclose vegetal pole material. Increase in areal extent of ectoderm.

Epibranchial Placode — placode (thickening) external to gills related to lateral line organs and tenth cranial nerves, (Syn., suprabranchial placode.)

Epidermis — ectodermal portion of skin including cutaneous glands, hair, feathers, nails, hoofs, and some types of horns and scales.

Epigenesis — development of systems starting with primitive, homogeneous, lowly organized condition and achieving great diversification.

Epimere — most dorsal mesoderm, that lying on either side of nerve and notochord, which gives rise to somites. (Syn., axial mesoderm.)

Epiphysis — evagination of anterior diencephalon of vertebrates which becomes separated from brain as pineal (endocrine) gland of adult.

Epithelioid Bodies — endodermal masses arising from second and third visceral pouches of amphibia.

Epithelium — thin covering layer of cells; may be ectodermal, endodermal, or mesodermal.

Equational Maturation Division — maturational divisions in which there is no (qualitative) reduction in chromosomal complex, similar in results to mitosis.

Equatorial Plate — lateral view of chromosomes, lined up on mitotic spindle, prior to any anaphase movement.

Eustachian Tube — vestige of endodermal portion of hyomandibular pouch connecting middle ear and pharyngeal cavities and lined with endoderm.

Evagination — growth from any surface outward.

"Ex Ovo Omnia" — -all life comes from the egg (Harvey, 1657).

Exogastrula — gastrulation modified experimentally by abnormal conditions so that invagination is partially or totally hindered and there remains some mesendoderm not enclosed by ectoderm.

Experimental Method — concerted, organized, and scientific analysis of the causes, forces, and factors operating in any (embryological) system.

External Gills — outgrowths of (amphibian) branchial arches which function as temporary (anura) or permanent neotonic (urodela) respiratory organs.

Extra-Embryonic — refers to structures apart from embryonic body, such as membranes.

F

Fate Map — map of blastula or early gastrula stage which indicates prospective significance of various surface areas, based upon previously established studies of normal development aided by means of vital dye markings.

Fate, Prospective — destination toward which we know, from previous experience, that a given part would develop under normal conditions; lineage of each part of egg through its cell descendants into a definite region or portion of adult organism.

Fertilization — activation of egg by sperm and syngamy of pronuclei; union of male and female gamete nuclei.

Fertilization, Anti "eggs contain within their interior a substance capable of combining with the agglutinating group of the fertilizin, but which is separate from it as long as the egg is inactive" (Lillie).

Feulgen Reaction — chemical test for thymo-nucleic acid, used as a specific staining test for chromatin.

Field — mosaic of spatio-temporal activities within developing organism.

Field, Morphogenetic — embryonic field out of which will normally develop certain specific structures.

Flexure — refers to a bending such as cranial, cervical, and pontine flexures. Also dorsal and lumbo-sacral flexures of the pig.

Follicle — cellular sac within which egg generally goes through early maturation stages.

Forebrain — most anterior of first three primary brain vesicles, associated with lateral opticoels. (Syn., prosencephalon.)

Foregut — more anterior portion of enteric canal, first to appear, aided by development of pharyngeal derivatives.

Fovea Germinativa — pigment-free spot of animal hemisphere where amphibian germinal vesicle gives off its polar bodies.

Frontal — plane at right angles to both transverse and sagittal, dividing dorsal from ventral. (Syn., coronal.)

G

Gamete — differentiated (matured) germ cell, capable of functioning in fertilization (e.g., sperm or egg cell, germ cell).

Gametogenesis — process of developing and maturing germ cells.

Ganglion — aggregation of neurons, generally derived from a neural crest (e.g., cranial and spinal ganglia).

Ganglion, Acoustic — eighth cranial ganglion from which fibers of eighth cranial nerve arise, purely sensory.

Ganglion, Acustico-facialis — early undifferentiated association of seventh and eighth cranial ganglia.

Ganglion, Gasserian — fifth cranial ganglion, carrying both sensory and motor fibers. (Syn., trigeminal ganglion, semilunar ganglion.)

Ganglion, Geniculate — ganglion at root of facial (VII) cranial nerve, carrying both sensory and motor fibers.

Ganglion, Nodosal — ganglion associated with vagus (X) cranial nerve which carries afferent fibers to pharynx, larynx, trachea, oesophagus, and thoracic and abdominal viscera.

Ganglion, Petrosal — ganglion associated with glossopharyngeal (IX) cranial nerve, more peripheral than superior ganglion carrying sensory fibers from pharynx and root of tongue.

Ganglion, Superior — ganglion associated with glossopharyngeal (X) cranial nerve, mesial to petrosal ganglion.

Gasserian Ganglion— fifth cranial or trigeminal ganglion, derived from midbrain.

Gastraea Theory — theory of Haeckel that since all higher forms have gastrula stages there may have existed a common ancestor built on the plan of a permanent gastrula, as are the recent Coeloenterata.

Gastral Mesoderm — mesoderm derived from dorso-lateral bands (enterocoelic) in Amphioxus or from dorsal lip in frog. Opposed to peristomial mesoderm.

Gastrocoel — cavity formed during process of gastrulation. (Syn., archenteron.)

Gastrula — didermic embryo, possessing a newly formed cavity, gastrocoel or archenteron. The two layers are ectoderm and endoderm.

Gastrular Cleavage — separation of ectoderm and endoderm, during gastrulation, by a slit-like crevice, actually compressed blastocoel.

Gastrulation — dynamic process involving cell movements which change embryo from a monodermic to either a di- or tridermic form. Generally involves inward movement of cells to form enteric endoderm. Description includes epiboly, concrescence, confluence, involution, invagination, extension, and convergence.

Genital — refers to reproductive organs or processes, or both.

Genital Ducts — any ducts which convey gametes from their point of origin to region of insemination (e.g., collecting tubules, vas deferens, vas efTerens, seminal vesicle, oviduct, uterus, etc.).

Genital Ridge — initial elevation or thickening for development of external genitalia.

Germ — egg throughout its development, or at any stage.

Germ Cell — cell capable of sharing in reproductive process, in contrast with a somatic cell (e.g., sperm or egg cell). (Syn., gamete.)

Germ Layer — more or less artificial spatial and histogenic distinction of cell groups beginning in gastrula stage, consisting of ectodermal, endodermal, and mesodermal layers. No permanent and clear-cut distinction, as shown by transplantation experiments.

Germ Plasm — hereditary material, generally referring specifically to the genotype. Opposed to somatoplasm.

Germ Ring — ring of cells showing accelerated mitotic activity, generally a synonym for lips of blastopore. The rapidly advancing cells in epiboly.

Germinal Epithelium — peritoneal epithelium out of which reproductive cells of both male and female presumably develop. (Syn., germinal ridges, gonadal ridges.)

Germinal Localization — every area of blastoderm or of unfertilized egg, corresponds to some future organ. Unequal growth produces differentiation of parts (His, 1874). This concept led to Mosaic Theory of Roux (see Fate Map, p. 101).

Germinal Spot — nucleolus of ovum.

Germinal Vesicle — pre-maturation nucleus of egg.

Gestalten — system of configuration consisting of a ladder of levels; electrons, atom, molecule, cell tissue, organ, and organism, each one of which exhibits specifically new modes of action that cannot be understood as mere additive phenomena of the previous levels. With each higher level new concepts become necessary. The parts of the cell cannot exist independently, hence the cell is more than a mere aggregation of its parts — it is a patterned whole. A coherent unit reaching a final configuration in space (W. Kohler). Gestaltung means formation.

Gill — See Branchial Arch, Branchial Chamber, Branchial Cleft.

Gill Plate — elevated and thickened areas of ectoderm posterior to sense plate of embryo where visceral grooves will subsequently form.

Gill Rakers — ectodermal, finger-like obstructions which sift water as it passes from oral cavity to gill chambers of frog tadpole.

Glia Cells — small rounded supporting cells of spinal cord, derived from germinal cells of neural ectoderm.

Glomerulus — aggregation of capillaries associated with branches of dorsal aorta but lying within substance of functional kidney; function is excretory.

Glomus — vascular aggregations within head kidney or pronephros, never to become a glomerulus.

Glottis — opening between pharynx and larynx.

Gonad — organ within which germ cells are produced and generally matured (e.g., ovary or testis). (Syn., sex or germ gland.)

Gonadromorph — condition in which part of an animal may be male and another part female; not to be confused with hermaphroditism.

Gonium — suffix referring to a stage in maturation of a germ cell prior to any maturation division (e.g., spermatogonium, or oogonium).

Gonoduct — See Genital Ducts.

Gradient — gradual variation of developmental forces along an axis; scaled regions of preference. (See Axis.)

Gray Crescent — See Crescent, Gray.

Growth — developmental increase in total mass of protoplasm at expense of raw materials; an embryonic process, generally differentiation; cell proliferation.

Gynogenesis — development of sperm activated egg but without benefit of sperm nucleus.

H

Haploid — having a single set of chromosomes not appearing in allelomorphic pairs, as in mature gametes. Opposed to diploid, or the condition in somatic cells.

Harmonious-Equipotential System — embryonic system in which all parts are equally ready to respond to organism as a whole. Isolated blastomeres of such a system may give rise to complete embryos.

Hatching — beginning of larval life of amphibian, accomplished by temporarily secreted hatching enzymes which aid embryo to escape from its gelatinous capsule.

Hepatic Sinusoids — maze of dilated and irregular capillaries between loosely packed framework of hepatic tubules.

Hepatic Veins — veins from liver to heart, originating as anterior portions of vitelline veins of amphibia.

Hepatic Veins, Portal — remnants of posterior portions of left vitelline vein.

Hermaphrodite — individual capable of producing both spermatozoa and ova.

Hermaphrodite, Protandrous — male elements mature prior to female elements in hermaphrodite.

Hermaphrodite, Protogynous — female elements mature prior to male elements in hermaphrodite.

Hertwig's Law — nucleus tends to place itself in center of its sphere of activity; longitudinal axis of mitotic spindle tends to lie in longitudinal axis of yolk-free cytoplasm of cell.

Heteroagglutinin — agglutinin (fertilizin) of eggs which acts on sperm of different species, substance extractable from egg water which causes irreversible agglutination of foreign species.

Heterozygous — condition in which zygote is composed of gametes bearing allelomorphic genes. Opposed to homozygous.

Hibernation — spending the cold (winter) period in a state of reduced activity.

Hindbrain — most posterior of the three original brain divisions. (Syn., rhombencephalon. )

Hindgut — portion of amphibian embryonic gut just anterior to neurenteric canal. Level of origin of rectum, cloaca, post-anal gut, and caudal portions of urogenital systems.

Histogenesis — development of tissues.

Homoiothermal — pertaining to a condition in which temperature of body of organism is under control of an internal mechanism; body temperature regulated. Opposed to poikilothermal.

Homology — similarity in structure based upon similar embryonic origin.

Homoplastic — pertaining to a graft to an organism of same species, or even to another position on the same individual. (Syn., autoplastic.)

Homozygous — condition in which zygote is composed of gametes bearing identical rather than allelomorphic genes.

Horizontal — unsatisfactory term sometimes used synonymously with frontal, longitudinal, and even sagittal plane or section. Actually means across the lines of gravitational forces.

Hormone — secretion of a ductless (endocrine) gland which can stimulate or inhibit activity of distant parts of biological system already formed.

Hyaloplasm — viscid liquid regarded as essential living protoplasm.

Hybrid — successful cross between different species (e.g., horse and ass give a mule, which is sterile).

Hyoid Arch — mesodermal mass between hyomandibular and first branchial clefts, or between first and second visceral pouches or clefts which give rise to columella and parts of hyoid apparatus. (Syn., second visceral arch. )

Hyomandibular — pertaining to pouch, cleft, or slit between mandibular and hyoid arches.

Hyperplasia — overgrowth; abnormal or unusual increase in elements composing a part.

Hypertrophy — increase in size due to increase in demands upon part concerned.

Hypochordal Rod — transitory string of cells constricted off between dorsal wall of midgut and notochord of amphibian embryo, between level of pancreas and tail, and disappearing before hatching time. (Syn., sLib-notochordal rod.)

Hypomere — most ventral segment of mesoderm out of which develop somatopleure, splanchnopleure, and coelom. (Syn., lateral plate mesoderm.)

Hypophysis — ectodermally derived solid structure arising anterior to stomodeum and growing inwardly toward infundibulum to give rise to anterior and intermediate parts of pituitary gland.

Hypoplasia — undergrowth or deficiency in elements composing a part.

Hypothesis — complemental supposition; presumption based on fragmentary but suggestive data offered to bridge a gap in incomplete knowledge of the facts. May be offered as an explanation of facts unproved, until subjected to verification or disproof.

I

Idiosome — material out of which acrosome is formed during metamorphosis of spermatid to spermatozoon. (Syn., spermatosphere, centrosphere. )

Induction — successive and purposeful influences which bring about morphogenetic changes within embryo.

Inductor — a loose word which includes both organizer and evocator (Needham). Generally means a piece of living tissue which brings about differentiation within otherwise indifferent tissue.

Infundibulum of the Brain — funnel-like evagination of floor of diencephalon which, along with hypophysis, will give rise to pituitary gland of adult.

Infundibulum of the Oviduct — See Ostium Abdominale Tubae.

Ingression — inward movement of yolk endoderm of amphibian blastula (Nicholas, 1945).

Insemination — process of impregnation; fertilization.

Interauricular Septum — longitudinal sheet of mesodermal tissue which grows ventrally from roof of atrial chamber to divide it into right and left halves.

Interkinesis — resting stage between mitotic divisions.

Intermediate Cell Mass — narrow strip of mesoderm which, for a time, joins dorsal epimere with ventral hypomere, being made up of a dorsal portion continuous with dorsal wall of somite and somatic mesoderm and a ventral portion continuous with ventral wall of somite and splanchnic mesoderm. Source of origin of excretory system. (Syn., nephrotome or middle plate.)

Internal Gills — filamentous outgrowths on posterior side of first three pairs of branchial arches and a single row on anterior side of fourth pair of branchial arches of frog tadpole, which have a respiratory function concurrent with and following absorption of external gills.

Internal Limiting Membrane — membrane which develops on innermost surface of inner wall of optic cup during fourth day of chick development.

Intersex — individual without typical sexual differentiation.

Interstitial Cells — specialized cells between seminiferous tubules of testis which produce hormones.

Interstitial Tissue of Testis — cell aggregations between seminiferous tubules of testis which elaborate a male sex hormone.

Invagination — folding or inpushing of a layer of cells into a preformed cavity, as in one of the processes of gastrulation. Opposed to involution.

Involution — rolling inward or turning in of cells over a rim, as in gastrulation of chick embryo.

Iris — ^narrow zone bounding pupil of eye in which two layers of optic cup become blended so that pigment from outer layer invades material of inner layer, giving eye a specific color by variable reflection.

Isogamy — similar gametes, without differentiations into spermatozoa and ova.

Isolation Culture — removal of a part of an organism and its maintenance in a suitable medium in living condition.

Isthmus of the Brain — depression in dorsal wall of embryonic brain which partially separates mesencephalon from metencephalon.

Isthmus of the Oviduct — short, tubular, posterior end of oviduct (e.g., chick) in which fluid albumen and shell membranes are applied to egg.

Iter — See Aqueduct of Sylvius.

J

Jacobson's Organ — ventro-medial evaginations from olfactory pits (amphibia and reptilia) which later become glandular and sensitive olfactory epithelia.

Jelly — mucin covering of amphibian egg derived from oviduct and applied outside vitelline membrane.

Jugular Veins — veins which bring blood from head, superior or internal jugular being anterior cardinal veins and inferior jugular veins growing toward lower jaw and mouth from base of each ductus Cuvieri.

K

Karyoplasm — protoplasm within confines of nucleus.

Kern-Plasma Relation — ratio of amount of nuclear and of cytoplasmic materials present in the cell. It seems to be a function of cleavage to restore kern-plasma relation from unbalanced condition of ovum with its excessive yolk and cytoplasm to new ratio of gastrula or somatic cell.

L

Lamina Terminalis — point of suture of anterior neural folds (i.e., anterior neuropore) where they are finally separated from head ectoderm; it consists of a median ventral thickening at anterior limit of telencephalon (from anterior side of optic recess to beginning of velum transversum) and includes anterior commissure of torus transversus.

Larva — stage in development when organism has emerged from its membranes and is able to lead an independent existence, but may not have completed its development. Generally (except in cases of neoteny or paedogenesis) larvae cannot reproduce.

Larynx — anterior part of original laryngo-tracheal groove which becomes a tube opening into pharynx by way of glottis.

Lateral — either right (dextral) or left (sinistral) side; laterad means toward the side.

Lateral Line Organs (or System) — line of sensory structures along side of body of fishes and amphibia, generally embedded in skin and innervated by a branch from vagus ganglion, presumably concerned with recognition of low vibrations in water. Appears first at about 4 mm. stage in frog embryo. (Syn., ramus lateralis.)

Lateral Mesocardium — septum posterior to heart extending from base of each vitelline vein obliquely upward to dorso-lateral body wall, representing one of the three parts of septum transversum.

Lateral Mesoderm — See Lateral Plate Mesoderm.

Lateral Neural Folds — See Neural Fold.

Lateral Plates or Lateral Plate Mesoderm — lateral mesoblast within which body cavity (coelom and exocoel) arises. (Syn., lateral mesoderm.)

Lateral Ventricles of the Brain — thick-walled and laterally compressed cavities of prosencephalon which open into third ventricle by way of foramen of Monro; walls will become cerebral hemispheres.

Lecithin — fat from an. animal organism which is phosphorized in form of phosphatides.

Lens — thickening in head ectoderm opposite optic cup at about time of hatching in frog embryo; it becomes a placode, invaginates to acquire a vesicle, and then pinches off into space of optic cup as a lens. Inner surface convex; substance fibrous.

Lens Placode — early thickened ectodermal primordium of lens.

Leptotene — stage in maturation which follows last -gonial division and prior to synaptene stage, structurally similar to resting cell stage. The chromatin material in form of a spireme. Term means thin, diffuse.

Lipids — fats and fatty substances such as oil and yolk (lecithin) found in eggs (e.g., cholesterol, ergosterol).

Lips of the Blastopore — See Blastopore, Lips of.

Localization — cytological separation of parts of the mosaic egg, each of which has a known specific subsequent differentiation. There is often a substratum associated with these areas, made up of pigmented granules, but it is cytoplasm rather than pigmented elements in which localization occurs.

M

Macromere — ^larger of the blastomeres where there is a conspicuous size difference.

Malpighian Body — unit of functional kidney including Bowman's capsule and glomerulus. (Syn., renal corpuscle, Malpighian corpuscle.)

Mandibular Arch — rudiment of lower jaw, mesodermal, and anterior to first or hyomandibular pouch.

Mantle Fibers — those fibers of mitotic spindle which attach chromosomes to centrosomes.

Mantle Layer of the Cord — zone of developing spinal cord with densely packed nuclei slightly peripheral to germinal cells from which they are derived. Includes elongated cells of ependyma.

Maturation — process of transformation of a primordial germ cell (spermatogonium or oogonium) into a functionally mature germ cell, the process involving two special divisions, one of which is always meiotic. Divisions known as equational and reductional.

Mechanism — assumption that biological processes do not violate physical and chemical laws but that they are more than the mere functioning of a machine because material taken into the organism becomes an integral part of the organism, through chemical changes. (Syn., the scientific attitude.) (See Vitalism.)

Meckel's Cartilage — core of lower jaw derived from ventral part of cartilaginous mandibular arch.

Median plane — "middle" plane, as of an embryo. May be median sagittal or median frontal.

Medulla Oblongata — that portion of adult brain derived from rhombencephalon.

Medullarin — sex differentiating substance spread in some amphibia by blood stream as a hormone and in other forms by diffusion. (See Corticin.)

Medullary — See terms under Neural, such as canal, fold, groove, plate, tube.

Medullary Cords — that portion of suprarenal glands derived from sympathetic nervous system; central cords. Also that portion of embryonic gonad presumably derived from pre-migratory germ cells upon reaching genital ridge.

Meiosis — process of nuclear division found in maturation of germ cells, involving a separation of members of pairs of chromosomes. (Syn., reductional division.)

Melanophore — well with black or brown pigment (melanin), derived from neural crests and migrating throughout body.

Membrane Bone — bone developed in regions occupied by connective tissue, not cartilage.

Membrane, Vitelline — See Vitelline Membrane.

Membranes — See Egg Membranes.

Meroblastic Cleavage or Ova — See under Cleavage or Egg.

Mesencephalon — section of primary brain between posterior level of prosencephalon and an imaginary line drawn from tuberculum posterius to a point just posterior to dorsal thickening. Gives rise to optic lobes, crura cerebri, and aqueduct of Sylvius. (Syn., midbrain.)

Mesenchyme — form of embryonic mesoderm or mesoblast in which migrating cells unite secondarily to form a syncitium or network having nuclei in thickened nodes between intercellular spaces filled with fluid; often derived from mesothelium.

Mesendoderm — newly formed layer of (urodele) gastmla before there has been any separation of endoderm and mesoderm. (Syn., mesentoderm, mesentoblast, ento-mesoblast.)

Mesentery — sheet of (mesodermal) tissue generally supporting organ systems (e.g., mesorchium, mesocardium).

Mesial — median, medial, middle.

Mesoblast, Gastral — See Gastral Mesoderm.

Mesoblast, Peristomial — involuted, ventral lip mesoderm, continuous with gastral mesoderm from dorsal lip.

Mesocardium — mesentery of heart; may be dorsal, ventral, or lateral. (See under Lateral Mesocardium.)

Mesoderm — the third primary germ layer developed in point of time, may be derived from endoderm in some forms and from ectoderm in others. (See other terms such as Mesoblast, Mesenchyme, Lateral Plate Mesoderm, Epimere, Mesomere, Hypomere, Gastral Mesoderm, Peristomial Mesoderm, Axial Mesoderm, etc.)

Mesomere — cell of intermediate size where there are conspicuous size differences in an early embryo; also refers to intermediate cell mass: intermediate mesoderm.

Mesonephric Duct — duct which grows posteriorly from mesonephros to cloaca and functions also as vas deferens in male. (Syn., Wolffian duct.)

Mesonephric Tubules — primary, secondary, and sometimes tertiary tubules developing in Wolffian body, functioning in adult amphibia.

Mesonephros — Wolffian body, or intermediate kidney, functional as kidney in adult fish and amphibian.

Mesorchium — mesentery (mesodermal) which surrounds and supports testis to body wall.

Mesothelium — epithelial layers or membranes of mesodermal origin.

Mesovarium — mesentery (mesodermal) which suspends ovary from dorsal body wall.

Metamerism — serial segmentation, as seen in nervous, muscular, and circulatory systems.

Metamorphosis — end of larval period of amphibia when growth is suspended temporarily. There is autolysis and resorption of old tissues and organs such as gills, and development of new structures such as eyelids and limbs; changes in structure correlated with changes in habitat from one that is aquatic to one that is terrestrial; change in structure without retention of original form, as in change from spermatid to spermatozoon.

Metaphase — stage in mitosis when paired chromosomes are lined up on equatorial plate midway between amphiasters, supported by mitotic spindle, prior to any anaphase movement.

Micromere — smaller of cells when there is a conspicuous difference in size, characteristic of Annelids and Molluscs.

Micropyle — aperture in egg covering through which spermatozoa may enter; in such eggs the only possible point of insemination (e.g., many fish eggs).

Midbrain — See Mesencephalon.

Midgut — that portion of archenteron which will give rise to intestines.

Milieu — Term used to include all of the physico-chemical and biological factors surrounding a living system (e.g., external or internal milieu).

Mitochondria — small, permanent, cytoplasmic granules which stain with Janus green B and Janus red; granules which have powers of growth and division; probably lipoid.

Mitosis — cytoplasmic division involving a nucleus and spindle apparatus.

Mitotic Index — proportion in any tissue and at any specified time of the dividing cells; percentage of actively dividing cells.

Monospermy — fertilization accomplished by only one sperm. Opposed to polyspermy.

Monro, Foramina of — tubular connections between single third and paired lateral ventricles of forebrain.

Morphogenesis — all of the topogenetic processes which result in structure formation; origin of characteristic structure (form) in an organ or in an organism compounded of organs.

Morphogenetic Movements — cell or cell area movements concerned with formation of germ layers (e.g., during gastrulation) or of organ primordia.

Morula — spherical mass of cells, as yet without segmentation cavity.

Mosaic — type of egg or development in which fate of all parts is fixed at an early stage, possibly even at time of fertilization. Local injury or excisions generally result in loss of specific organs in developing embryo. Opposed to regulative development.

Miillerian Duct — See Oviducts.

Muscle Plate — See Myotome.

Myeloblasts — muscle-forming (embryonic) cells.

Myoblasts— formative cells within myotome or muscle plate which will give rise to true striated muscles of adult.

Myocardium — muscular part of heart arising from splanchnic mesoblast.

Myocoel — cavity within which ovaries of Amphioxus develop; temporary cavities within myotomes which may have been connected with coelom.

Myotome — thickened primordium of muscle found in each somite, (Syn., muscle plate.)

N

Nares, External — external openings of tubes which are connected with olfactory vesicles.

Nares, Internal — openings of tubular organ from olfactory placodes into anterior part of pharynx of 12 mm. frog tadpole. (Syn., choanae.)

Nasal Choanae — openings of olfactory chambers into mouth.

Nasal Pit — See Olfactory Pit.

Nebenkern — cytological structure near nucleus of early spermatid.

Neoteny — condition of many urodeles and of experimentally produced (thyroidless) anuran embryos in which larval period is extended or retained, i.e., larvae fail to go through normal metamorphosis. Sexual maturity in larval stage (e.g., axolotl, Necturus).

Nephrocoel — cavity, found in nephrotome or intermediate cell mass, which temporarily joins myocoel and coelom.

Nephrogenic Cord — continuous band of intermediate mesoderm (mesomere) without apparent segmentation, prior to budding off of mesonephric tubules.

Nephrogenic Tissue — intermediate cell mass, mesomere, or nephrotome which will give rise to excretory system.

Nephrostome — funnel-shaped opening of kidney tubules into coelom; outer tubules of amphibian mesonephric kidney acquire ciliated nephrostomal openings from coelom and shift their connections to renal portal sinus.

Nephrotome — intermediate cell mass.

Nephrotomic Plate — intermediate mesoderm, mesomere.

Nerve, Abducens — sixth (VI) cranial nerve arising from basal plate of rhombencephalon which controls external rectus muscles of eye.

Nerve, Auditory — eighth ( VllI ) cranial nerve, purely sensory, arising from acoustic ganghon and associated with geniculate ganglion of seventh nerve.

Nerve, Facial — seventh (VII) cranial nerve, both sensory and motor, related to taste buds and facial muscles.

Nerve, Glossopharyngeal — ninth (IX) cranial nerve, mixed, associated with superior and petrosal ganglia.

Nerve, Oculomotor — third (III) cranial nerve which arises from neuroblasts in ventral zone of midbrain near median line just before hatching in frog tadpole.

Nerve, Vagus — tenth (X) cranial nerve, mixed, arising from rhombencephalon and associated with jugular ganglion.

Nervous Layer — innermost of two layers found in roof of segmentation cavity of amphibian blastula, from which bulk of central nervous system is developed.

Neural Arch — ossified cartilages which extend dorsally from centrum around nerve cord.

Neural Canal — See Neurocoel and Neural Tube.

Neural Crest — continuous cord of ectodermally derived cells lying on each side in angle between neural tube and body ectoderm, separated from ectoderm at time of closure of neural tube and extending from extreme anterior to posterior end of embryo; material out of which spinal and possibly some cranial ganglia develop, and related to development of sympathetic ganglia by cell migration.

Neural Fold — elevation of ectoderm on either side of thickened and depressing medullary plate; folds which close dorsally to form neural tube. (Syn., medullary folds.)

Neural Groove — depression caused by sinking in of center of medullary plate to form a longitudinal groove, later to be incorporated within neural tube (spinal cord). (Syn., medullary groove.)

Neural Plate — thickened broad strip of ectoderm along future dorsal side of all vertebrate embryos, later to give rise to central nervous system. (Syn., medullary plate.)

Neural Tube — tube formed by dorsal fusion of neural folds, rudiment of nerve or spinal cord.

Neurenteric Canal — posterior neurocoel where it is connected with closing blastopore and posterior enteron of amphibian; the large common nervous and enteric chamber of Amphioxus; the Kupffer's vesicle of fish embryo; possibly the primitive pit of chick embryo. (Syn., notochordal canal, primitive pit.)

Neuroblasts — primitive or formative nerve cells, probably derived (along with epithelial and glia cells) from germinal cells of embryonic neural tube.

Neurocoel — cavity of neural tube, formed simultaneously with closure of neural folds. (Syn., central canal, neural canal.)

Neurocranium — dorsal portion of skull associated with brain and sense organs.

Neuroglia — see Glia Cells.

Neuropore — temporary opening into neural canal due to a lag in fusion of neural folds at anterior extremity; permanent in Amphioxus and in vicinity of epiphysis of higher vertebrates.

Neurula — stage in embryonic development which follows gastrulation and during which neural axis is formed and histogenesis proceeds rapidly.

Notochord and neural plate are already differentiated, and basic vertebrate pattern is indicated,

Notochord — rod of vacuolated cells representing axis of all vertebrates, found beneath neural tube and dorsal to archenteron. Thought to be derived from or simultaneously with endoderm.

Notochordal Sheath — double mesodermal sheath around notochord consisting of an outer elastic sheath developed from superficial chorda cells and an inner secondary or fibrous sheath from chorda epithelium.

Nucleolus — the body generally within the nucleus which has no affinity for chromatin dyes, but stains with acid or cytoplasmic dyes. Function unknown. (Syn., plasmosome.)

O

Oesophagus — elongated portion of foregut between future glottis and opening of bile duct of frog embryo; temporarily occluded just behind glottis but opens again.

Olfactory Lobes — anterior extremities of telencephalic cerebral lobes, partially constricted, associated with first pair of cranial nerves.

Olfactory Pit — depressions within olfactory placodes of 6 mm. frog embryo which will become olfactory organs (external nares).

Olfactory Placode — thickened ectoderm lateral to stomodeal region found in 5 mm. frog embryo, primordia of olfactory pits.

"Omne Vivum e Vivo" — all life is derived from preexisting life (Pasteur).

Omnipotent — term used in connection with a cell which could, under various conditions, assume every cytological differentiation known to the species or which, by division, could give rise to such varied differentiations.

"Omnis Cellula e Cellula" — all cells from preexisting cells (Virchow).

Ontogeny — developmental history of an organism; sequence of stages in early development.

Oocyte — presumptive egg cell after initiation of growth phase of maturation. (Syn., ovocyte.)

Oogenesis— process of maturation of ovum; transformation of oogonium to mature ovum. (Syn., ovogenesis.)

Oogonia — multiplication (mitotic) stage prior to maturation of presumptive egg cell (ovum), found most frequently in peripheral germinal epithelium.

Ooplasm — cytoplasmic substances connected with building rather than reserve materials utilized in developmental processes.

Opercular Chamber — See Branchial Chamber.

Operculum — integumentary growth posteriorly from each of the hyoid arches of frog embryo, which covers and encloses gills.

Optic Chiasma — thickening in forebrain ventral to infundibulum, found as a bunch of optic nerve fibers in future diencephalon.

Optic Cup — invagination of outer wall of primary optic vesicle to form a secondary optic vesicle made up of two layers; a thick internal or retinal layer continuous at pupil and choroid fissure, and a thin external layer which is pigmented. Cavity of cup becomes future posterior chamber of eye.

Optic Lobes — thickened, evaginated, dorso-lateral walls of mesencephalon.

Optic Recess — depression in forebrain anterior to optic chiasma which leads to optic stalks.

Optic Stalk — attachment of optic vesicle to forebrain, at first a tubular connection between optic vesicle and diencephalon. Lumen is later obliterated by development of optic nerve fibers.

Optic Vesicle — evagination of forebrain ectoderm to form primary optic vesicles which in turn invaginate to form secondary optic vesicles or optic cups of eyes.

Opticoel — cavity of primary optic cup.

Oral Plate — stomodeal ectoderm and pharyngeal endoderm fused to form oral membrane. Breaks through to form mouth. (Syn., pharyngeal membrane, oral membrane, stomodeal plate.)

Oral Suckers — elongated, pigmented depressions at antero-ventral ends of mandibular arches of frog embryo which give rise to mucous glands; with adhesive function.

Organization — indicated by interdependence of parts and the whole. "When elements of a certain degree of complexity become organized into an entity belonging to a higher level of organization," says Waddington, "we must suppose that the coherence of the higher level depends on properties which the isolated elements indeed possessed but which could not be entered into certain relations with one another." See Gestalten.

Organizer — chorda-mesodermal field of amphibian embryo; a tissue area which has power of organizing indifferent tissue into a neural axis; possibly comparable to Henson's node of chick embryo.

Osteoblasts — mesenchymal cells which actively secrete a calcareous material in formation of bone; bone-forming cells.

Osteoclasts — bone-destroying cells; cells which appear in and tend to destroy formed bone; constantly active, even in embryo.

Ostium Abdominale Tubae — most anterior, fimbriated end of oviduct in female vertebrates; point of entrance of ovulated egg into oviduct; double in amphibia. (Syn., infundibulum of oviduct, tubal ridge.)

Otic Vesicle — auditory vesicle, otocyst.

Otocyst — original auditory vesicle appearing at level of rhombencephalon in amphibian embryo just before hatching, forming first as a placode. (Syn., auditory vesicle.)

Oviducal Membranes of Ovum — tertiary membranes applied over egg as it passes through oviduct.

Oviducts — paired MUllerian ducts in both males and females, which generally persist in males.

Ovigerous Cords — columns or strands of tissue which divide germinal epithelium of primordium of ovary, carrying primordial germ cells with them and later breaking up into nests of cells, each of which contains an oogonium. (Syn., egg tubes or cords of Pfliiger [mammal].)

Oviposition — process of laying eggs.

Ovocyte — See Oocyte.

Ovogenesis — See Oogenesis.

Ovogonia — See Oogonia.

Ovulation — release of egg from ovary, not necessarily from body.

Ovum — Latin for egg.

P

Pachytene — stage in maturation when allelomorphic pairs of chromosomes are fused (telosynapsis or parasynapsis) so as to appear haploid, during which process crossing over may occur; stage just prior to diplotene. Term means thick or condensed. (Syn., diplonema.)

Paedogenesis — reproduction during larval stage; precocious sex development.

Pancreas — digestive and endocrine glands arising as single posterior and single anterior primordia in vicinity of liver.

Parthenogenesis — development of an egg without benefit of spermatozoon.

Parthenogenesis, Artificial — initiation of development of an egg by artificial means.

Parthenogenesis, Natural — maturation of eggs of some forms leads directly to development without aid of spermatozoa.

Parthenogenetic Cleavage — fragmentation of protoplasm of old and unfertilized chick eggs, originally thought to be true cleavage.

Path, Copulation — See Copulation Path.

Path, Penetration — initial direction of sperm entrance into egg, often shifting toward egg nucleus along a new copulation path. (Syn., entrance path.)

Perforatorium — See A crosome.

Pericardial Cavity — cavity or membrane sac which encloses heart, representing a cephalic portion of coelom within embryonic body. (Syn., parietal cavity.)

Pericardium — thin mesodermal membrane which encloses pericardial cavity and heart.

Perichondrium — mesenchymal layer immediately around forming cartilage.

Perichordal Sheath — thin, mesodermal (sclerotomal), continuous sheet of tissue immediately around notochord.

Periosteum — mesenchymal layer, often originally perichondrium, which will be found immediately around forming bone.

Peristomiai Mesoderm — mesoderm of amphibian gastrula derived from (ventral) lips of blastopore. Opposed to gastral mesoderm.

Peritoneal cavity — body cavity (coelom).

Peritoneum — coelomic mesothelium of abdominal region reinforced by connective tissue.

Perivitelline Membrane — See Vitelline Membrane.

Perivitelline Space — space between vitelline (fertilization) membrane and contained egg, generally filled with a fluid.

Pfliiger's Law — dividing nucleus elongates in direction of least resistance.

Phenotype — outward appearance of an organism regardless of its genetic make-up. Opposed to genotype.

Pigment Layer of Optic Cup — thin outer wall of primary optic cup, posterior to retina, which never fuses with rods and cones of retina.

Pineal — See Epiphysis.

Pituitary — See Hypophysis.

Placode — Plate-like thickening of ectoderm from which arise sensory or nervous structures (e.g., olfactory placode).

Plane — imaginary two-dimensional surface; may be frontal, sagittal, transverse, median, or lateral.

Plasmosome — a true nucleolus. (See Nucleolus.)

Plectrum — See Columella.

Plexus Choroid — Vascular folds in roof of prosencephalon, diencephalon, and rhombencephalon.

Poikilothermal — cold-blooded; animals whose body temperatures are subject to environmental changes because they lack regulating mechanisms. Opposed to homoiothermal.

Polar — pertaining, in most cases, to animal pole, although may refer to vegetal pole, or both.

Polar Body — relatively minute, discarded nucleus of maturing oocyte (generally three). (Syn., polocytes.)

Polarity — axial distribution of component parts; animal and vegetal poles; stratification.

Pole, Animal — region of egg where polar bodies are eliminated; ectoderm forming portion of pre-cleaved egg. (Syn., apical or animal hemisphere.)

Pole, Vegetal — region of egg opposite animal pole; region of lowest metabolic rate; pole with greater density of yolk in telolecithal eggs; generally endoderm-forming region of egg.

Polyembryony — production of several separate individuals from one egg by an early separation of its blastomeres; possible origin of some identical twins.

Polyploid — possessing a multiple number of chromosomes, such as triploid (three times the haploid number), tetraploid (four times the haploid number), etc. Alwavs more than the normal diploid of the typical zygote.

Polyspermy — insemination of an egg with more than a single sperm, occurring generally in chick egg, although but a single sperm nucleus is functional, in syngamy.

Post-Ana! Gut — posteriorly projecting blind pocket of hindgut, that portion of hindgut posterior to anal plate or proctodeal plate. (Syn., postcloacal gut.)

Post-Reduction — maturation in which equational and reductional divisions occur in that order.

Posterior Tubercle — See Tuberculum posterius.

Potency, Prospective — sum total of developmental possibilities, the full range of developmental performance of which a given area (or germ) is capable. Not to be confused with competence.

Preformation — theory that adult is represented in miniature within egg or sperm and that development is simply enlargement.

Pre-migratory Germ Cell — yolk-laden cells of splanchnopleuric origin which migrate by way of blood vessels to gonad primordia. Believed by some to be precursors of gonad stroma or functional germ cells.

Pre-Reduction — maturation in which reductional and equational divisions occur in that order.

Presumptive — expected or predicted outcome of development of a given area (e.g., fate of a part in question) based on previous fate map studies.

Primary Oocyte — termination of growth phase in maturation of ovum from oogonial stage, prior to any maturational divisions.

Primary Spermatocyte — stage in spermatogenesis in which division results in secondary spermatocytes; stage beginning with growth of spermatogonia.

Primitive Groove — groove through center of primitive streak, bounded by primitive folds and terminated anteriorly by primitive pit and posteriorly by primitive plate.

Primordial Germ Cells — diploid cells which are destined to become germ cells (e.g., oogonia and spermatogonia). (Syn., primitive germ cells.)

Primordium — See Anlage.

Proctodeum — ectodermal pit in region of future cloaca which invaginates to fuse with hindgut endoderm to form anal or proctodeal plate, later to rupture and form anus.

Pronephric Capsule — mesodermal connective tissue covering of pronephric masses derived from adjacent myotomes and somatic mesoderm.

Pronephric Chamber — portion of amphibian coelomic cavity open anteriorly and posteriorly but closed ventrally by development of lungs.

Pronephric Duct — outer portion of pronephric nephrotomes which develops a lumen connected posteriorly with mesonephric or Wolffian duct. (Syn., segmental duct.)

Pronephric Tubules — lateral outgrowths of the most anterior nephrotomal masses which acquire cavities in amphibia, connected with pronephric duct. Possibly become infundibulum of oviduct.

Pronephros — embryonic kidney of all vertebrates, extending from second to fourth somites of frog embryo and consisting of as many primitive tubules as somites concerned; completely lost in all adult vertebrates except a few bony fish. (Syn., head kidney.)

Pronucleus — egg nucleus after polar body formation and sperm nucleus after entrance of spermatozoon into egg.

Prophase — first stage in mitotic cycle when spireme is broken up into definite chromosomes, prior to lining up on metaphase (equatorial) plate.

Prosencephalon — See Forebrain.

Prosocoel — cavity of prosencephalon.

Proximal — nearer the point of reference, toward main body mass.

Pupil — opening into secondary optic vesicle, occluded in part by lens, and regulated in diameter by ciliary muscles of iris.

R

Ramus Communicans — connection between sympathetic ganglion and spinal nerve, as numerous as ganglia in any vertebrate; probably originating from crest cells. Ramus means branch.

Recapitulation Theory — theory that embryonic development reviews major steps in evolutionary history. (See qualifications under Biogenetic Law.)

Rectum — narrowed posterior portion of hindgut, lined with thickened endodermal epithelium, which opens directly into cloaca.

Reductional Maturation Division — one of the two important divisions in the maturation of gametes which results in separation of allelomorphic (homologous) pairs of chromosomes so that resulting cells are invariably haploid. Opposed to equational division. (Syn., meiotic division, disjunctional division.)

Regeneration — repair or replacement of lost part or parts, a power gradually lost in the ontogeny of most animals.

Regions, Presumptive — regions of blastula which, by previous experimentation, have been demonstrated to develop in certain specific directions under normal ontogenetic influences.

Regulation — reorganization toward the whole; power of pre-gastrula embryos to utilize materials remaining, after partial excision, to bring about normal conditions; more flexible power than regeneration.

Renal Portal System- — venous system which carries blood to kidneys, involving lateral portions of caval veins (really parts of posterior cardinals), iliacs, and dorso-lumbars. Found in adult amphibia as the most striking evidences of recapitulation.

Rete Cords — strands of epithelial cells containing many primordial germ cells which connect with seminiferous tubules and later become vasa efferentia, in the bird. (Syn., rete testis.)

Retinal Zone — ectodermal derivatives of optic cup consisting of internal limiting membrane, retinal and lenticular zones, and outer pigmented layer. Retina proper includes portions from internal limiting membrane to rods and cones, inclusive.

Rhombencephalon — See Hindbrain.

S

Saccule — -outer and ventral portion of inner ear from which are derived cochlea associated with eighth or auditory nerve. (Syn., sacculus.)

Saccus Endolymphaticus — original endolymphatic duct, closed off from exterior, which (in 20 mm. stage of tadpole) grows up over rhombencephalon to join other sac and form a vascular covering of the brain.

Sachs' Law — all cells tend to divide into equal parts and each new plane of division tends to intersect the preceding one at right angles.

Sagittal — mesial plane, or any plane parallel to it, dividing right parts of body from left. Right angles to both frontal and transverse planes.

Sclerotic Coat — tough mesenchymatous and partially cartilaginous coat outside of choroid coat of vertebrate eye. (Syn., sclera.)

Sclerotome — loose mesenchymal cells proliferated off from inner and ventral edges of myotomes (5 mm. frog) which contribute to formation of axial skeleton.

Secondary Oocyte — stage in oogenesis between primary oocyte and ovum; may be either haploid or diploid, depending upon species considered.

Secondary Spermatocyte — stage in spermatogenesis in which next division results in haploid spermatids, these spermatocytes being either haploid or diploid, depending upon species considered. (See Post- and Prereduction.)

Secretory Tubule — portion of kidney tubule actually involved in excretory process.

Section — generally a slice of an embryo, often of microscopic dimensions, taken in any one of the various planes such as frontal, transverse, or sagittal. (See Serial Sections.)

Segmental Plate — See Axial Mesoderm.

Segmentation — repetition of structural pattern; used as synonym for cleavage as well as for metamerism.

Segmentation Cavity — cavity of blastula. (Syn., subgerminal cavity, blastocoel.)

Semi-Circular Canals — tubular derivatives of utricle lined with ectoderm from otocyst, which constitute accessory balancing mechanisms of vertebrates.

Seminal Vesicle — glandular dilation of distal end of ductus deferens (Wolffian duct) where spermatozoa are temporarily collected prior to ejaculation.

Semination — act 'of fertilizing by discharge of spermatozoa.

Seminiferous Tubule — tubular divisions of testis derived from rete cords, covered by a connective tissue theca and containing supporting (Sertoli) cells and all stages of spermatogenesis.

Sense Plate — narrow band of elevated ectodermal tissue which passes transversely across anterior end of amphibian embryo, ventral to level of fused neural folds, with ends of band bending dorsally to merge with neural folds. Lower margins represent mandibular arch, the plate giving rise to mucous glands (oral suckers) of amphibia and to parts of olfactory organs, lens of eye, and possibly to part of inner ear.

Septum — partition.

Serial Sections — thin (often of microscopic dimensions) sections of embryos which are mounted on slides in order of their removal from the embryo, so that a study in sequence will provide an understanding of all organ systems from one region of embryo to the other.

Sertoli Cell — derivative of sexual cords of testis, found within seminiferous tubule and functionally similar to follicle cell in ovary in that it is the nutritive, supporting, or nurse cell of the maturing spermatozoa. The heads of adult spermatozoa may be seen embedded in the cytoplasm of Sertoli cells.

Sex Cell Cord — division of sex cell ridge or gonad primordium, not to be confused with sexual (rete) cords.

Sex Determination — See Determination of Sex.

Sexual Cords — derivatives of germinal epithelium from which they become separated and give rise to bulk of gonads of both sexes.

Sexual Cords of the Ovary — sex cords of the originally indifferent gonad primordium which form only cords of ovary, the functional follicles coming from germinal epithelium.

Sexual Cords of the Testis — sex cords of the originally indifferent gonad primordium which give rise to seminiferous tubules of testis, forming a rather solid mesenchymatous reticulum when cavities begin to appear lined with spermatogonia (from primordial germ cells) and Sertoli cells, the whole constituting seminiferous tubules.

Sheath, Myelin — myelin covering of axons in so-called white matter of spinal cord.

Sinus Venosus — point of fusion of vitelline veins of amphibian embryo bilaterally symmetrical and related to ducts of Cuvieri and ductus venosus.

Skeletogenous Sheath — sclerotomal cells which first form a continuous layer around both notochord and nerve cord.

Skin — See Dermis and Epidermis. (Syn., integument.)

Somatic — relating to body in contrast to germinal cells; or relating to outer body in contrast to inner splanchnic mesoderm.

Somatoblast — blastomeres with specific germ layer predisposition, i.e., ectodermal somatoblasts.

Somatopleure — layer of somatic mesoderm and closely associated ectoderm, extension of which (from body wall) gives rise to both amnion and chorion.

Somite — blocks of paraxial mesoblast, metamerically separated by transverse clefts, derived from enterocoelic or gastral mesoderm and giving rise to dermatome, myotome, and sclerotome.

Spawning — act of expelling eggs from uteri of anamniota (e.g., amphibia).

Sperm — germ cell characteristically produced by the male. (Syn., spermatozoon, sperm cell, male gamete, spermatosome.)

Spermatid — products of the second maturation division in spermatogenesis, the spermatids having certain cytological characteristics and being invariably haploid; cells which go through a metamorphosis into functionally mature spermatozoa.

Spermatocyte — stages in spermatogenesis between the time the primordial germ cell (spermatogonium) begins to grow, without division, until after the division which results in spermatids. (See Primary Spermatocyte, Secondary Spermatocyte.)

Spermatogenesis — entire process which results in maturation of spermatozoon.

Spermatogonium — primordial germ cell of male gonad, indistinguishable from somatic cells, both of which are diploid; stage prior to maturation when the presumptive spermatozoon undergoes rapid multiplication by mitosis.

Spermatophore — sperm-bearing bundle, such as that which is shed by male urodele, the bundles later to be picked up by cloacal lips of female.

Spermatosphere — See Idiosome.

Spermatozoon — functionally mature male gamete. (Syn., sperm.)

Spina Bifida — split tail, generally involving spine, in developing embryo caused by a variety of environmental conditions, most of which act through interference with normal gastrulation and neurulation.

Spinal Cord — that portion of central nervous system, excluding brain, which is derived from epithelial and neural ectoderm of original blastula, consisting of ependyma, glia, neuroblasts and their derivatives, and connecting cells.

Spindle — group of fibers between centrosomes during mitosis, to which chromosomes are attached and by means of which (mantle fiber portion) chromosomes are drawn to their respective poles.

Spinous Process — prolongation of neural processes fused dorsally to neural canal; becomes dorsal spine of vertebra.

Spiracle — short funnel between body wall and operculum on left side of head of frog tadpole, the only exit for water passing through gill chambers to exterior.

Spireme — continuous chromatin thread characteristic of so-called resting cell nucleus. Existence questioned by current cytologists.

Splanchnic — refers to viscera, opposed to somatic or body.

Splanchnic Mesoderm — visceral mesoderm, or that nearest embryonic axis in lateral plate.

Splanchnocoel — that portion of enterocoel (of Amphioxus) which lies between somatic and splanchnic mesoderm within body. (Syn., coelom.)

Splanchnocranium — that portion of skull which is preformed in cartilage and which arises from the first three pairs of visceral arches. Opposed to neurocranium.

Splanchnopleure — layer of endoderm and inner (splanchnic) mesoderm within which develop the numerous blood vessels of area vasculosa and later yolk sac septa; layers within the body which give rise to lining and to musculature of alimentary canal.

Spongioblasts- — cells of mantle layer of developing spinal cord destined to form merely supporting tissue.

Stereoblastula — solid blastula as found in Crepidula.

Stomodeum — ectodermal invagination (pit) which fuses with pharyngeal endoderm to form oral plate, which later ruptures to form margins of mouth cavity. Stomodeal portion of mouth lining is therefore ectodermal.

Stroma — mesodermally derived, medullary, supporting tissues of an organ.

Sub-Germinal Cavity — See Blastocoel, Segmentation Cavity.

Sub-Notochordal Rod or Bar — hypochordal rod of amphibian embryo, found dorsal to midgut. Transitory.

Sucker — adhesive, connecting organ of oral region (larval stage).

Sustentacular Cell — cell which provides nourishment for another, such as Sertoli or follicle cells of gonads.

Sylvius, Aqueduct of — See Aqueduct of Sylvius.

Sympathetic System — originating either from mesenchymal element arising In situ or, more probably, from ectodermal elements emanating from neural crests, to organize as a chain of ganglia near dorsal aorta and controlling involuntary (visceral) musculature.

Synapsis — union, such as the lateral (parasynapsis) or terminal (telosynapsis) union of embryos; or pairing of homologous chromosomes.

Synaptene Stage — stage in maturation between leptotene and synizesis (contraction) stage wherein chromatin is in form of long threads, intertwined in homologous pairs. (Syn., zygotene, amphitene.)

Syncytium — nuclei and cytoplasm without cellular boundaries; multinucleate protoplasm without cell boundaries.

Syngamy — specifically the fusion of the gamete pronuclei, but also the union of gametes at fertilization. (Syn., zygotogenesis, fertilization.)

Synizesis — stage in maturation between synaptene and pachytene when chromatin threads are short and thick and ends away from centrosome are tangled.

T

Telencephalon — portion of forebrain (ventricle) anterior to a plane which includes posterior side of choroid plexus and anterior side of optic recess of 5 mm. frog embryo. Gives rise to torus transversus (anterior commissure), cerebral hemispheres, corpora striata, anterior choroid plexus, olfactory lobes, lateral ventricles, and part of foramina of Monro.

Telobiosis — fusion of embryos end-to-end. (Syn., parabiosis.)

Telocoel — cavity of telencephalon.

Telolecithal — See Egg, telolecithal.

Telophase — last phase in mitosis when respective chromosome groups have reached their respective astral centers and are beginning to reform a resting cell nucleus; stage often accompanied by beginning of cytoplasmic division.

Telosynapsis — end-to-end fusion of chromosomes. (Syn., parasynapsis.)

Teratology — study of causes of monster formation.

Tetrads — paired (homologous) chromosomes which have become duplicated longitudinally in anticipation of the meiotic (reductional) division. When viewed from end will appear as a group of four chromosomes, hence a tetrad.

Thalamus — dorso-lateral wall of diencephalon which becomes thickened by development of fibers passing from cord to more posterior parts of cerebral hemispheres.

Theca externa- — outermost of coverings of ovarian follicle, rather loose connective tissue with abundant blood supply. Continuous with peritoneum.

Theca interna — layer of connective tissue consisting of closely packed fibers, possibly some of smooth muscle, immediately external to egg. Consists of cyst wall.

Thymus — derivatives of first pair of branchial pouches of frog embryo which separate from pouches (12 mm.) and migrate to a position posterior to auditory capsules near surface of the head. Endocrine functions.

Thyroid (Body or Gland) — originates as an endodermal thickening in floor of pharynx between second pair of visceral arches; evaginates to form a vesicle temporarily connected with gut by a duct; separates from gut; becomes divided; and migrates to position near hyoglossus muscle. Somewhat similar history in all vertebrate embryos. Endocrine function.

Tissue Culture — in vitro culturing of isolated tissues; excision of tissues or organs and their maintenance in an artificial medium, generally consisting in part of embryonic extracts or blood plasma.

Tongue — solid mesodermal mass, covered with endoderm, derived by cell proliferation from floor of pharynx beginning in the 9 mm. frog tadpole.

Tonsils — lymphatic structures derived from endoderm and mesoderm of second pair of visceral pouches.

Torus Transversus — thickening in median ventro-anterior wall of lamina terminalis of telencephalon, just exterior to optic recess, representing rudiment of anterior commissure.

Totipotency — related to theory that isolated blastomere is capable of producing a complete embryo.

Trachea — that portion of respiratory tract between larynx and lung buds, lined with endoderm, probably derived from posterior portion of original laryngotracheal groove.

Tracheal Groove — Syn., laryngotracheal groove.

Transplant — an embryonic area (cell, tissue, or organ) removed to a different environment.

Transverse — a plane (or sections) which divides antero-posterior axis at right angles, separating more anterior from more posterior. (Syn., cross section, but this synonym is not generally satisfactory.)

Transverse Neural Fold — continuation of lateral neural folds (ridge) of early frog embryo around anterior neuropore. (Syn., transverse medullary fold or ridge.)

Trigeminal Ganglion — cranial (V) ganglia which consist of motor and sensory portions and arise from segments of the most anterior crest in conjunction with cells from inner (ganglionic) portion of corresponding placode. Give rise to ophthalmic, mandibular, and maxillary branches, associated with rhombencephalon at level of greatest width of fourth ventricle.

Trochlearis Nerves — cranial (IV) motor nerves which arise from dorsal surface of brain near isthmus, coming from medullary neuroblasts and innervating superior oblique muscles of eye.

Truncus Arteriosus — anterior continuation of buibus arteriosus beneath foregut, divided in antero-posterior direction by a septum which is continuous through buibus to ventricle; gives off external carotids to mandibular arches and second, third, and fourth aortic arches which join dorsal aorta. (Syn., ventral aorta.)

Tuberculum Posterius— a thickening in floor of brain at region of anterior end of notochord, representing posterior margin of diencephalon.

Tubo-tympanic Cavity — remnants of dorsal parts of first pair of visceral (hyomandibular) pouches and lateral walls of pharynx, connecting pharynx and middle ear, represented by Eustachian tube of adult bird or mammal.

Tubules — See under specific names such as Collecting, Mesonephric, Pronephric. Seminiferous.

Tunica Albuginea — See Albuginea of Testis.

Tympanic Cavity — cavity of middle ear, a vestige of hyomandibular pouch. (See Tubo-tympanic Cavity.)

Tympanic Membrane — membrane made up of ectoderm, mesenchyme, and endoderm which separates tympanic cavity from exterior. (Syn., ear drum.)

U

Urinary Bladder- — endodermally lined vesicle derived from hindgut, homologous to allantois of chick. Connected with mesonephric (excretory) ducts of frog only through cloaca.

Uriniferous Tubule — functional kidney tubule of mesonephros.

Urodele — tailed amphibia (e.g., salamanders). (Syn., caudata.)

Urogenital Duct — ducts which open into cloaca of male amphibia and convey both excretory and genital products, derived from mesonephric (Wolffian) ducts.

Urogenital System — entire excretory and reproductive systems, some embryonic parts of which degenerate before hatching. Shows various degrees of common origin and ultimate function. (See specific excretory and reproductive components.)

Urostyle — fused skeletogenous elements of the last two somites in frog embryo which surround end of notochord as cartilage and finally ossify.

Utricle— a vesicle, generally referring to superior portion of otocyst which gives rise to the various semi-circular canals of the ear, and into which these canals open. Lined with ectoderm.

V

Vasa Deferentia — mesonephric or Wolffian ducts of frog, which persist as male gonoducts of bird and mammal, connecting with testes through vasa efferentia and epididymis and functioning as sperm ducts after degeneration of embryonic mesonephros and development of gonads. (Sing., vas deferens.)

Vasa Efferentia — ducts which convey frog sperm from collecting tubules through mesorchium to Malpighian corpuscles of mesonephric kidney; derived from rete cords and connected with mesonephric tubules of anterior (sexual) half of the mesonephric or Wolffian body.

Vegetal Pole — pole of a telolecithal egg where there is greatest concentration of yolk, usually opposite animal pole and location of germinal vesicle. (Syn., vegetal or vegetative hemisphere; abapical or antipolar hemisphere.) (See Animal Pole.)

Vein — See under specific names.

Velar Plate — folds or flaps developing anterior and posterior to branchial regions of frog (anuran) embryo derived from pharyngeal wall and serving as a gross sifting organ between pharynx and gill (branchial) chamber.

Velum Transversum — depressed roof of telencephalon just anterior to lamina terminalis, which later becomes much folded and vascular as anterior roof of third ventricle.

Vena Cava Anterior — junction of inferior jugular (anterior cardinal) and subclavian and vertebral veins which empty into ductus Cuvieri, and later the right auricle. (Syn., superior vena cava, superior caval veins.)

Vena Cava Posterior — single median ventral vein which represents remnant of anterior right cardinal and which later receives hepatic vein prior to joining ductus Cuvieri, and later joins right auricle directly.

Ventral — belly surface. Ventrad means toward belly surface.

Ventral Mesentery — double layer of mesoblast which connects alimentary canal with splanchnopleure in embryo.

Ventricle III — main cavity (diocoel) of forebrain, related to paired lateral ventricles or telocoels, by way of foramina of Monro.

Ventricle IV — main cavity of hindbrain (rhombencephalon) connected anteriorly with aqueduct of Sylvius and posteriorly with neural canal, having as a roof the vascular posterior choroid plexus.

Ventricle, Lateral — See Lateral Ventricles of the Brain.

Ventricle of the Heart — chamber of the heart, single in frog and very muscular, developing from anterior myocardium and provided with valves; connected with bulbus arteriosus anteriorly.

Vertebra — derivatives of sclerotome which surround nerve cord and notochord, and finally incorporate notochord by chondrification and ossification (centrum).

Vertebral Arch — See Neural Arch.

Vertebral Plate — See Axial Mesoderm. (Syn., segmental plate.)

Vesicle, Germinal — nucleus of egg while it is a distinct entity and before elimination of either of the polar bodies.

Visceral — pertaining to viscera.

Visceral Arches — mesodermal masses (usually six pairs) between visceral pouches and lateral to pharynx of all vertebrate embryos, including mandibular, hyoid, and four branchial arches. Each arch is bounded by endoderm on pharyngeal side and ectoderm on outside. (Syn,, visceral arches III to VI are also called branchial arches I to IV, respectively; pharyngeal arch.)

Visceral Clefts — slit-like openings between pharynx and outside, found in vertebrate embryos on either side of visceral arches II to V, or less, consisting of peripheral lining of ectoderm and mesial lining of endoderm. (Syn., pharyngeal, and some may be called gill or branchial clefts.)

Visceral Furrow — ectodermal invaginations which may meet endodermal pharyngeal evaginations to form visceral clefts. (Syn., visceral groove.)

Visceral Groove — See Visceral Furrow.

Visceral Mesoderm — See Splanchnic Mesoderm, Splanchnopleure.

Visceral Plexus — aggregation of sympathetic neurons which control viscera, having migrated posteriorly from tenth (vagus) cranial ganglia.

Visceral Pouch — endodermal evagination of pharynx which, if it meets corresponding visceral furrow, often breaks through to form visceral cleft. (Syn., pharyngeal pouch.)

Vital Stain — localized staining of living embryonic areas with vital, nontoxic dyes.

Vitalism — a philosophical approach to biological phenomena which bases its proof on present inability of scientists to explain all phenomena of development. Idea that biological activities are directed by forces neither physical nor chemical but which must be supra-scientific or supernatural. Effective guidance in development by some non-material agency. (See Mechanism.)

Vitelline — pertains to yolk (e.g., vitelline vein brings blood from yolk; vitelline membrane is that which covers yolked egg).

Vitelline Artery — paired off shoots of dorsal aorta which take blood to belly yolk of early embryo, later to become coeliac and mesenteric arteries.

Vitelline Membrane — delicate, outer, non-living egg covering derived while egg is still within ovary, probably by joint action of egg and its follicle cells; probably same membrane that is elevated as the fertilization membrane after successful insemination. (Syn., fertilization membrane.)

Vitelline Substance — yolk.

Vitelline Vein — paired veins, first to be formed in embryo, found in ventrolateral splanchnopleure, carrying nutritious blood from yolk region to their junction with sinus venosus prior to the full development and function of heart.

Vitreous Humor — the rather viscous fluid of eye chamber posterior to lens, formed by cells budded from retinal wall and from inner side of lens, hence ectodermal and probably also mesenchymal in origin. (See Aqueous Humor.)

W

Wolffian Body — See Mesonephros.

Wolffian Duct — See Mesonephric Duct, Urogenital Duct, Vasa Deferentia.

Y

Yolk — highly nutritious food (metaplasm) consisting of non-nucleated spheres and globules of fatty material found in all except alecithal eggs.

Yolk Nuclei — darkly staining chromatin-like substances within cytoplasm of young (immature) eggs around which yolk is accumulated during growth phase of oogenesis. May be derived from nucleoli which escape from nucleus.

Yolk Plug — a plug formed by large yolk cells which are too large to be incorporated immediately in floor of archenteron of amphibian embryo, hence are found protruding slightly from blastopore. Size of plug is often used to determine approximate stage of gastrulation.

Z

Zone, Marginal — presumptive chorda-mesodermal complex at junction of roof and floor of early gastrula. (Syn., germ ring.)

Historic Disclaimer - information about historic embryology pages

Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Reference

Rugh R. Book - The Frog Its Reproduction and Development. (1951) The Blakiston Company.

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Cite this page: Hill, M.A. (2024, April 19) Embryology Book - The Frog Its Reproduction and Development 15. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Frog_Its_Reproduction_and_Development_15

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B

Balancers — cylindrical and paired projections of ectoderm with mesenchymatous cores, used as adhesive organs in place of (anuran) suckers by many urodele amphibia.

Balfour's Law — "The velocity of segmentation in any part of the ovum is, roughly speaking, proportional to the concentration of the protoplasm there; and the size of the segments is inversely proportional to the concentration of the protoplasm." The intervals between cleavages increase in proportion to the amount of yolk which a cell contains in its protoplasm.

Basal Plate — ventro-lateral wall of myelencephalon, separated from dorsolateral alar plate by sulcus limitans.

Basophil — cell constituents having an affinity for basic dyes, often used as an adjective for an entire cell. (See Acidophil.)

Bidder's Organ — anterior portion of anuran pro-gonad, somewhat ovarian in character, developing from part of gonad rudiment consisting wholly of cortex; its development indicates failure of medullary substance to diffuse to anterior extremity of gonad rudiment.

Biogenetic Law — embryos of higher species tend to resemble embryos of lower species in certain respects but are never like adults of lower species. Embryonic development is a gradual deviation from the more general (phylogenetic) to the more specific characters of the individual species. Not to be confused with recapitulation theory.

Blastema — indifferent group of cells about to be organized into definite tissue; nev/ly formed cells covering a cut surface, functional in regeneration of tissues.

Blastocoel — cavity of blastula. (Syn., segmentation or subgerminal cavity.)

Blastoderm — living portion of egg from which both embryo and all of its membranes are derived. The cellular blastodiscs. "Because the embryo chooses this as its seat and its domicile, contributing much to its configuration out of its own substance, therefore, in the future we shall call it blastoderm" (Pander, 1817).

Blastomere — cellular unit of developing egg or early embryo, prior to time of gastrulation. Smaller blastomeres are micromeres; intermediate ones are mesomeres; larger ones are macromeres, where there is great disparity in size.

Blastopore — opening of archenteron (gastrocoel) to exterior, occluded by yoliv plug in amphibian embryos; consisting of a slit-like space between elevated margin of blastoderm and underlying yolk of chick egg; represented in amniota as primitive streak.

Blastopore, Dorsal Lip of — region of first involution of cells in amphibian gastrula; general area of the "organizer"; original gray crescent area; cells which turn in beneath potential central nervous system (Amphioxus) and form roof of archenteron. (Syn., germ ring or marginal zone.)

Blastopore, Ventral Lip of — region of blastopore opposite dorsal lip; region which gives rise to peristomial mesoderm of frog. (Syn., germ ring.)

Blastula — stage in embryonic development between appearance of distinct blastomeres and end of cleavage (i.e., beginning of gastrulation); a stage generally possessing a primary embryonic cavity or blastocoel; invariably monodermic. (See specific types under specific names.)

Blood Islands — pre-vascular groups of mesodermal cells found in splanchnopleure, from which will arise blood vessels and corpuscles.

Bowman's Capsule — double-walled glomerular cup associated with uriniferous tubule.

Branchial — having to do with respiration. (Syn., gill.)

Branchial Arch — visceral arches, beginning with third pair, which contain blood vessels which (phylogenetically) have respiratory function during embryonic development. Mesodermal components which support those blood vessels are branchial arches. (Syn,, gill arch.) (See Visceral Arches.)

Branchial Artery — blood vessel which actually passes through gills (external or internal) of frog embryo. (Syn., gill artery.)

Branchial Chamber — closed chamber (except for a single spiracular opening on left side) which encloses internal gills of frog embryo. (Syn., opercular or gill chamber.)

Branchial Cleft — opening between branchial arches formed by invaginating head ectoderm and evaginating pharyngeal endoderm (pouch) through which water passes from pharynx to outside of frog. (Syn., gill cleft or slit, some visceral clefts.)

Branchial Groove — ectodermal invagination anterior or posterior to visceral arch, which joins branchial pouch to form branchial cleft, in most instances.

Branchiomery — type of serial metamerism involving respiratory structures exemplified by visceral arches.

Bud — undeveloped branch, generally an anlage of an appendage (e.g., limb or wing bud).

Budding — reproductive process by which a small secondary part is produced from parent organism, and which gradually grows to independence.

Bulbus Arteriosus — most anterior division of early, tubular, embryonic heart which leads from ventricle to truncus arteriosus.

C

Cardinal Veins — anterior, posterior, and sub-cardinal veins; anterior veins receive blood from head, including first three segmental veins; posterior veins receive blood from all pairs of trunk segmental veins and from veins of Wolffian bodies; paired cardinals enlarge and fuse, left half degenerates, and balance fuses with developing inferior (posterior) vena cava.

Cell — protoplasmic territory under control of a single nucleus, whether or not territory is bounded by a discrete membrane. By this definition a syncytium is made up of many cells with physiological rather than morphological boundaries.

Cell Lineage — study of origin and fate of specific blastomeres in embryonic development. (Syn., cytogeny.)

Cell Theory — body of any living organism is composed of structural and functional units, the primary agents of organization called cells. Each cell consists of a nucleus and its sphere of influence, including the cytoplasm, generally circumscribed by a membrane. "Omnis cellula e cellula" (Virchow).

Central Canal — See Neurocoel.

Centriole — granular core of centrosome.

Centrosome — granule (centriole) and surrounding sphere of rays (centrosphere) which function as kinetic centers in mitosis. Center of aster which does not disappear when astral rays disappear. Dynamic center of mitosis.

Centrosphere — rayed portion of centrosome; structure in spermatid which gives rise to acrosome. (Syn., spermatosphere, idiosome, attraction sphere. )

Cephalic Flexure — ventral bending of embryonic head at level of midbrain and hindbrain.

Chimera — compound embryo generally derived by grafting major portions of two embryos, usually of different species; may be derived by abnormal chromosome distribution in cleavage after normal fertilization.

Choana — openings of olfactory organ into pharynx, internal nares. Sometimes also used in connection with external olfactory opening.

Chondrification — process of forming cartilage, by secretion of a homogeneous matrix between the more primitive cells.

Chondrin — chemical substance in cartilage which makes it increasingly susceptible to basic stains.

Chondrocranium — that portion of skull which is originally cartilaginous.

Chorda Dorsalis — Syn., notochord.

Chorda Mesoderm — region of the late (amphibian) blastula, arising from gray crescent area, which will give rise to notochord and mesoderm and will, if transplanted, induce formation of secondary medullary folds.

Choroid Coat — mesenchymatous and sometimes pigmented coat within sclerotic coat but surrounding pigmented layer of eye in vertebrate embryos.

Choroid Fissure — inverted groove in optic stalk whose lips later close around blood vessels and nerves that enter eyeball.

Choroid Knot — thickened region of fused lips of choroid fissure, near pupil, from which arise cells of iris.

Chromatid — one of the parts of a tetrad (McClung, 1900); really a longitudinal half of a chromosome.

Chromatin — deeply staining substance of nuclear network and chromosomes, consisting of nuclein; gives Feulgen reaction and stains with basic dyes.

Chromatophore — pigment-bearing cell frequently capable of changing size, shape, and color; cells responsible for superficial color changes in animals; behavior under control of sympathetic nervous system or neurohumors.

Chromidia — granules within cytoplasm which stain like chromatin and which may actually be extruded chromatin granules.

Chromomere — unit of chromosome recognized as a chromatin granule.

Chromonema — slender thread of chromatin which is core of chromosome during mitosis.

Chromophil — cells which have an affinity for dyes.

Chromophobe — cells whose constituents are non-stainable; have no affinity for dyes.

Chromosome — chromatic or deeply staining bodies derived from nuclear network and containing a matrix and one or more chromonemata during process of mitosis; bodies found in all somatic cells of normal organism in a number characteristic of the species; bearers of gene.

Cleavage — mitotic division of egg resulting in blastomeres. (Syn., segmentation. )

Cleavage, Accessory — cleavage in peripheral or deeper portions of (chick) germinal disc caused by supernumerary sperm nuclei following (normal) polyspermy, sometimes occurring in urodeles.

Cleavage, Asymmetrical — extremely unequal divisions of egg as in Ctenophore.

Cleavage, Bilateral — cleavage in which egg substances are distributed symmetrically with respect to median plane of future embryo.

Cleavage, Determinate — cleavage in which certain parts of future embryo may be circumscribed in certain specific (early) blastomeres; cleavage which produces blastomeres that are not qualitatively equipotential, i.e., when such blastomeres are isolated they will not give rise to entire embryos. (Syn., mosaic development.)

Cleavage, Dexiotropic — cleavage resulting in a right-handed production of daughter blastomere(s), as in spiral cleavage.

Cleavage, Discoidal — See Cleavage, Meroblastic.

Cleavage, Equatorial — cleavage at right angles to egg axis, opposed to vertical or meridional; often the typical third cleavage plane. (Syn., latitudinal or horizontal cleavage.)

Cleavage, Holoblastic — complete division of egg into blastomeres, generally equal in size although not necessarily so (e.g., Amphioxus). (Syn., total cleavage.)

Cleavage, Horizontal — See Cleavage, Equatorial.

Cleavage, Indeterminate — cleavage resulting in qualitatively equipotential blastomeres in early stages of development. When such blastomeres are isolated from each other they give rise to complete embryos. Opposed to mosaic development. (Syn., regulatory development.)

Cleavage, Latitudinal — See Cleavage, Equatorial.

Cleavage Laws — See specific laws under names of Balfour, Hertwig, and Sachs.

Cleavage, Levotropic — cleavage resulting in left-handed or counterclockwise production of daughter blastomere(s) as in some cases of spiral cleavage.

Cleavage, Meridional — cleavage along egg axis, opposed to equatorial; generally the first two cleavages on any egg. (Syn., vertical cleavage.)

Cleavage, Meroblastic — cleavage restricted to peripherally located protoplasm, as in chick egg. (Syn., discoidal cleavage.)

Cleavage Nucleus — nucleus which controls cleavage. This may be syngamic nucleus of normal fertilization, egg nucleus of parthenogenetic or gynogenetic eggs, or sperm nucleus of androgenetic eggs.

Cleavage Path — path taken by syngamic nuclei to position awaiting first division.

Cleavage, Radial — holoblastic cleavage which results in tiers of cells.

Cleavage, Spiral — cleavage at an oblique angle with respect to egg axis so that resulting blastomeres (generally micromeres) lie in an interlocking fashion within furrows of original blastomeres, due to intrinsic genetic factors (e.g., Mollusca).

Cleavage, Superficial — cleavage around periphery of centrolecithal eggs. (Syn., peripheral cleavage.)

Cochlea — portion of original otic vesicle associated with sense of hearing; supplied by vestibular ganglion of eighth cranial nerve, having to do with equilibration.

Coeloblastula — spherical ball of blastomeres with a central cavity (e.g., Echinoderms).

Coelom — mesodermal cavity from walls of which gonads develop; cavity subdivided in higher forms into pericardial, pleural, and peritoneal cavities. (Syn., extra-embryonic body cavity and exocoel.)

Coitus — copulation of male and female, term generally used in connection with mammals. Comparable situation in amphibia is called amplexus.

Collecting Tubule — portion of nephric tubule system leading to nephric duct (Wolffian, etc.); term also used to refer to tubules which conduct spermatozoa from seminiferous tubule to vasa efferentia, within testis.

Colloid — dispersed substance whose particles are not smaller than 1 ^ and not larger than 100 jx, approximately. Physical state of protoplasm.

Columella — bone in tubo-tympanic cavity of frog which aids in auditory sensations. (Syn., plectrum, malleus.)

Competence — ability of embryonic area to react to stimulus (e.g., evocator).

Concrescence — coming together of previously separate parts (cell areas) of embryo, generally resulting in a piling up of parts. One of the corollaries of gastrulation where a bottle-neck of cell movements occurs at lips of blastopore. Original meaning (His, 1874) referred to presumed preformed parts of fish germ ring. (See Confluence.)

Cone, Fertilization — conical projection of cytoplasm from surface of egg to meet spermatozoon which is to invade egg cortex. Cone makes contact and then draws sperm into egg. Not universally demonstrated or seen in frog, but seen in starfish (Chambers). (Syn., exudation cone.)

Cones of Growth — enlarged outgrowth of neuroblast forms axis cylinder or axon of nerve fiber and is termed cone of growth because growth processes by which axon increases in length are supposed to be located there.

Confluence — similar to concrescence except that this term refers specifically to "flow" of cells (or areas) together, whether or not they are piled up.

Constriction — gradual closure of blastopore (diametrical reduction of germ ring) over yolk toward vegetal pole. May be due to stretching of marginal zone, to pull or tension of dorsal lip, or even to narrowing of marginal zone. (Syn., convergence [Jordan] or Konzentrisches Urmundschluss [Vogt].)

Convergence, Dorsal — material of marginal zone moves toward dorsal mid-line as it involutes during gastrulation, resulting in a compensatory ventral divergence. (Syn., confluence [Smith] or dorsal Reffung [Vogt].)

Copulation Path — second portion of sperm migration path through egg toward egg nucleus, when there is any deviation from entrance or penetration path; path of spermatozoon which results in syngamy.

Cords, Medullary — structures which give rise to urogenital connections and take part in formation of seminiferous tubules, and are derived from blastema of mesonephric cords.

Cords, Sex — strands of somatic cells and primordial germ cells growing from cortex toward medulla of gonad primordium. Best seen in early stages of testes development.

Cornea — transparent head ectoderm plus underlying mesenchyme form a layer directly over eye of vertebrates, known as cornea.

Corticin — sex-differentiating substances which spread in some amphibia by blood stream and in other forms by diffusion and act as a hormone. (See Medullarin.)

Cranial — relative to head; "craniad" means toward head. (Syn., rostral, cephalad.)

Cranial Flexure — bending of forebrain forward with angle of bend occurring transversely at level of midbrain. (See Cephalic Flexure.)

Crescent, Gray — crescentic area between original animal and vegetal pole regions on surface of frog's egg, gray in color because of migration of pigment away from area and toward sperm entrance point (Roux, 1888); region of presumptive chorda-mesoderm, future blastopore, and anus.

Crest, Neural — paired cell masses derived from ectoderm cells along edge of former neural plate, and wedged into space between dorso-lateral wall of closed neural tube and integument. Gives rise to spinal ganglia, sympathetic ganglia, and chromatophores.

Crest Segment — original neural crest becomes divided into segments from which develop spinal and possibly cranial ganglia.

Cross-Fertilization — union of gametes produced by different individuals which, if they are of different species, may produce hybrids.

Crossing Over — mutual exchange of portions of allelomorphic pairs of chromosomes during process of synapsis in maturation.

Cyclopia — failure of eyes to separate; median fusion of eyes which may be due to suppression of rostral block of tissue which ordinarily separates eyes; exaggeration of vegetativization tendencies.

Cyst — tubular portions of testis within which aggregations of germ cells mature, often (e.g., Rhomaleum) containing cells all in same stage of maturation.

Cystic Duct — narrow, proximal portion of embryonic bile duct leading from gallbladder to common bile duct.

Cytasters — asters arising apart from nucleus in cytoplasm.

Cyte — suffix meaning cell (e.g., osteocyte for bone cell, oocyte for egg cell). (See specific definitions.)

Cytology — study of cells.

Cytolysis — breakdown of cell indicated by dispersal of formed components.

Cytoplasm — material of cell exclusive of nucleus; protoplasm apart from nucleoplasm.

D

Delamination — separation (of cell layers) by splitting, a process in mesoderm formation.

Dermal Bones — bony plates which originate in dermis and cover cartilaginous skull.

Dermatome — outer unthickened wall of somite which gives rise to dermis. (Syn., cutis plate.)

Dermis — deeper layers of skin entirely derived from mesoderm (dermatome).

Dermocranium — portion of skull which does not go through an intermediate cartilaginous stage in development. (Syn., membranocranium.)

Determination — process of development indicated when a tissue, whether treated as an isolated unit or as a transplant, still develops in the originally predicted manner.

Determination of Sex — mechanism by which realization of sex differences is achieved, generally thought to be associated with chromosomal relations.

Deutencephalon — caudal region of brain which later forms mesencephalon and rhombencephalon.

Deutoplasm — yolk or secondary food substances of egg; non-living.

Development — gradual transformation of dependent differentiation into self-differentiation; transformation of invisible multiplicity into a visible mosaic elaboration of components in successive spatial hierarchies.

Development, Mosaic — "all the single primordia stand side by side, separate from each other like the stones of a mosaic work, and develop independently although in perfect harmony with each other, into the finished organism" (Spemann, 1938). Some believe there is prelocalization of embryonic potencies within egg, test for which would be self-differentiation.

Development, Regulative — type of development requiring organizer or inductor influences since each of the early blastomeres could develop into whole embryos. Structures are progressively determined through action of evocators.

Diencephalon — portion of forcbrain posterior to telencephalon, including second and third neuromeres.

Differentiation — acquisition of specialized features which distinguish areas from each other; progressive increase in complexity and organization, visible and invisible; elaboration of diversity through determination leading to histogenesis; production of morphogenetic heterogeneity; process of change from a simple to a complex organism. (Syn., diff erenzierung. )

Differentiation, Axial — variations in density of chemical and often indefinable inclusion in direction of one diameter of the egg, called egg axis.

Differentiation, Dependent — all difi[erentiation that is not self-diflferentiation; development of parts of organism under mutual influences, such influences being activating, limiting, or inhibiting. Inability of parts of organism to develop independently of other parts.

Differentiation, Self perseverance in a definite course of development of a part of an embryo, regardless of its altered surroundings (Roux, 1912).

Diocoel — cavity of diencephalon, ultimate third ventricle.

Diploid — normal complement of chromosomes in somatic and primordial germ cells, twice the haploid number characteristic of mature gametes.

Diplotene — stage in maturation following pachytene when chromosomes again appear double and do not converge toward centrosome. Sometimes refers to split individual chromosomes.

Discoblastula — disc-shaped blastula found in cases of discoidal (meroblastic) cleavage (e.g.. Cephalopoda and chick).

Distal — farther from any point of reference, away from main body mass.

Divergence, Ventral — divergence of material from mid-ventral line, compensatory to process of dorsal convergence in gastrulation (Vogt).

Diverticulum — blind outpocketing of a tubular structure (e.g., liver or thyroid anlage).

Dominance — parts of a system which have greater growth momentum and also which gather strength from the rest, such as dorsal lip of blastopore.

Dorsal Mesentery — membrane formed by doubling of peritoneum from mid-dorsal line of body cavity, which supports intestine.

Dorsal Root Ganglion — aggregation of neuroblasts which are derived from neural crests and which send their processes into dorsal horns of spinal cord.

Dorsal Thickening — roof of mesencephalon which gives rise to optic lobes.

'Duct — See ducts under specific names.

Ductus Arteriosus — See Ductus Botalli.

Ductus Botalli — dorsal portion of sixth pair of aortic arches which normally becomes occluded after birth, remainder of arch giving rise to pulmonary arteries. (Syn:, ductus arteriosus.)

Ductus Cuvieri — union of all somatic veins which empty directly into heart, specifically the vein which unites common cardinals and sinus venosus. Sometimes regarded as synonymous with common cardinal.

Ductus Endolymphaticus — dorsal portion of original otic vesicle which has lost all connections with epidermis, and which is partially constricted from region which will form semi-circular canals.

Duodenum — portion of embryonic gut associated with outgrowths of pancreas and liver (bile) ducts.

Dyads — aggregations of chromosomes consisting of two rather than four (tetrad) parts, term used to describe condition during maturation process.

E

Ecdysis — process of molting a cuticular layer, shedding of epithelium.

Ectoblast — See Epiblast.

Ectoderm — outermost layer of didermic gastrula. (Syn., epiblast.)

Ectoplasm — external layer of protoplasm of egg cell; layer immediately beneath cell membrane. (Syn., egg cortex.)

Edema — condition in which tissues hold an excess of water, common in parthenogenetic tadpoles. (Older spelling: oedema.)

Egg, Alecithal — eggs with little or no yolk. Literally means "without yolk."

Egg, Cleidoic — eggs, such as those of reptiles, birds, and oviparous mammals, which are covered by a protective shell.

Egg, Ectolecithal — egg having yolk around formative protoplasm. Opposed to centrolecithal.

Egg Envelope — material enveloping egg but not necessarily a part of the egg, such as vitelline membrane, chorion, jelly, albumen.

Egg, Giant — abnormal polyploid condition where chromosome complexes are multiplied, resulting in giant cells and embryos.

Egg, Homolecithal — egg (e.g., mammal) in which but little yolk is scattered throughout cytoplasm.

Egg, Isolecithal — eggs with homogeneous distribution of yolk; may be isolecithal, alecithal, or homolecithal.

Egg Jelly — mucin covering deposited on amphibian egg as it passes through oviduct.

Egg, Macrolecithal — egg with large amount of yolk, generally telolecithal.

Egg Membranes — include all egg coverings such as vitelline membrane, chorion, and tertiary membranes.

Egg, Microlecithal — egg with small amount of yolk. (Syn., meiolecithal egg, oligolecithal egg.)

Egg Receptor — part of Lillie's scheme picturing parts that go into the fertilization reaction involving fertilizin. Egg receptor plus amboceptor plus sperm receptor gives fertilization.

Egg, Telolecithal — egg with large amount of yolk concentrated at one pole.

Egg Water — watery extract of materials diffusing from living eggs, presumably the "fertilizin" of Lillie. (Syn., egg water extract.)

Ejaculation — forcible emission of mature spermatozoa from body of male.

Ejaculatory Duct — short portion of mesonephric duct (mammal) between seminal vesicles and urethra.

Emboitement — preformationist theory of Bonnet and others based on idea that ovary of first female (Eve?) contained the miniatures of all subsequently existing human beings. (Syn., encasement theory.)

Embryo — any stage in ontogeny of fertilized egg, generally limited to period prior to independent food-getting. Stage between second week and second month of human embryo.

Endocardium — delicate endothelial tissue forming lining of heart.

Endochondral Bone — bone preformed in cartilage. (Syn., cartilage bone.)

Endoderm — innermost layer of didermic gastmla. (Syn., entoderm.)

Endolymphatic Duct — See Ductus Endolymphaticus.

Endolymphatic Sac — See Saccus Endolymphaticus.

Endoplasm — inner medullary substance of (egg) cell which is generally granular, soft, watery, and less refractive than ectoplasm.

Entelchy — Driesch's theory of an (intangible) agent controlling development. (Syn., elan vital.)

Enterocoel — cavity or pouch within mesoderm just formed by evagination of gut (enteron) endoderm as in Amphioxus. (Syn., gut pouch, coelomic pouch, archenteric pouch.)

Enteron — definitive gut of embryo, always lined with endoderm.

Ento-mesoderm — refers to portion of invaginating blastoporal lips which will induce formation of medullary fields in amphibian embryo.

Entrance Cone — temporary depression on surface of egg following entrance of spermatozoon.

Entrance Path — See Path, penetration.

Ependymal Cells — narrow zone of non-nervous and ciliated cells which surround central canal (neurocoel), from outer ends of which branching processes extend to periphery, such processes forming a framework for other cellular elements in spinal cord and brain.

Epiblast — outermost layer of early embryo from which the various germ layers may be derived.

Epiboly — growing, spreading, or flowing over; process by which rapidly dividing animal pole cells or micromeres grow over and enclose vegetal pole material. Increase in areal extent of ectoderm.

Epibranchial Placode — placode (thickening) external to gills related to lateral line organs and tenth cranial nerves, (Syn., suprabranchial placode.)

Epidermis — ectodermal portion of skin including cutaneous glands, hair, feathers, nails, hoofs, and some types of horns and scales.

Epigenesis — development of systems starting with primitive, homogeneous, lowly organized condition and achieving great diversification.

Epimere — most dorsal mesoderm, that lying on either side of nerve and notochord, which gives rise to somites. (Syn., axial mesoderm.)

Epiphysis — evagination of anterior diencephalon of vertebrates which becomes separated from brain as pineal (endocrine) gland of adult.

Epithelioid Bodies — endodermal masses arising from second and third visceral pouches of amphibia.

Epithelium — thin covering layer of cells; may be ectodermal, endodermal, or mesodermal.

Equational Maturation Division — maturational divisions in which there is no (qualitative) reduction in chromosomal complex, similar in results to mitosis.

Equatorial Plate — lateral view of chromosomes, lined up on mitotic spindle, prior to any anaphase movement.

Eustachian Tube — vestige of endodermal portion of hyomandibular pouch connecting middle ear and pharyngeal cavities and lined with endoderm.

Evagination — growth from any surface outward.

"Ex Ovo Omnia" — -all life comes from the egg (Harvey, 1657).

Exogastrula — gastrulation modified experimentally by abnormal conditions so that invagination is partially or totally hindered and there remains some mesendoderm not enclosed by ectoderm.

Experimental Method — concerted, organized, and scientific analysis of the causes, forces, and factors operating in any (embryological) system.

External Gills — outgrowths of (amphibian) branchial arches which function as temporary (anura) or permanent neotonic (urodela) respiratory organs.

Extra-Embryonic — refers to structures apart from embryonic body, such as membranes.

F

Fate Map — map of blastula or early gastrula stage which indicates prospective significance of various surface areas, based upon previously established studies of normal development aided by means of vital dye markings.

Fate, Prospective — destination toward which we know, from previous experience, that a given part would develop under normal conditions; lineage of each part of egg through its cell descendants into a definite region or portion of adult organism.

Fertilization — activation of egg by sperm and syngamy of pronuclei; union of male and female gamete nuclei.

Fertilization, Anti "eggs contain within their interior a substance capable of combining with the agglutinating group of the fertilizin, but which is separate from it as long as the egg is inactive" (Lillie).

Feulgen Reaction — chemical test for thymo-nucleic acid, used as a specific staining test for chromatin.

Field — mosaic of spatio-temporal activities within developing organism.

Field, Morphogenetic — embryonic field out of which will normally develop certain specific structures.

Flexure — refers to a bending such as cranial, cervical, and pontine flexures. Also dorsal and lumbo-sacral flexures of the pig.

Follicle — cellular sac within which egg generally goes through early maturation stages.

Forebrain — most anterior of first three primary brain vesicles, associated with lateral opticoels. (Syn., prosencephalon.)

Foregut — more anterior portion of enteric canal, first to appear, aided by development of pharyngeal derivatives.

Fovea Germinativa — pigment-free spot of animal hemisphere where amphibian germinal vesicle gives off its polar bodies.

Frontal — plane at right angles to both transverse and sagittal, dividing dorsal from ventral. (Syn., coronal.)

G

Gamete — differentiated (matured) germ cell, capable of functioning in fertilization (e.g., sperm or egg cell, germ cell).

Gametogenesis — process of developing and maturing germ cells.

Ganglion — aggregation of neurons, generally derived from a neural crest (e.g., cranial and spinal ganglia).

Ganglion, Acoustic — eighth cranial ganglion from which fibers of eighth cranial nerve arise, purely sensory.

Ganglion, Acustico-facialis — early undifferentiated association of seventh and eighth cranial ganglia.

Ganglion, Gasserian — fifth cranial ganglion, carrying both sensory and motor fibers. (Syn., trigeminal ganglion, semilunar ganglion.)

Ganglion, Geniculate — ganglion at root of facial (VII) cranial nerve, carrying both sensory and motor fibers.

Ganglion, Nodosal — ganglion associated with vagus (X) cranial nerve which carries afferent fibers to pharynx, larynx, trachea, oesophagus, and thoracic and abdominal viscera.

Ganglion, Petrosal — ganglion associated with glossopharyngeal (IX) cranial nerve, more peripheral than superior ganglion carrying sensory fibers from pharynx and root of tongue.

Ganglion, Superior — ganglion associated with glossopharyngeal (X) cranial nerve, mesial to petrosal ganglion.

Gasserian Ganglion— fifth cranial or trigeminal ganglion, derived from midbrain.

Gastraea Theory — theory of Haeckel that since all higher forms have gastrula stages there may have existed a common ancestor built on the plan of a permanent gastrula, as are the recent Coeloenterata.

Gastral Mesoderm — mesoderm derived from dorso-lateral bands (enterocoelic) in Amphioxus or from dorsal lip in frog. Opposed to peristomial mesoderm.

Gastrocoel — cavity formed during process of gastrulation. (Syn., archenteron.)

Gastrula — didermic embryo, possessing a newly formed cavity, gastrocoel or archenteron. The two layers are ectoderm and endoderm.

Gastrular Cleavage — separation of ectoderm and endoderm, during gastrulation, by a slit-like crevice, actually compressed blastocoel.

Gastrulation — dynamic process involving cell movements which change embryo from a monodermic to either a di- or tridermic form. Generally involves inward movement of cells to form enteric endoderm. Description includes epiboly, concrescence, confluence, involution, invagination, extension, and convergence.

Genital — refers to reproductive organs or processes, or both.

Genital Ducts — any ducts which convey gametes from their point of origin to region of insemination (e.g., collecting tubules, vas deferens, vas efTerens, seminal vesicle, oviduct, uterus, etc.).

Genital Ridge — initial elevation or thickening for development of external genitalia.

Germ — egg throughout its development, or at any stage.

Germ Cell — cell capable of sharing in reproductive process, in contrast with a somatic cell (e.g., sperm or egg cell). (Syn., gamete.)

Germ Layer — more or less artificial spatial and histogenic distinction of cell groups beginning in gastrula stage, consisting of ectodermal, endodermal, and mesodermal layers. No permanent and clear-cut distinction, as shown by transplantation experiments.

Germ Plasm — hereditary material, generally referring specifically to the genotype. Opposed to somatoplasm.

Germ Ring — ring of cells showing accelerated mitotic activity, generally a synonym for lips of blastopore. The rapidly advancing cells in epiboly.

Germinal Epithelium — peritoneal epithelium out of which reproductive cells of both male and female presumably develop. (Syn., germinal ridges, gonadal ridges.)

Germinal Localization — every area of blastoderm or of unfertilized egg, corresponds to some future organ. Unequal growth produces differentiation of parts (His, 1874). This concept led to Mosaic Theory of Roux (see Fate Map, p. 101).

Germinal Spot — nucleolus of ovum.

Germinal Vesicle — pre-maturation nucleus of egg.

Gestalten — system of configuration consisting of a ladder of levels; electrons, atom, molecule, cell tissue, organ, and organism, each one of which exhibits specifically new modes of action that cannot be understood as mere additive phenomena of the previous levels. With each higher level new concepts become necessary. The parts of the cell cannot exist independently, hence the cell is more than a mere aggregation of its parts — it is a patterned whole. A coherent unit reaching a final configuration in space (W. Kohler). Gestaltung means formation.

Gill — See Branchial Arch, Branchial Chamber, Branchial Cleft.

Gill Plate — elevated and thickened areas of ectoderm posterior to sense plate of embryo where visceral grooves will subsequently form.

Gill Rakers — ectodermal, finger-like obstructions which sift water as it passes from oral cavity to gill chambers of frog tadpole.

Glia Cells — small rounded supporting cells of spinal cord, derived from germinal cells of neural ectoderm.

Glomerulus — aggregation of capillaries associated with branches of dorsal aorta but lying within substance of functional kidney; function is excretory.

Glomus — vascular aggregations within head kidney or pronephros, never to become a glomerulus.

Glottis — opening between pharynx and larynx.

Gonad — organ within which germ cells are produced and generally matured (e.g., ovary or testis). (Syn., sex or germ gland.)

Gonadromorph — condition in which part of an animal may be male and another part female; not to be confused with hermaphroditism.

Gonium — suffix referring to a stage in maturation of a germ cell prior to any maturation division (e.g., spermatogonium, or oogonium).

Gonoduct — See Genital Ducts.

Gradient — gradual variation of developmental forces along an axis; scaled regions of preference. (See Axis.)

Gray Crescent — See Crescent, Gray.

Growth — developmental increase in total mass of protoplasm at expense of raw materials; an embryonic process, generally differentiation; cell proliferation.

Gynogenesis — development of sperm activated egg but without benefit of sperm nucleus.

H

Haploid — having a single set of chromosomes not appearing in allelomorphic pairs, as in mature gametes. Opposed to diploid, or the condition in somatic cells.

Harmonious-Equipotential System — embryonic system in which all parts are equally ready to respond to organism as a whole. Isolated blastomeres of such a system may give rise to complete embryos.

Hatching — beginning of larval life of amphibian, accomplished by temporarily secreted hatching enzymes which aid embryo to escape from its gelatinous capsule.

Hepatic Sinusoids — maze of dilated and irregular capillaries between loosely packed framework of hepatic tubules.

Hepatic Veins — veins from liver to heart, originating as anterior portions of vitelline veins of amphibia.

Hepatic Veins, Portal — remnants of posterior portions of left vitelline vein.

Hermaphrodite — individual capable of producing both spermatozoa and ova.

Hermaphrodite, Protandrous — male elements mature prior to female elements in hermaphrodite.

Hermaphrodite, Protogynous — female elements mature prior to male elements in hermaphrodite.

Hertwig's Law — nucleus tends to place itself in center of its sphere of activity; longitudinal axis of mitotic spindle tends to lie in longitudinal axis of yolk-free cytoplasm of cell.

Heteroagglutinin — agglutinin (fertilizin) of eggs which acts on sperm of different species, substance extractable from egg water which causes irreversible agglutination of foreign species.

Heterozygous — condition in which zygote is composed of gametes bearing allelomorphic genes. Opposed to homozygous.

Hibernation — spending the cold (winter) period in a state of reduced activity.

Hindbrain — most posterior of the three original brain divisions. (Syn., rhombencephalon. )

Hindgut — portion of amphibian embryonic gut just anterior to neurenteric canal. Level of origin of rectum, cloaca, post-anal gut, and caudal portions of urogenital systems.

Histogenesis — development of tissues.

Homoiothermal — pertaining to a condition in which temperature of body of organism is under control of an internal mechanism; body temperature regulated. Opposed to poikilothermal.

Homology — similarity in structure based upon similar embryonic origin.

Homoplastic — pertaining to a graft to an organism of same species, or even to another position on the same individual. (Syn., autoplastic.)

Homozygous — condition in which zygote is composed of gametes bearing identical rather than allelomorphic genes.

Horizontal — unsatisfactory term sometimes used synonymously with frontal, longitudinal, and even sagittal plane or section. Actually means across the lines of gravitational forces.

Hormone — secretion of a ductless (endocrine) gland which can stimulate or inhibit activity of distant parts of biological system already formed.

Hyaloplasm — viscid liquid regarded as essential living protoplasm.

Hybrid — successful cross between different species (e.g., horse and ass give a mule, which is sterile).

Hyoid Arch — mesodermal mass between hyomandibular and first branchial clefts, or between first and second visceral pouches or clefts which give rise to columella and parts of hyoid apparatus. (Syn., second visceral arch. )

Hyomandibular — pertaining to pouch, cleft, or slit between mandibular and hyoid arches.

Hyperplasia — overgrowth; abnormal or unusual increase in elements composing a part.

Hypertrophy — increase in size due to increase in demands upon part concerned.

Hypochordal Rod — transitory string of cells constricted off between dorsal wall of midgut and notochord of amphibian embryo, between level of pancreas and tail, and disappearing before hatching time. (Syn., sLib-notochordal rod.)

Hypomere — most ventral segment of mesoderm out of which develop somatopleure, splanchnopleure, and coelom. (Syn., lateral plate mesoderm.)

Hypophysis — ectodermally derived solid structure arising anterior to stomodeum and growing inwardly toward infundibulum to give rise to anterior and intermediate parts of pituitary gland.

Hypoplasia — undergrowth or deficiency in elements composing a part.

Hypothesis — complemental supposition; presumption based on fragmentary but suggestive data offered to bridge a gap in incomplete knowledge of the facts. May be offered as an explanation of facts unproved, until subjected to verification or disproof.

I

Idiosome — material out of which acrosome is formed during metamorphosis of spermatid to spermatozoon. (Syn., spermatosphere, centrosphere. )

Induction — successive and purposeful influences which bring about morphogenetic changes within embryo.

Inductor — a loose word which includes both organizer and evocator (Needham). Generally means a piece of living tissue which brings about differentiation within otherwise indifferent tissue.

Infundibulum of the Brain — funnel-like evagination of floor of diencephalon which, along with hypophysis, will give rise to pituitary gland of adult.

Infundibulum of the Oviduct — See Ostium Abdominale Tubae.

Ingression — inward movement of yolk endoderm of amphibian blastula (Nicholas, 1945).

Insemination — process of impregnation; fertilization.

Interauricular Septum — longitudinal sheet of mesodermal tissue which grows ventrally from roof of atrial chamber to divide it into right and left halves.

Interkinesis — resting stage between mitotic divisions.

Intermediate Cell Mass — narrow strip of mesoderm which, for a time, joins dorsal epimere with ventral hypomere, being made up of a dorsal portion continuous with dorsal wall of somite and somatic mesoderm and a ventral portion continuous with ventral wall of somite and splanchnic mesoderm. Source of origin of excretory system. (Syn., nephrotome or middle plate.)

Internal Gills — filamentous outgrowths on posterior side of first three pairs of branchial arches and a single row on anterior side of fourth pair of branchial arches of frog tadpole, which have a respiratory function concurrent with and following absorption of external gills.

Internal Limiting Membrane — membrane which develops on innermost surface of inner wall of optic cup during fourth day of chick development.

Intersex — individual without typical sexual differentiation.

Interstitial Cells — specialized cells between seminiferous tubules of testis which produce hormones.

Interstitial Tissue of Testis — cell aggregations between seminiferous tubules of testis which elaborate a male sex hormone.

Invagination — folding or inpushing of a layer of cells into a preformed cavity, as in one of the processes of gastrulation. Opposed to involution.

Involution — rolling inward or turning in of cells over a rim, as in gastrulation of chick embryo.

Iris — ^narrow zone bounding pupil of eye in which two layers of optic cup become blended so that pigment from outer layer invades material of inner layer, giving eye a specific color by variable reflection.

Isogamy — similar gametes, without differentiations into spermatozoa and ova.

Isolation Culture — removal of a part of an organism and its maintenance in a suitable medium in living condition.

Isthmus of the Brain — depression in dorsal wall of embryonic brain which partially separates mesencephalon from metencephalon.

Isthmus of the Oviduct — short, tubular, posterior end of oviduct (e.g., chick) in which fluid albumen and shell membranes are applied to egg.

Iter — See Aqueduct of Sylvius.

J

Jacobson's Organ — ventro-medial evaginations from olfactory pits (amphibia and reptilia) which later become glandular and sensitive olfactory epithelia.

Jelly — mucin covering of amphibian egg derived from oviduct and applied outside vitelline membrane.

Jugular Veins — veins which bring blood from head, superior or internal jugular being anterior cardinal veins and inferior jugular veins growing toward lower jaw and mouth from base of each ductus Cuvieri.

K

Karyoplasm — protoplasm within confines of nucleus.

Kern-Plasma Relation — ratio of amount of nuclear and of cytoplasmic materials present in the cell. It seems to be a function of cleavage to restore kern-plasma relation from unbalanced condition of ovum with its excessive yolk and cytoplasm to new ratio of gastrula or somatic cell.

L

Lamina Terminalis — point of suture of anterior neural folds (i.e., anterior neuropore) where they are finally separated from head ectoderm; it consists of a median ventral thickening at anterior limit of telencephalon (from anterior side of optic recess to beginning of velum transversum) and includes anterior commissure of torus transversus.

Larva — stage in development when organism has emerged from its membranes and is able to lead an independent existence, but may not have completed its development. Generally (except in cases of neoteny or paedogenesis) larvae cannot reproduce.

Larynx — anterior part of original laryngo-tracheal groove which becomes a tube opening into pharynx by way of glottis.

Lateral — either right (dextral) or left (sinistral) side; laterad means toward the side.

Lateral Line Organs (or System) — line of sensory structures along side of body of fishes and amphibia, generally embedded in skin and innervated by a branch from vagus ganglion, presumably concerned with recognition of low vibrations in water. Appears first at about 4 mm. stage in frog embryo. (Syn., ramus lateralis.)

Lateral Mesocardium — septum posterior to heart extending from base of each vitelline vein obliquely upward to dorso-lateral body wall, representing one of the three parts of septum transversum.

Lateral Mesoderm — See Lateral Plate Mesoderm.

Lateral Neural Folds — See Neural Fold.

Lateral Plates or Lateral Plate Mesoderm — lateral mesoblast within which body cavity (coelom and exocoel) arises. (Syn., lateral mesoderm.)

Lateral Ventricles of the Brain — thick-walled and laterally compressed cavities of prosencephalon which open into third ventricle by way of foramen of Monro; walls will become cerebral hemispheres.

Lecithin — fat from an. animal organism which is phosphorized in form of phosphatides.

Lens — thickening in head ectoderm opposite optic cup at about time of hatching in frog embryo; it becomes a placode, invaginates to acquire a vesicle, and then pinches off into space of optic cup as a lens. Inner surface convex; substance fibrous.

Lens Placode — early thickened ectodermal primordium of lens.

Leptotene — stage in maturation which follows last -gonial division and prior to synaptene stage, structurally similar to resting cell stage. The chromatin material in form of a spireme. Term means thin, diffuse.

Lipids — fats and fatty substances such as oil and yolk (lecithin) found in eggs (e.g., cholesterol, ergosterol).

Lips of the Blastopore — See Blastopore, Lips of.

Localization — cytological separation of parts of the mosaic egg, each of which has a known specific subsequent differentiation. There is often a substratum associated with these areas, made up of pigmented granules, but it is cytoplasm rather than pigmented elements in which localization occurs.

M

Macromere — ^larger of the blastomeres where there is a conspicuous size difference.

Malpighian Body — unit of functional kidney including Bowman's capsule and glomerulus. (Syn., renal corpuscle, Malpighian corpuscle.)

Mandibular Arch — rudiment of lower jaw, mesodermal, and anterior to first or hyomandibular pouch.

Mantle Fibers — those fibers of mitotic spindle which attach chromosomes to centrosomes.

Mantle Layer of the Cord — zone of developing spinal cord with densely packed nuclei slightly peripheral to germinal cells from which they are derived. Includes elongated cells of ependyma.

Maturation — process of transformation of a primordial germ cell (spermatogonium or oogonium) into a functionally mature germ cell, the process involving two special divisions, one of which is always meiotic. Divisions known as equational and reductional.

Mechanism — assumption that biological processes do not violate physical and chemical laws but that they are more than the mere functioning of a machine because material taken into the organism becomes an integral part of the organism, through chemical changes. (Syn., the scientific attitude.) (See Vitalism.)

Meckel's Cartilage — core of lower jaw derived from ventral part of cartilaginous mandibular arch.

Median plane — "middle" plane, as of an embryo. May be median sagittal or median frontal.

Medulla Oblongata — that portion of adult brain derived from rhombencephalon.

Medullarin — sex differentiating substance spread in some amphibia by blood stream as a hormone and in other forms by diffusion. (See Corticin.)

Medullary — See terms under Neural, such as canal, fold, groove, plate, tube.

Medullary Cords — that portion of suprarenal glands derived from sympathetic nervous system; central cords. Also that portion of embryonic gonad presumably derived from pre-migratory germ cells upon reaching genital ridge.

Meiosis — process of nuclear division found in maturation of germ cells, involving a separation of members of pairs of chromosomes. (Syn., reductional division.)

Melanophore — well with black or brown pigment (melanin), derived from neural crests and migrating throughout body.

Membrane Bone — bone developed in regions occupied by connective tissue, not cartilage.

Membrane, Vitelline — See Vitelline Membrane.

Membranes — See Egg Membranes.

Meroblastic Cleavage or Ova — See under Cleavage or Egg.

Mesencephalon — section of primary brain between posterior level of prosencephalon and an imaginary line drawn from tuberculum posterius to a point just posterior to dorsal thickening. Gives rise to optic lobes, crura cerebri, and aqueduct of Sylvius. (Syn., midbrain.)

Mesenchyme — form of embryonic mesoderm or mesoblast in which migrating cells unite secondarily to form a syncitium or network having nuclei in thickened nodes between intercellular spaces filled with fluid; often derived from mesothelium.

Mesendoderm — newly formed layer of (urodele) gastmla before there has been any separation of endoderm and mesoderm. (Syn., mesentoderm, mesentoblast, ento-mesoblast.)

Mesentery — sheet of (mesodermal) tissue generally supporting organ systems (e.g., mesorchium, mesocardium).

Mesial — median, medial, middle.

Mesoblast, Gastral — See Gastral Mesoderm.

Mesoblast, Peristomial — involuted, ventral lip mesoderm, continuous with gastral mesoderm from dorsal lip.

Mesocardium — mesentery of heart; may be dorsal, ventral, or lateral. (See under Lateral Mesocardium.)

Mesoderm — the third primary germ layer developed in point of time, may be derived from endoderm in some forms and from ectoderm in others. (See other terms such as Mesoblast, Mesenchyme, Lateral Plate Mesoderm, Epimere, Mesomere, Hypomere, Gastral Mesoderm, Peristomial Mesoderm, Axial Mesoderm, etc.)

Mesomere — cell of intermediate size where there are conspicuous size differences in an early embryo; also refers to intermediate cell mass: intermediate mesoderm.

Mesonephric Duct — duct which grows posteriorly from mesonephros to cloaca and functions also as vas deferens in male. (Syn., Wolffian duct.)

Mesonephric Tubules — primary, secondary, and sometimes tertiary tubules developing in Wolffian body, functioning in adult amphibia.

Mesonephros — Wolffian body, or intermediate kidney, functional as kidney in adult fish and amphibian.

Mesorchium — mesentery (mesodermal) which surrounds and supports testis to body wall.

Mesothelium — epithelial layers or membranes of mesodermal origin.

Mesovarium — mesentery (mesodermal) which suspends ovary from dorsal body wall.

Metamerism — serial segmentation, as seen in nervous, muscular, and circulatory systems.

Metamorphosis — end of larval period of amphibia when growth is suspended temporarily. There is autolysis and resorption of old tissues and organs such as gills, and development of new structures such as eyelids and limbs; changes in structure correlated with changes in habitat from one that is aquatic to one that is terrestrial; change in structure without retention of original form, as in change from spermatid to spermatozoon.

Metaphase — stage in mitosis when paired chromosomes are lined up on equatorial plate midway between amphiasters, supported by mitotic spindle, prior to any anaphase movement.

Micromere — smaller of cells when there is a conspicuous difference in size, characteristic of Annelids and Molluscs.

Micropyle — aperture in egg covering through which spermatozoa may enter; in such eggs the only possible point of insemination (e.g., many fish eggs).

Midbrain — See Mesencephalon.

Midgut — that portion of archenteron which will give rise to intestines.

Milieu — Term used to include all of the physico-chemical and biological factors surrounding a living system (e.g., external or internal milieu).

Mitochondria — small, permanent, cytoplasmic granules which stain with Janus green B and Janus red; granules which have powers of growth and division; probably lipoid.

Mitosis — cytoplasmic division involving a nucleus and spindle apparatus.

Mitotic Index — proportion in any tissue and at any specified time of the dividing cells; percentage of actively dividing cells.

Monospermy — fertilization accomplished by only one sperm. Opposed to polyspermy.

Monro, Foramina of — tubular connections between single third and paired lateral ventricles of forebrain.

Morphogenesis — all of the topogenetic processes which result in structure formation; origin of characteristic structure (form) in an organ or in an organism compounded of organs.

Morphogenetic Movements — cell or cell area movements concerned with formation of germ layers (e.g., during gastrulation) or of organ primordia.

Morula — spherical mass of cells, as yet without segmentation cavity.

Mosaic — type of egg or development in which fate of all parts is fixed at an early stage, possibly even at time of fertilization. Local injury or excisions generally result in loss of specific organs in developing embryo. Opposed to regulative development.

Miillerian Duct — See Oviducts.

Muscle Plate — See Myotome.

Myeloblasts — muscle-forming (embryonic) cells.

Myoblasts— formative cells within myotome or muscle plate which will give rise to true striated muscles of adult.

Myocardium — muscular part of heart arising from splanchnic mesoblast.

Myocoel — cavity within which ovaries of Amphioxus develop; temporary cavities within myotomes which may have been connected with coelom.

Myotome — thickened primordium of muscle found in each somite, (Syn., muscle plate.)

N

Nares, External — external openings of tubes which are connected with olfactory vesicles.

Nares, Internal — openings of tubular organ from olfactory placodes into anterior part of pharynx of 12 mm. frog tadpole. (Syn., choanae.)

Nasal Choanae — openings of olfactory chambers into mouth.

Nasal Pit — See Olfactory Pit.

Nebenkern — cytological structure near nucleus of early spermatid.

Neoteny — condition of many urodeles and of experimentally produced (thyroidless) anuran embryos in which larval period is extended or retained, i.e., larvae fail to go through normal metamorphosis. Sexual maturity in larval stage (e.g., axolotl, Necturus).

Nephrocoel — cavity, found in nephrotome or intermediate cell mass, which temporarily joins myocoel and coelom.

Nephrogenic Cord — continuous band of intermediate mesoderm (mesomere) without apparent segmentation, prior to budding off of mesonephric tubules.

Nephrogenic Tissue — intermediate cell mass, mesomere, or nephrotome which will give rise to excretory system.

Nephrostome — funnel-shaped opening of kidney tubules into coelom; outer tubules of amphibian mesonephric kidney acquire ciliated nephrostomal openings from coelom and shift their connections to renal portal sinus.

Nephrotome — intermediate cell mass.

Nephrotomic Plate — intermediate mesoderm, mesomere.

Nerve, Abducens — sixth (VI) cranial nerve arising from basal plate of rhombencephalon which controls external rectus muscles of eye.

Nerve, Auditory — eighth ( VllI ) cranial nerve, purely sensory, arising from acoustic ganghon and associated with geniculate ganglion of seventh nerve.

Nerve, Facial — seventh (VII) cranial nerve, both sensory and motor, related to taste buds and facial muscles.

Nerve, Glossopharyngeal — ninth (IX) cranial nerve, mixed, associated with superior and petrosal ganglia.

Nerve, Oculomotor — third (III) cranial nerve which arises from neuroblasts in ventral zone of midbrain near median line just before hatching in frog tadpole.

Nerve, Vagus — tenth (X) cranial nerve, mixed, arising from rhombencephalon and associated with jugular ganglion.

Nervous Layer — innermost of two layers found in roof of segmentation cavity of amphibian blastula, from which bulk of central nervous system is developed.

Neural Arch — ossified cartilages which extend dorsally from centrum around nerve cord.

Neural Canal — See Neurocoel and Neural Tube.

Neural Crest — continuous cord of ectodermally derived cells lying on each side in angle between neural tube and body ectoderm, separated from ectoderm at time of closure of neural tube and extending from extreme anterior to posterior end of embryo; material out of which spinal and possibly some cranial ganglia develop, and related to development of sympathetic ganglia by cell migration.

Neural Fold — elevation of ectoderm on either side of thickened and depressing medullary plate; folds which close dorsally to form neural tube. (Syn., medullary folds.)

Neural Groove — depression caused by sinking in of center of medullary plate to form a longitudinal groove, later to be incorporated within neural tube (spinal cord). (Syn., medullary groove.)

Neural Plate — thickened broad strip of ectoderm along future dorsal side of all vertebrate embryos, later to give rise to central nervous system. (Syn., medullary plate.)

Neural Tube — tube formed by dorsal fusion of neural folds, rudiment of nerve or spinal cord.

Neurenteric Canal — posterior neurocoel where it is connected with closing blastopore and posterior enteron of amphibian; the large common nervous and enteric chamber of Amphioxus; the Kupffer's vesicle of fish embryo; possibly the primitive pit of chick embryo. (Syn., notochordal canal, primitive pit.)

Neuroblasts — primitive or formative nerve cells, probably derived (along with epithelial and glia cells) from germinal cells of embryonic neural tube.

Neurocoel — cavity of neural tube, formed simultaneously with closure of neural folds. (Syn., central canal, neural canal.)

Neurocranium — dorsal portion of skull associated with brain and sense organs.

Neuroglia — see Glia Cells.

Neuropore — temporary opening into neural canal due to a lag in fusion of neural folds at anterior extremity; permanent in Amphioxus and in vicinity of epiphysis of higher vertebrates.

Neurula — stage in embryonic development which follows gastrulation and during which neural axis is formed and histogenesis proceeds rapidly. Notochord and neural plate are already differentiated, and basic vertebrate pattern is indicated,

Notochord — rod of vacuolated cells representing axis of all vertebrates, found beneath neural tube and dorsal to archenteron. Thought to be derived from or simultaneously with endoderm.

Notochordal Sheath — double mesodermal sheath around notochord consisting of an outer elastic sheath developed from superficial chorda cells and an inner secondary or fibrous sheath from chorda epithelium.

Nucleolus — the body generally within the nucleus which has no affinity for chromatin dyes, but stains with acid or cytoplasmic dyes. Function unknown. (Syn., plasmosome.)

O

Oesophagus — elongated portion of foregut between future glottis and opening of bile duct of frog embryo; temporarily occluded just behind glottis but opens again.

Olfactory Lobes — anterior extremities of telencephalic cerebral lobes, partially constricted, associated with first pair of cranial nerves.

Olfactory Pit — depressions within olfactory placodes of 6 mm. frog embryo which will become olfactory organs (external nares).

Olfactory Placode — thickened ectoderm lateral to stomodeal region found in 5 mm. frog embryo, primordia of olfactory pits.

"Omne Vivum e Vivo" — all life is derived from preexisting life (Pasteur).

Omnipotent — term used in connection with a cell which could, under various conditions, assume every cytological differentiation known to the species or which, by division, could give rise to such varied differentiations.

"Omnis Cellula e Cellula" — all cells from preexisting cells (Virchow).

Ontogeny — developmental history of an organism; sequence of stages in early development.

Oocyte — presumptive egg cell after initiation of growth phase of maturation. (Syn., ovocyte.)

Oogenesis— process of maturation of ovum; transformation of oogonium to mature ovum. (Syn., ovogenesis.)

Oogonia — multiplication (mitotic) stage prior to maturation of presumptive egg cell (ovum), found most frequently in peripheral germinal epithelium.

Ooplasm — cytoplasmic substances connected with building rather than reserve materials utilized in developmental processes.

Opercular Chamber — See Branchial Chamber.

Operculum — integumentary growth posteriorly from each of the hyoid arches of frog embryo, which covers and encloses gills.

Optic Chiasma — thickening in forebrain ventral to infundibulum, found as a bunch of optic nerve fibers in future diencephalon.

Optic Cup — invagination of outer wall of primary optic vesicle to form a secondary optic vesicle made up of two layers; a thick internal or retinal layer continuous at pupil and choroid fissure, and a thin external layer which is pigmented. Cavity of cup becomes future posterior chamber of eye.

Optic Lobes — thickened, evaginated, dorso-lateral walls of mesencephalon.

Optic Recess — depression in forebrain anterior to optic chiasma which leads to optic stalks.

Optic Stalk — attachment of optic vesicle to forebrain, at first a tubular connection between optic vesicle and diencephalon. Lumen is later obliterated by development of optic nerve fibers.

Optic Vesicle — evagination of forebrain ectoderm to form primary optic vesicles which in turn invaginate to form secondary optic vesicles or optic cups of eyes.

Opticoel — cavity of primary optic cup.

Oral Plate — stomodeal ectoderm and pharyngeal endoderm fused to form oral membrane. Breaks through to form mouth. (Syn., pharyngeal membrane, oral membrane, stomodeal plate.)

Oral Suckers — elongated, pigmented depressions at antero-ventral ends of mandibular arches of frog embryo which give rise to mucous glands; with adhesive function.

Organization — indicated by interdependence of parts and the whole. "When elements of a certain degree of complexity become organized into an entity belonging to a higher level of organization," says Waddington, "we must suppose that the coherence of the higher level depends on properties which the isolated elements indeed possessed but which could not be entered into certain relations with one another." See Gestalten.

Organizer — chorda-mesodermal field of amphibian embryo; a tissue area which has power of organizing indifferent tissue into a neural axis; possibly comparable to Henson's node of chick embryo.

Osteoblasts — mesenchymal cells which actively secrete a calcareous material in formation of bone; bone-forming cells.

Osteoclasts — bone-destroying cells; cells which appear in and tend to destroy formed bone; constantly active, even in embryo.

Ostium Abdominale Tubae — most anterior, fimbriated end of oviduct in female vertebrates; point of entrance of ovulated egg into oviduct; double in amphibia. (Syn., infundibulum of oviduct, tubal ridge.)

Otic Vesicle — auditory vesicle, otocyst.

Otocyst — original auditory vesicle appearing at level of rhombencephalon in amphibian embryo just before hatching, forming first as a placode. (Syn., auditory vesicle.)

Oviducal Membranes of Ovum — tertiary membranes applied over egg as it passes through oviduct.

Oviducts — paired MUllerian ducts in both males and females, which generally persist in males.

Ovigerous Cords — columns or strands of tissue which divide germinal epithelium of primordium of ovary, carrying primordial germ cells with them and later breaking up into nests of cells, each of which contains an oogonium. (Syn., egg tubes or cords of Pfliiger [mammal].)

Oviposition — process of laying eggs.

Ovocyte — See Oocyte.

Ovogenesis — See Oogenesis.

Ovogonia — See Oogonia.

Ovulation — release of egg from ovary, not necessarily from body.

Ovum — Latin for egg.

P

Pachytene — stage in maturation when allelomorphic pairs of chromosomes are fused (telosynapsis or parasynapsis) so as to appear haploid, during which process crossing over may occur; stage just prior to diplotene. Term means thick or condensed. (Syn., diplonema.)

Paedogenesis — reproduction during larval stage; precocious sex development.

Pancreas — digestive and endocrine glands arising as single posterior and single anterior primordia in vicinity of liver.

Parthenogenesis — development of an egg without benefit of spermatozoon.

Parthenogenesis, Artificial — initiation of development of an egg by artificial means.

Parthenogenesis, Natural — maturation of eggs of some forms leads directly to development without aid of spermatozoa.

Parthenogenetic Cleavage — fragmentation of protoplasm of old and unfertilized chick eggs, originally thought to be true cleavage.

Path, Copulation — See Copulation Path.

Path, Penetration — initial direction of sperm entrance into egg, often shifting toward egg nucleus along a new copulation path. (Syn., entrance path.)

Perforatorium — See A crosome.

Pericardial Cavity — cavity or membrane sac which encloses heart, representing a cephalic portion of coelom within embryonic body. (Syn., parietal cavity.)

Pericardium — thin mesodermal membrane which encloses pericardial cavity and heart.

Perichondrium — mesenchymal layer immediately around forming cartilage.

Perichordal Sheath — thin, mesodermal (sclerotomal), continuous sheet of tissue immediately around notochord.

Periosteum — mesenchymal layer, often originally perichondrium, which will be found immediately around forming bone.

Peristomiai Mesoderm — mesoderm of amphibian gastrula derived from (ventral) lips of blastopore. Opposed to gastral mesoderm.

Peritoneal cavity — body cavity (coelom).

Peritoneum — coelomic mesothelium of abdominal region reinforced by connective tissue.

Perivitelline Membrane — See Vitelline Membrane.

Perivitelline Space — space between vitelline (fertilization) membrane and contained egg, generally filled with a fluid.

Pfliiger's Law — dividing nucleus elongates in direction of least resistance.

Phenotype — outward appearance of an organism regardless of its genetic make-up. Opposed to genotype.

Pigment Layer of Optic Cup — thin outer wall of primary optic cup, posterior to retina, which never fuses with rods and cones of retina.

Pineal — See Epiphysis.

Pituitary — See Hypophysis.

Placode — Plate-like thickening of ectoderm from which arise sensory or nervous structures (e.g., olfactory placode).

Plane — imaginary two-dimensional surface; may be frontal, sagittal, transverse, median, or lateral.

Plasmosome — a true nucleolus. (See Nucleolus.)

Plectrum — See Columella.

Plexus Choroid — Vascular folds in roof of prosencephalon, diencephalon, and rhombencephalon.

Poikilothermal — cold-blooded; animals whose body temperatures are subject to environmental changes because they lack regulating mechanisms. Opposed to homoiothermal.

Polar — pertaining, in most cases, to animal pole, although may refer to vegetal pole, or both.

Polar Body — relatively minute, discarded nucleus of maturing oocyte (generally three). (Syn., polocytes.)

Polarity — axial distribution of component parts; animal and vegetal poles; stratification.

Pole, Animal — region of egg where polar bodies are eliminated; ectoderm forming portion of pre-cleaved egg. (Syn., apical or animal hemisphere.)

Pole, Vegetal — region of egg opposite animal pole; region of lowest metabolic rate; pole with greater density of yolk in telolecithal eggs; generally endoderm-forming region of egg.

Polyembryony — production of several separate individuals from one egg by an early separation of its blastomeres; possible origin of some identical twins.

Polyploid — possessing a multiple number of chromosomes, such as triploid (three times the haploid number), tetraploid (four times the haploid number), etc. Alwavs more than the normal diploid of the typical zygote.

Polyspermy — insemination of an egg with more than a single sperm, occurring generally in chick egg, although but a single sperm nucleus is functional, in syngamy.

Post-Ana! Gut — posteriorly projecting blind pocket of hindgut, that portion of hindgut posterior to anal plate or proctodeal plate. (Syn., postcloacal gut.)

Post-Reduction — maturation in which equational and reductional divisions occur in that order.

Posterior Tubercle — See Tuberculum posterius.

Potency, Prospective — sum total of developmental possibilities, the full range of developmental performance of which a given area (or germ) is capable. Not to be confused with competence.

Preformation — theory that adult is represented in miniature within egg or sperm and that development is simply enlargement.

Pre-migratory Germ Cell — yolk-laden cells of splanchnopleuric origin which migrate by way of blood vessels to gonad primordia. Believed by some to be precursors of gonad stroma or functional germ cells.

Pre-Reduction — maturation in which reductional and equational divisions occur in that order.

Presumptive — expected or predicted outcome of development of a given area (e.g., fate of a part in question) based on previous fate map studies.

Primary Oocyte — termination of growth phase in maturation of ovum from oogonial stage, prior to any maturational divisions.

Primary Spermatocyte — stage in spermatogenesis in which division results in secondary spermatocytes; stage beginning with growth of spermatogonia.

Primitive Groove — groove through center of primitive streak, bounded by primitive folds and terminated anteriorly by primitive pit and posteriorly by primitive plate.

Primordial Germ Cells — diploid cells which are destined to become germ cells (e.g., oogonia and spermatogonia). (Syn., primitive germ cells.)

Primordium — See Anlage.

Proctodeum — ectodermal pit in region of future cloaca which invaginates to fuse with hindgut endoderm to form anal or proctodeal plate, later to rupture and form anus.

Pronephric Capsule — mesodermal connective tissue covering of pronephric masses derived from adjacent myotomes and somatic mesoderm.

Pronephric Chamber — portion of amphibian coelomic cavity open anteriorly and posteriorly but closed ventrally by development of lungs.

Pronephric Duct — outer portion of pronephric nephrotomes which develops a lumen connected posteriorly with mesonephric or Wolffian duct. (Syn., segmental duct.)

Pronephric Tubules — lateral outgrowths of the most anterior nephrotomal masses which acquire cavities in amphibia, connected with pronephric duct. Possibly become infundibulum of oviduct.

Pronephros — embryonic kidney of all vertebrates, extending from second to fourth somites of frog embryo and consisting of as many primitive tubules as somites concerned; completely lost in all adult vertebrates except a few bony fish. (Syn., head kidney.)

Pronucleus — egg nucleus after polar body formation and sperm nucleus after entrance of spermatozoon into egg.

Prophase — first stage in mitotic cycle when spireme is broken up into definite chromosomes, prior to lining up on metaphase (equatorial) plate.

Prosencephalon — See Forebrain.

Prosocoel — cavity of prosencephalon.

Proximal — nearer the point of reference, toward main body mass.

Pupil — opening into secondary optic vesicle, occluded in part by lens, and regulated in diameter by ciliary muscles of iris.

R

Ramus Communicans — connection between sympathetic ganglion and spinal nerve, as numerous as ganglia in any vertebrate; probably originating from crest cells. Ramus means branch.

Recapitulation Theory — theory that embryonic development reviews major steps in evolutionary history. (See qualifications under Biogenetic Law.)

Rectum — narrowed posterior portion of hindgut, lined with thickened endodermal epithelium, which opens directly into cloaca.

Reductional Maturation Division — one of the two important divisions in the maturation of gametes which results in separation of allelomorphic (homologous) pairs of chromosomes so that resulting cells are invariably haploid. Opposed to equational division. (Syn., meiotic division, disjunctional division.)

Regeneration — repair or replacement of lost part or parts, a power gradually lost in the ontogeny of most animals.

Regions, Presumptive — regions of blastula which, by previous experimentation, have been demonstrated to develop in certain specific directions under normal ontogenetic influences.

Regulation — reorganization toward the whole; power of pre-gastrula embryos to utilize materials remaining, after partial excision, to bring about normal conditions; more flexible power than regeneration.

Renal Portal System- — venous system which carries blood to kidneys, involving lateral portions of caval veins (really parts of posterior cardinals), iliacs, and dorso-lumbars. Found in adult amphibia as the most striking evidences of recapitulation.

Rete Cords — strands of epithelial cells containing many primordial germ cells which connect with seminiferous tubules and later become vasa efferentia, in the bird. (Syn., rete testis.)

Retinal Zone — ectodermal derivatives of optic cup consisting of internal limiting membrane, retinal and lenticular zones, and outer pigmented layer. Retina proper includes portions from internal limiting membrane to rods and cones, inclusive.

Rhombencephalon — See Hindbrain.

S

Saccule — -outer and ventral portion of inner ear from which are derived cochlea associated with eighth or auditory nerve. (Syn., sacculus.)

Saccus Endolymphaticus — original endolymphatic duct, closed off from exterior, which (in 20 mm. stage of tadpole) grows up over rhombencephalon to join other sac and form a vascular covering of the brain.

Sachs' Law — all cells tend to divide into equal parts and each new plane of division tends to intersect the preceding one at right angles.

Sagittal — mesial plane, or any plane parallel to it, dividing right parts of body from left. Right angles to both frontal and transverse planes.

Sclerotic Coat — tough mesenchymatous and partially cartilaginous coat outside of choroid coat of vertebrate eye. (Syn., sclera.)

Sclerotome — loose mesenchymal cells proliferated off from inner and ventral edges of myotomes (5 mm. frog) which contribute to formation of axial skeleton.

Secondary Oocyte — stage in oogenesis between primary oocyte and ovum; may be either haploid or diploid, depending upon species considered.

Secondary Spermatocyte — stage in spermatogenesis in which next division results in haploid spermatids, these spermatocytes being either haploid or diploid, depending upon species considered. (See Post- and Prereduction.)

Secretory Tubule — portion of kidney tubule actually involved in excretory process.

Section — generally a slice of an embryo, often of microscopic dimensions, taken in any one of the various planes such as frontal, transverse, or sagittal. (See Serial Sections.)

Segmental Plate — See Axial Mesoderm.

Segmentation — repetition of structural pattern; used as synonym for cleavage as well as for metamerism.

Segmentation Cavity — cavity of blastula. (Syn., subgerminal cavity, blastocoel.)

Semi-Circular Canals — tubular derivatives of utricle lined with ectoderm from otocyst, which constitute accessory balancing mechanisms of vertebrates.

Seminal Vesicle — glandular dilation of distal end of ductus deferens (Wolffian duct) where spermatozoa are temporarily collected prior to ejaculation.

Semination — act 'of fertilizing by discharge of spermatozoa.

Seminiferous Tubule — tubular divisions of testis derived from rete cords, covered by a connective tissue theca and containing supporting (Sertoli) cells and all stages of spermatogenesis.

Sense Plate — narrow band of elevated ectodermal tissue which passes transversely across anterior end of amphibian embryo, ventral to level of fused neural folds, with ends of band bending dorsally to merge with neural folds. Lower margins represent mandibular arch, the plate giving rise to mucous glands (oral suckers) of amphibia and to parts of olfactory organs, lens of eye, and possibly to part of inner ear.

Septum — partition.

Serial Sections — thin (often of microscopic dimensions) sections of embryos which are mounted on slides in order of their removal from the embryo, so that a study in sequence will provide an understanding of all organ systems from one region of embryo to the other.

Sertoli Cell — derivative of sexual cords of testis, found within seminiferous tubule and functionally similar to follicle cell in ovary in that it is the nutritive, supporting, or nurse cell of the maturing spermatozoa. The heads of adult spermatozoa may be seen embedded in the cytoplasm of Sertoli cells.

Sex Cell Cord — division of sex cell ridge or gonad primordium, not to be confused with sexual (rete) cords.

Sex Determination — See Determination of Sex.

Sexual Cords — derivatives of germinal epithelium from which they become separated and give rise to bulk of gonads of both sexes.

Sexual Cords of the Ovary — sex cords of the originally indifferent gonad primordium which form only cords of ovary, the functional follicles coming from germinal epithelium.

Sexual Cords of the Testis — sex cords of the originally indifferent gonad primordium which give rise to seminiferous tubules of testis, forming a rather solid mesenchymatous reticulum when cavities begin to appear lined with spermatogonia (from primordial germ cells) and Sertoli cells, the whole constituting seminiferous tubules.

Sheath, Myelin — myelin covering of axons in so-called white matter of spinal cord.

Sinus Venosus — point of fusion of vitelline veins of amphibian embryo bilaterally symmetrical and related to ducts of Cuvieri and ductus venosus.

Skeletogenous Sheath — sclerotomal cells which first form a continuous layer around both notochord and nerve cord.

Skin — See Dermis and Epidermis. (Syn., integument.)

Somatic — relating to body in contrast to germinal cells; or relating to outer body in contrast to inner splanchnic mesoderm.

Somatoblast — blastomeres with specific germ layer predisposition, i.e., ectodermal somatoblasts.

Somatopleure — layer of somatic mesoderm and closely associated ectoderm, extension of which (from body wall) gives rise to both amnion and chorion.

Somite — blocks of paraxial mesoblast, metamerically separated by transverse clefts, derived from enterocoelic or gastral mesoderm and giving rise to dermatome, myotome, and sclerotome.

Spawning — act of expelling eggs from uteri of anamniota (e.g., amphibia).

Sperm — germ cell characteristically produced by the male. (Syn., spermatozoon, sperm cell, male gamete, spermatosome.)

Spermatid — products of the second maturation division in spermatogenesis, the spermatids having certain cytological characteristics and being invariably haploid; cells which go through a metamorphosis into functionally mature spermatozoa.

Spermatocyte — stages in spermatogenesis between the time the primordial germ cell (spermatogonium) begins to grow, without division, until after the division which results in spermatids. (See Primary Spermatocyte, Secondary Spermatocyte.)

Spermatogenesis — entire process which results in maturation of spermatozoon.

Spermatogonium — primordial germ cell of male gonad, indistinguishable from somatic cells, both of which are diploid; stage prior to maturation when the presumptive spermatozoon undergoes rapid multiplication by mitosis.

Spermatophore — sperm-bearing bundle, such as that which is shed by male urodele, the bundles later to be picked up by cloacal lips of female.

Spermatosphere — See Idiosome.

Spermatozoon — functionally mature male gamete. (Syn., sperm.)

Spina Bifida — split tail, generally involving spine, in developing embryo caused by a variety of environmental conditions, most of which act through interference with normal gastrulation and neurulation.

Spinal Cord — that portion of central nervous system, excluding brain, which is derived from epithelial and neural ectoderm of original blastula, consisting of ependyma, glia, neuroblasts and their derivatives, and connecting cells.

Spindle — group of fibers between centrosomes during mitosis, to which chromosomes are attached and by means of which (mantle fiber portion) chromosomes are drawn to their respective poles.

Spinous Process — prolongation of neural processes fused dorsally to neural canal; becomes dorsal spine of vertebra.

Spiracle — short funnel between body wall and operculum on left side of head of frog tadpole, the only exit for water passing through gill chambers to exterior.

Spireme — continuous chromatin thread characteristic of so-called resting cell nucleus. Existence questioned by current cytologists.

Splanchnic — refers to viscera, opposed to somatic or body.

Splanchnic Mesoderm — visceral mesoderm, or that nearest embryonic axis in lateral plate.

Splanchnocoel — that portion of enterocoel (of Amphioxus) which lies between somatic and splanchnic mesoderm within body. (Syn., coelom.)

Splanchnocranium — that portion of skull which is preformed in cartilage and which arises from the first three pairs of visceral arches. Opposed to neurocranium.

Splanchnopleure — layer of endoderm and inner (splanchnic) mesoderm within which develop the numerous blood vessels of area vasculosa and later yolk sac septa; layers within the body which give rise to lining and to musculature of alimentary canal.

Spongioblasts- — cells of mantle layer of developing spinal cord destined to form merely supporting tissue.

Stereoblastula — solid blastula as found in Crepidula.

Stomodeum — ectodermal invagination (pit) which fuses with pharyngeal endoderm to form oral plate, which later ruptures to form margins of mouth cavity. Stomodeal portion of mouth lining is therefore ectodermal.

Stroma — mesodermally derived, medullary, supporting tissues of an organ.

Sub-Germinal Cavity — See Blastocoel, Segmentation Cavity.

Sub-Notochordal Rod or Bar — hypochordal rod of amphibian embryo, found dorsal to midgut. Transitory.

Sucker — adhesive, connecting organ of oral region (larval stage).

Sustentacular Cell — cell which provides nourishment for another, such as Sertoli or follicle cells of gonads.

Sylvius, Aqueduct of — See Aqueduct of Sylvius.

Sympathetic System — originating either from mesenchymal element arising In situ or, more probably, from ectodermal elements emanating from neural crests, to organize as a chain of ganglia near dorsal aorta and controlling involuntary (visceral) musculature.

Synapsis — union, such as the lateral (parasynapsis) or terminal (telosynapsis) union of embryos; or pairing of homologous chromosomes.

Synaptene Stage — stage in maturation between leptotene and synizesis (contraction) stage wherein chromatin is in form of long threads, intertwined in homologous pairs. (Syn., zygotene, amphitene.)

Syncytium — nuclei and cytoplasm without cellular boundaries; multinucleate protoplasm without cell boundaries.

Syngamy — specifically the fusion of the gamete pronuclei, but also the union of gametes at fertilization. (Syn., zygotogenesis, fertilization.)

Synizesis — stage in maturation between synaptene and pachytene when chromatin threads are short and thick and ends away from centrosome are tangled.

T

Telencephalon — portion of forebrain (ventricle) anterior to a plane which includes posterior side of choroid plexus and anterior side of optic recess of 5 mm. frog embryo. Gives rise to torus transversus (anterior commissure), cerebral hemispheres, corpora striata, anterior choroid plexus, olfactory lobes, lateral ventricles, and part of foramina of Monro.

Telobiosis — fusion of embryos end-to-end. (Syn., parabiosis.)

Telocoel — cavity of telencephalon.

Telolecithal — See Egg, telolecithal.

Telophase — last phase in mitosis when respective chromosome groups have reached their respective astral centers and are beginning to reform a resting cell nucleus; stage often accompanied by beginning of cytoplasmic division.

Telosynapsis — end-to-end fusion of chromosomes. (Syn., parasynapsis.)

Teratology — study of causes of monster formation.

Tetrads — paired (homologous) chromosomes which have become duplicated longitudinally in anticipation of the meiotic (reductional) division. When viewed from end will appear as a group of four chromosomes, hence a tetrad.

Thalamus — dorso-lateral wall of diencephalon which becomes thickened by development of fibers passing from cord to more posterior parts of cerebral hemispheres.

Theca externa- — outermost of coverings of ovarian follicle, rather loose connective tissue with abundant blood supply. Continuous with peritoneum.

Theca interna — layer of connective tissue consisting of closely packed fibers, possibly some of smooth muscle, immediately external to egg. Consists of cyst wall.

Thymus — derivatives of first pair of branchial pouches of frog embryo which separate from pouches (12 mm.) and migrate to a position posterior to auditory capsules near surface of the head. Endocrine functions.

Thyroid (Body or Gland) — originates as an endodermal thickening in floor of pharynx between second pair of visceral arches; evaginates to form a vesicle temporarily connected with gut by a duct; separates from gut; becomes divided; and migrates to position near hyoglossus muscle. Somewhat similar history in all vertebrate embryos. Endocrine function.

Tissue Culture — in vitro culturing of isolated tissues; excision of tissues or organs and their maintenance in an artificial medium, generally consisting in part of embryonic extracts or blood plasma.

Tongue — solid mesodermal mass, covered with endoderm, derived by cell proliferation from floor of pharynx beginning in the 9 mm. frog tadpole.

Tonsils — lymphatic structures derived from endoderm and mesoderm of second pair of visceral pouches.

Torus Transversus — thickening in median ventro-anterior wall of lamina terminalis of telencephalon, just exterior to optic recess, representing rudiment of anterior commissure.

Totipotency — related to theory that isolated blastomere is capable of producing a complete embryo.

Trachea — that portion of respiratory tract between larynx and lung buds, lined with endoderm, probably derived from posterior portion of original laryngotracheal groove.

Tracheal Groove — Syn., laryngotracheal groove.

Transplant — an embryonic area (cell, tissue, or organ) removed to a different environment.

Transverse — a plane (or sections) which divides antero-posterior axis at right angles, separating more anterior from more posterior. (Syn., cross section, but this synonym is not generally satisfactory.)

Transverse Neural Fold — continuation of lateral neural folds (ridge) of early frog embryo around anterior neuropore. (Syn., transverse medullary fold or ridge.)

Trigeminal Ganglion — cranial (V) ganglia which consist of motor and sensory portions and arise from segments of the most anterior crest in conjunction with cells from inner (ganglionic) portion of corresponding placode. Give rise to ophthalmic, mandibular, and maxillary branches, associated with rhombencephalon at level of greatest width of fourth ventricle.

Trochlearis Nerves — cranial (IV) motor nerves which arise from dorsal surface of brain near isthmus, coming from medullary neuroblasts and innervating superior oblique muscles of eye.

Truncus Arteriosus — anterior continuation of buibus arteriosus beneath foregut, divided in antero-posterior direction by a septum which is continuous through buibus to ventricle; gives off external carotids to mandibular arches and second, third, and fourth aortic arches which join dorsal aorta. (Syn., ventral aorta.)

Tuberculum Posterius— a thickening in floor of brain at region of anterior end of notochord, representing posterior margin of diencephalon.

Tubo-tympanic Cavity — remnants of dorsal parts of first pair of visceral (hyomandibular) pouches and lateral walls of pharynx, connecting pharynx and middle ear, represented by Eustachian tube of adult bird or mammal.

Tubules — See under specific names such as Collecting, Mesonephric, Pronephric. Seminiferous.

Tunica Albuginea — See Albuginea of Testis.

Tympanic Cavity — cavity of middle ear, a vestige of hyomandibular pouch. (See Tubo-tympanic Cavity.)

Tympanic Membrane — membrane made up of ectoderm, mesenchyme, and endoderm which separates tympanic cavity from exterior. (Syn., ear drum.)

U

Urinary Bladder- — endodermally lined vesicle derived from hindgut, homologous to allantois of chick. Connected with mesonephric (excretory) ducts of frog only through cloaca.

Uriniferous Tubule — functional kidney tubule of mesonephros.

Urodele — tailed amphibia (e.g., salamanders). (Syn., caudata.)

Urogenital Duct — ducts which open into cloaca of male amphibia and convey both excretory and genital products, derived from mesonephric (Wolffian) ducts.

Urogenital System — entire excretory and reproductive systems, some embryonic parts of which degenerate before hatching. Shows various degrees of common origin and ultimate function. (See specific excretory and reproductive components.)

Urostyle — fused skeletogenous elements of the last two somites in frog embryo which surround end of notochord as cartilage and finally ossify.

Utricle— a vesicle, generally referring to superior portion of otocyst which gives rise to the various semi-circular canals of the ear, and into which these canals open. Lined with ectoderm.

V

Vasa Deferentia — mesonephric or Wolffian ducts of frog, which persist as male gonoducts of bird and mammal, connecting with testes through vasa efferentia and epididymis and functioning as sperm ducts after degeneration of embryonic mesonephros and development of gonads. (Sing., vas deferens.)

Vasa Efferentia — ducts which convey frog sperm from collecting tubules through mesorchium to Malpighian corpuscles of mesonephric kidney; derived from rete cords and connected with mesonephric tubules of anterior (sexual) half of the mesonephric or Wolffian body.

Vegetal Pole — pole of a telolecithal egg where there is greatest concentration of yolk, usually opposite animal pole and location of germinal vesicle. (Syn., vegetal or vegetative hemisphere; abapical or antipolar hemisphere.) (See Animal Pole.)

Vein — See under specific names.

Velar Plate — folds or flaps developing anterior and posterior to branchial regions of frog (anuran) embryo derived from pharyngeal wall and serving as a gross sifting organ between pharynx and gill (branchial) chamber.

Velum Transversum — depressed roof of telencephalon just anterior to lamina terminalis, which later becomes much folded and vascular as anterior roof of third ventricle.

Vena Cava Anterior — junction of inferior jugular (anterior cardinal) and subclavian and vertebral veins which empty into ductus Cuvieri, and later the right auricle. (Syn., superior vena cava, superior caval veins.)

Vena Cava Posterior — single median ventral vein which represents remnant of anterior right cardinal and which later receives hepatic vein prior to joining ductus Cuvieri, and later joins right auricle directly.

Ventral — belly surface. Ventrad means toward belly surface.

Ventral Mesentery — double layer of mesoblast which connects alimentary canal with splanchnopleure in embryo.

Ventricle III — main cavity (diocoel) of forebrain, related to paired lateral ventricles or telocoels, by way of foramina of Monro.

Ventricle IV — main cavity of hindbrain (rhombencephalon) connected anteriorly with aqueduct of Sylvius and posteriorly with neural canal, having as a roof the vascular posterior choroid plexus.

Ventricle, Lateral — See Lateral Ventricles of the Brain.

Ventricle of the Heart — chamber of the heart, single in frog and very muscular, developing from anterior myocardium and provided with valves; connected with bulbus arteriosus anteriorly.

Vertebra — derivatives of sclerotome which surround nerve cord and notochord, and finally incorporate notochord by chondrification and ossification (centrum).

Vertebral Arch — See Neural Arch.

Vertebral Plate — See Axial Mesoderm. (Syn., segmental plate.)

Vesicle, Germinal — nucleus of egg while it is a distinct entity and before elimination of either of the polar bodies.

Visceral — pertaining to viscera.

Visceral Arches — mesodermal masses (usually six pairs) between visceral pouches and lateral to pharynx of all vertebrate embryos, including mandibular, hyoid, and four branchial arches. Each arch is bounded by endoderm on pharyngeal side and ectoderm on outside. (Syn,, visceral arches III to VI are also called branchial arches I to IV, respectively; pharyngeal arch.)

Visceral Clefts — slit-like openings between pharynx and outside, found in vertebrate embryos on either side of visceral arches II to V, or less, consisting of peripheral lining of ectoderm and mesial lining of endoderm. (Syn., pharyngeal, and some may be called gill or branchial clefts.)

Visceral Furrow — ectodermal invaginations which may meet endodermal pharyngeal evaginations to form visceral clefts. (Syn., visceral groove.)

Visceral Groove — See Visceral Furrow.

Visceral Mesoderm — See Splanchnic Mesoderm, Splanchnopleure.

Visceral Plexus — aggregation of sympathetic neurons which control viscera, having migrated posteriorly from tenth (vagus) cranial ganglia.

Visceral Pouch — endodermal evagination of pharynx which, if it meets corresponding visceral furrow, often breaks through to form visceral cleft. (Syn., pharyngeal pouch.)

Vital Stain — localized staining of living embryonic areas with vital, nontoxic dyes.

Vitalism — a philosophical approach to biological phenomena which bases its proof on present inability of scientists to explain all phenomena of development. Idea that biological activities are directed by forces neither physical nor chemical but which must be supra-scientific or supernatural. Effective guidance in development by some non-material agency. (See Mechanism.)

Vitelline — pertains to yolk (e.g., vitelline vein brings blood from yolk; vitelline membrane is that which covers yolked egg).

Vitelline Artery — paired off shoots of dorsal aorta which take blood to belly yolk of early embryo, later to become coeliac and mesenteric arteries.

Vitelline Membrane — delicate, outer, non-living egg covering derived while egg is still within ovary, probably by joint action of egg and its follicle cells; probably same membrane that is elevated as the fertilization membrane after successful insemination. (Syn., fertilization membrane.)

Vitelline Substance — yolk.

Vitelline Vein — paired veins, first to be formed in embryo, found in ventrolateral splanchnopleure, carrying nutritious blood from yolk region to their junction with sinus venosus prior to the full development and function of heart.

Vitreous Humor — the rather viscous fluid of eye chamber posterior to lens, formed by cells budded from retinal wall and from inner side of lens, hence ectodermal and probably also mesenchymal in origin. (See Aqueous Humor.)

W

Wolffian Body — See Mesonephros.

Wolffian Duct — See Mesonephric Duct, Urogenital Duct, Vasa Deferentia.

Y

Yolk — highly nutritious food (metaplasm) consisting of non-nucleated spheres and globules of fatty material found in all except alecithal eggs.

Yolk Nuclei — darkly staining chromatin-like substances within cytoplasm of young (immature) eggs around which yolk is accumulated during growth phase of oogenesis. May be derived from nucleoli which escape from nucleus.

Yolk Plug — a plug formed by large yolk cells which are too large to be incorporated immediately in floor of archenteron of amphibian embryo, hence are found protruding slightly from blastopore. Size of plug is often used to determine approximate stage of gastrulation.

Z

Zone, Marginal — presumptive chorda-mesodermal complex at junction of roof and floor of early gastrula. (Syn., germ ring.)

Historic Disclaimer - information about historic embryology pages

Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Reference

Rugh R. Book - The Frog Its Reproduction and Development. (1951) The Blakiston Company.

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