Book - The Frog Its Reproduction and Development 15: Difference between revisions

From Embryology
mNo edit summary
Line 6: Line 6:


==A==
==A==
Acidophil — oxyphil: cell constituents which stain with acid dyes, often used to designate an entire cell type. (See Basophil.)  
'''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.)  
'''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.  
'''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.  
'''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.  
'''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).  
'''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.  
'''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.  
'''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''' — 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  
'''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.  
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.  
'''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).  
'''Amphiblastula''' — double-structured blastula as in Porifera (sponges).  


Amphimixis — mixing of germinal substances accomplished during fertilization.  
'''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.  
'''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.  
'''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.)  
'''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.  
'''Analogy''' — similarity of parts in respect to function rather than to structure.  


Anamniota — forms which never develop an amnion, e.g., cyclostomes, fishes, amphibia.  
'''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.  
'''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.  
'''Anastomosis''' — joining together, as of blood vessels and nerves, generally forming a network.  


Androgen — hormonal secretion of interstitial tissue of testis.  
'''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.  
'''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.  
'''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.)  
'''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.)  
'''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.)  
'''Anterior''' — toward head; head end. (Syn., cephalic, cranial, rostral.)  


Anura — tailless amphibia (e.g., frogs and toads). (Syn., Salientia.)  
'''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.  
'''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.)  
'''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.  
'''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.  
'''Archencephalon''' — pre-chordal brain, e.g., forebrain. Brain anterior to anterior end of notochord.  


Archenteric Pouch — See Enterocoel.  
'''Archenteric Pouch''' — See Enterocoel.  


Archenteron — primitive gut found in gastrula and communicating with outside by blastopore; precursor of embryonic gut. (Syn., gastrocoel, enteron.)  
'''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.  
'''Arcualia''' — small blocks of sclerotomal connective tissue involved in formation of vertebrae.  


Asexual Reproduction — reproduction without union of gametes; generally with no maturation divisions.  
'''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.  
'''Aster''' — "star-shaped structure" surrounding centrosome (Fol, 1877); lines radiating in all directions from centrosome during mitosis.  


Astral Rays — lines which make up aster.  
'''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.  
'''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.  
'''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.)  
'''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.  
'''Attraction Sphere''' — See Centrosphere.  


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


Autogamy — self-fertilization.  
'''Autogamy''' — self-fertilization.  


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


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


Axial Filament — central fiber in tail of a spermatozoon.  
'''Axial Filament''' — central fiber in tail of a spermatozoon.  


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


Axis — imaginary central or median line, generally correlated with a gradient.  
'''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 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.  
Axis of the Embryo — imaginary line representing antero-posterior axis of the future embryo.  


==B==
==B==
Balancers — cylindrical and paired projections of ectoderm with mesenchymatous cores, used as adhesive organs in place of (anuran) suckers by many urodele amphibia.  
'''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.  
'''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.  
'''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.)  
'''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.  
'''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.  
'''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.  
'''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.)  
'''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).  
'''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.  
'''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''' — 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, 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.)  
'''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.)  
'''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.  
'''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.  
'''Bowman's Capsule''' — double-walled glomerular cup associated with uriniferous tubule.  


Branchial — having to do with respiration. (Syn., gill.)  
'''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 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 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 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 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.  
'''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  
'''Branchiomery''' — type of serial metamerism involving respiratory structures  
exemplified by visceral arches.  
exemplified by visceral arches.  


Bud — undeveloped branch, generally an anlage of an appendage (e.g., limb or wing bud).  
'''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.  
'''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.  
'''Bulbus Arteriosus''' — most anterior division of early, tubular, embryonic heart which leads from ventricle to truncus arteriosus.  


==C==
==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.  
'''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''' — 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 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).  
'''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.  
'''Central Canal''' — See Neurocoel.  


Centriole — granular core of centrosome.  
'''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.  
'''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. )  
'''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.  
'''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.  
'''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.  
'''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.  
'''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.  
'''Chondrin''' — chemical substance in cartilage which makes it increasingly susceptible to basic stains.  


Chondrocranium — that portion of skull which is originally cartilaginous.  
'''Chondrocranium''' — that portion of skull which is originally cartilaginous.  


Chorda Dorsalis — Syn., notochord.  
'''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.  
'''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  
'''Choroid Coat''' — mesenchymatous and sometimes pigmented coat within sclerotic coat but surrounding pigmented layer of eye in vertebrate  
embryos.  
embryos.  


Choroid Fissure — inverted groove in optic stalk whose lips later close around blood vessels and nerves that enter eyeball.  
'''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.  
'''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.  
'''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.  
'''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.  
'''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.  
'''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.  
'''Chromomere''' — unit of chromosome recognized as a chromatin granule.  


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


Chromophil — cells which have an affinity for dyes.  
'''Chromophil''' — cells which have an affinity for dyes.  


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


Chromophobe — cells whose constituents are non-stainable; have no affinity for dyes.
'''Chromosome''' — chromatic or deeply staining bodies derived from nuclear  
 
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  
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.  
organism in a number characteristic of the species; bearers of gene.  


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


Cleavage, Accessory — cleavage in peripheral or deeper portions of (chick)  
'''Cleavage, Accessory''' — cleavage in peripheral or deeper portions of (chick)  
germinal disc caused by supernumerary sperm nuclei following (normal) polyspermy, sometimes occurring in urodeles.  
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, 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, 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  
'''Cleavage, Determinate''' — cleavage in which certain parts of future embryo  
may be circumscribed in certain specific (early) blastomeres; cleavage  
may be circumscribed in certain specific (early) blastomeres; cleavage  
which produces blastomeres that are not qualitatively equipotential,  
which produces blastomeres that are not qualitatively equipotential,  
Line 240: Line 238:
entire embryos. (Syn., mosaic development.)  
entire embryos. (Syn., mosaic development.)  


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


Cleavage, Discoidal — See Cleavage, Meroblastic.  
'''Cleavage, Discoidal''' — See Cleavage, Meroblastic.  


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


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


Cleavage, Horizontal — See Cleavage, Equatorial.  
'''Cleavage, Horizontal''' — See Cleavage, Equatorial.  


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


Cleavage, Latitudinal — See Cleavage, Equatorial.  
'''Cleavage, Latitudinal''' — See Cleavage, Equatorial.  


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


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


Cleavage, Meridional — cleavage along egg axis, opposed to equatorial;  
'''Cleavage, Meridional''' — cleavage along egg axis, opposed to equatorial;  
generally the first two cleavages on any egg. (Syn., vertical cleavage.)  
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, 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  
'''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.  
or gynogenetic eggs, or sperm nucleus of androgenetic eggs.  


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


Cleavage, Radial — holoblastic cleavage which results in tiers of cells.  
'''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, 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.)  
'''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.  
'''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).  
'''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.)  
'''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  
'''Coitus''' — copulation of male and female, term generally used in connection with mammals. Comparable situation in amphibia is called  
amplexus.  
amplexus.  


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


Colloid — dispersed substance whose particles are not smaller than 1 ^ and not larger than 100 jx, approximately. Physical state of protoplasm.  
'''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.)  
'''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).  
'''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.)  
'''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.)  
'''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.  
'''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.  
'''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].)  
'''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].)  
'''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.  
'''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, 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.  
'''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.  
'''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.)  
'''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''' — 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.)  
'''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.  
'''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  
'''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.  
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.  
'''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.  
'''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.  
'''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.  
'''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.  
'''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  
'''Cystic Duct''' — narrow, proximal portion of embryonic bile duct leading from gallbladder to common bile duct.  
from gallbladder to common bile duct.  


Cytasters — asters arising apart from nucleus in cytoplasm.  
'''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.)  
'''Cyte''' — suffix meaning cell (e.g., osteocyte for bone cell, oocyte for egg cell). (See specific definitions.)  


Cytology — study of cells.  
'''Cytology''' — study of cells.  


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


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


==D==
==D==


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


Dermal Bones — bony plates which originate in dermis and cover cartilaginous skull.  
'''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.)  
'''Dermatome''' — outer unthickened wall of somite which gives rise to dermis. (Syn., cutis plate.)  


Dermis — deeper layers of skin entirely derived from mesoderm (dermatome).  
'''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.)  
'''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''' — 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.  
'''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.  
'''Deutencephalon''' — caudal region of brain which later forms mesencephalon and rhombencephalon.  


Deutoplasm — yolk or secondary food substances of egg; non-living.  
'''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''' — 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  
'''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  
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  
finished organism" (Spemann, 1938). Some believe there is prelocalization of embryonic potencies within egg, test for which would  
be self-differentiation.  
be self-differentiation.  


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




Diencephalon — portion of forcbrain posterior to telencephalon, including second and third neuromeres.  
'''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,  
'''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. )  
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, 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,  
'''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  
such influences being activating, limiting, or inhibiting. Inability of  
parts of organism to develop independently of other parts.  
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,  
'''Differentiation, Self perseverance in a definite course of development of a part of an embryo, regardless of its altered surroundings (Roux,  
1912).  
1912).  


Diocoel — cavity of diencephalon, ultimate third ventricle.  
'''Diocoel''' — cavity of diencephalon, ultimate third ventricle.  


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


Diplotene — stage in maturation following pachytene when chromosomes  
'''Diplotene''' — stage in maturation following pachytene when chromosomes  
again appear double and do not converge toward centrosome. Sometimes refers to split individual 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).  
'''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.  
'''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).  
'''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  
'''Diverticulum''' — blind outpocketing of a tubular structure (e.g., liver or  
thyroid anlage).  
thyroid anlage).  


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


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


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


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


Duct. See ducts under specific names.  
'''Duct'' — See ducts under specific names.  


Ductus Arteriosus — See Ductus Botalli.  
'''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  
'''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.)  
pulmonary arteries. (Syn:, ductus arteriosus.)  


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


Ductus Endolymphaticus — dorsal portion of original otic vesicle which  
'''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.  
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.  
'''Duodenum''' — portion of embryonic gut associated with outgrowths of pancreas and liver (bile) ducts.  


Dyads — aggregations of chromosomes consisting of two rather than four  
'''Dyads''' — aggregations of chromosomes consisting of two rather than four  
(tetrad) parts, term used to describe condition during maturation  
(tetrad) parts, term used to describe condition during maturation  
process.  
process.  
Line 453: Line 449:
==E==
==E==


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


Ectoblast — See Epiblast.  
'''Ectoblast''' — See Epiblast.  


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


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


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


Egg, Alecithal — eggs with little or no yolk. Literally means "without yolk."  
'''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, 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, Ectolecithal''' — egg having yolk around formative protoplasm. Opposed to centrolecithal.  


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




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


Egg, Homolecithal — egg (e.g., mammal) in which but little yolk is scattered throughout cytoplasm.  
'''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, Isolecithal''' — eggs with homogeneous distribution of yolk; may be isolecithal, alecithal, or homolecithal.  


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


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


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


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


Egg Receptor — part of Lillie's scheme picturing parts that go into the  
'''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.  
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, 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.)  
'''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.  
'''Ejaculation''' — forcible emission of mature spermatozoa from body of male.  


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


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


Endocardium — delicate endothelial tissue forming lining of heart.  
'''Endocardium''' — delicate endothelial tissue forming lining of heart.  


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


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


Endolymphatic Duct — See Ductus Endolymphaticus.  
'''Endolymphatic Duct''' — See Ductus Endolymphaticus.  


Endolymphatic Sac — See Saccus Endolymphaticus.  
'''Endolymphatic Sac''' — See Saccus Endolymphaticus.  


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




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


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


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


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


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


Entrance Path — See Path, penetration.  
'''Entrance Path''' — See Path, penetration.  


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


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


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


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


Epidermis — ectodermal portion of skin including cutaneous glands, hair,  
'''Epidermis''' — ectodermal portion of skin including cutaneous glands, hair,  
feathers, nails, hoofs, and some types of horns and scales.  
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.  
'''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  
'''Epimere''' — most dorsal mesoderm, that lying on either side of nerve and  
notochord, which gives rise to somites. (Syn., axial mesoderm.)  
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.  
'''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  
'''Epithelioid Bodies''' — endodermal masses arising from second and third  
visceral pouches of amphibia.  
visceral pouches of amphibia.  


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




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


Equatorial Plate — lateral view of chromosomes, lined up on mitotic spindle, prior to any anaphase movement.  
'''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  
'''Eustachian Tube''' — vestige of endodermal portion of hyomandibular pouch  
connecting middle ear and pharyngeal cavities and lined with endoderm.  
connecting middle ear and pharyngeal cavities and lined with endoderm.  


Evagination — growth from any surface outward.  
'''Evagination''' — growth from any surface outward.  


"Ex Ovo Omnia" — -all life comes from the egg (Harvey, 1657).  
'''"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  
'''Exogastrula''' — gastrulation modified experimentally by abnormal conditions so that invagination is partially or totally hindered and there  
remains some mesendoderm not enclosed by ectoderm.  
remains some mesendoderm not enclosed by ectoderm.  


Experimental Method — concerted, organized, and scientific analysis of the  
'''Experimental Method''' — concerted, organized, and scientific analysis of the  
causes, forces, and factors operating in any (embryological) system.  
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.  
'''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  
'''Extra-Embryonic''' — refers to structures apart from embryonic body, such  
as membranes.  
as membranes.  


==F==
==F==


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


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


Fertilization — activation of egg by sperm and syngamy of pronuclei; union  
'''Fertilization''' — activation of egg by sperm and syngamy of pronuclei; union  
of male and female gamete nuclei.  
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  
'''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).  
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  
'''Feulgen Reaction''' — chemical test for thymo-nucleic acid, used as a specific  
staining test for chromatin.  
staining test for chromatin.  


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




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


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


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


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


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


Fovea Germinativa — pigment-free spot of animal hemisphere where amphibian germinal vesicle gives off its polar bodies.  
'''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.)  
'''Frontal''' — plane at right angles to both transverse and sagittal, dividing dorsal from ventral. (Syn., coronal.)  


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


Gametogenesis — process of developing and maturing germ cells.  
'''Gametogenesis''' — process of developing and maturing germ cells.  


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


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


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


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


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


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


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


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




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


Gastraea Theory — theory of Haeckel that since all higher forms have gastrula stages there may have existed a common ancestor built on the  
'''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.  
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.  
'''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.)  
'''Gastrocoel''' — cavity formed during process of gastrulation. (Syn., archenteron.)  


Gastrula — didermic embryo, possessing a newly formed cavity, gastrocoel  
'''Gastrula''' — didermic embryo, possessing a newly formed cavity, gastrocoel  
or archenteron. The two layers are ectoderm and endoderm.  
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.  
'''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  
'''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.  
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,  
Description includes epiboly, concrescence, confluence, involution,  
invagination, extension, and convergence.  
invagination, extension, and convergence.  


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


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


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


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


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


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


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


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


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


Germinal Localization — every area of blastoderm or of unfertilized egg,  
'''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  
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).  
Roux (see Fate Map, p. 101).  


Germinal Spot — nucleolus of ovum.  
'''Germinal Spot''' — nucleolus of ovum.  


Germinal Vesicle — pre-maturation nucleus of egg.  
'''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  
'''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  
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  
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  
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.  
a final configuration in space (W. Kohler). Gestaltung means formation.  


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


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


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


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


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


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


Glottis — opening between pharynx and larynx.  
'''Glottis''' — opening between pharynx and larynx.  


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


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


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




Gonoduct — See Genital Ducts.  
'''Gonoduct''' — See Genital Ducts.  


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


Gray Crescent — See Crescent, Gray.  
'''Gray Crescent''' — See Crescent, Gray.  


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


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


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


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


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


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


Hermaphrodite — individual capable of producing both spermatozoa and  
'''Hermaphrodite''' — individual capable of producing both spermatozoa and  
ova.  
ova.  


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


Hermaphrodite, Protogynous — female elements mature prior to male 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  
'''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.  
axis of yolk-free cytoplasm of cell.  


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




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


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


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


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


Histogenesis — development of tissues.  
'''Histogenesis''' — development of tissues.  


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


Homology — similarity in structure based upon similar embryonic origin.  
'''Homology''' — similarity in structure based upon similar embryonic origin.  


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


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


Horizontal — unsatisfactory term sometimes used synonymously with frontal,  
'''Horizontal''' — unsatisfactory term sometimes used synonymously with frontal,  
longitudinal, and even sagittal plane or section. Actually means across  
longitudinal, and even sagittal plane or section. Actually means across  
the lines of gravitational forces.  
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  
'''Hormone''' — secretion of a ductless (endocrine) gland which can stimulate or inhibit activity of distant parts of biological system already  
formed.  
formed.  


Hyaloplasm — viscid liquid regarded as essential living protoplasm.  
'''Hyaloplasm''' — viscid liquid regarded as essential living protoplasm.  


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


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


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


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


Hypertrophy — increase in size due to increase in demands upon part concerned.  
'''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  
'''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.,  
of pancreas and tail, and disappearing before hatching time. (Syn.,  
sLib-notochordal rod.)  
sLib-notochordal rod.)  


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


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


Hypoplasia — undergrowth or deficiency in elements composing a part.  
'''Hypoplasia''' — undergrowth or deficiency in elements composing a part.  


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


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


Induction — successive and purposeful influences which bring about  
'''Induction''' — successive and purposeful influences which bring about  
morphogenetic changes within embryo.  
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  
'''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.  
differentiation within otherwise indifferent tissue.  


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


Infundibulum of the Oviduct — See Ostium Abdominale Tubae.  
'''Infundibulum of the Oviduct''' — See Ostium Abdominale Tubae.  


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


Insemination — process of impregnation; fertilization.  
'''Insemination''' — process of impregnation; fertilization.  


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


Interkinesis — resting stage between mitotic divisions.  
'''Interkinesis''' — resting stage between mitotic divisions.  


Intermediate Cell Mass — narrow strip of mesoderm which, for a time, joins  
'''Intermediate Cell Mass''' — narrow strip of mesoderm which, for a time, joins  
dorsal epimere with ventral hypomere, being made up of a dorsal  
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.,  
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.)  
nephrotome or middle plate.)  


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


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


Intersex — individual without typical sexual differentiation.  
'''Intersex''' — individual without typical sexual differentiation.  


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


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


Invagination — folding or inpushing of a layer of cells into a preformed  
'''Invagination''' — folding or inpushing of a layer of cells into a preformed  
cavity, as in one of the processes of gastrulation. Opposed to involution.  
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.  
'''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  
'''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  
become blended so that pigment from outer layer invades material of  
inner layer, giving eye a specific color by variable reflection.  
inner layer, giving eye a specific color by variable reflection.  


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


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


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


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


Iter — See Aqueduct of Sylvius.  
'''Iter''' — See Aqueduct of Sylvius.  


==J==
==J==
Jacobson's Organ — ventro-medial evaginations from olfactory pits (amphibia and reptilia) which later become glandular and sensitive olfactory epithelia.  
'''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  
'''Jelly''' — mucin covering of amphibian egg derived from oviduct and applied  
outside vitelline membrane.  
outside vitelline membrane.  




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


==K==
==K==
Karyoplasm — protoplasm within confines of nucleus.  
'''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.  
'''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==
==L==


Lamina Terminalis — point of suture of anterior neural folds (i.e., anterior neuropore) where they are finally separated from head ectoderm; it  
'''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  
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.  
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  
'''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  
completed its development. Generally (except in cases of neoteny or  
paedogenesis) larvae cannot reproduce.  
paedogenesis) larvae cannot reproduce.  


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


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


Lateral Line Organs (or System) — line of sensory structures along 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  
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  
with recognition of low vibrations in water. Appears first at about 4  
mm. stage in frog embryo. (Syn., ramus lateralis.)  
mm. stage in frog embryo. (Syn., ramus lateralis.)  


Lateral Mesocardium — septum posterior to heart extending from base of  
'''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.  
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 Mesoderm''' — See Lateral Plate Mesoderm.  


Lateral Neural Folds — See Neural Fold.  
'''Lateral Neural Folds''' — See Neural Fold.  


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


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


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


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


Lens Placode — early thickened ectodermal primordium of lens.  
'''Lens Placode''' — early thickened ectodermal primordium of lens.  


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


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


Lips of the Blastopore — See Blastopore, Lips of.  
'''Lips of the Blastopore''' — See Blastopore, Lips of.  


Localization — cytological separation of parts of the mosaic egg, each of  
'''Localization''' — cytological separation of parts of the mosaic egg, each of  
which has a known specific subsequent differentiation. There is often  
which has a known specific subsequent differentiation. There is often  
a substratum associated with these areas, made up of pigmented  
a substratum associated with these areas, made up of pigmented  
Line 1,028: Line 1,020:


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


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


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


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


Mantle Layer of the Cord — zone of developing spinal cord with densely  
'''Mantle Layer of the Cord''' — zone of developing spinal cord with densely  
packed nuclei slightly peripheral to germinal cells from which they  
packed nuclei slightly peripheral to germinal cells from which they  
are derived. Includes elongated cells of ependyma.  
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  
'''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.  
process involving two special divisions, one of which is always meiotic.  
Divisions known as equational and reductional.  
Divisions known as equational and reductional.  


Mechanism — assumption that biological processes do not violate physical  
'''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  
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.)  
scientific attitude.) (See Vitalism.)  


Meckel's Cartilage — core of lower jaw derived from ventral part of cartilaginous mandibular arch.  
'''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  
'''Median plane''' — "middle" plane, as of an embryo. May be median sagittal  
or median frontal.  
or median frontal.  


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


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


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


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


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


Melanophore — well with black or brown pigment (melanin), derived from  
'''Melanophore''' — well with black or brown pigment (melanin), derived from neural crests and migrating throughout body.  
neural crests and migrating throughout body.
 
Membrane Bone — bone developed in regions occupied by connective tissue,
not cartilage.  


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


Membranes — See Egg Membranes.  
'''Membrane, Vitelline''' — See Vitelline Membrane.  


Meroblastic Cleavage or Ova — See under Cleavage or Egg.  
'''Membranes''' — See Egg Membranes.  


Mesencephalon section of primary brain between posterior level of
'''Meroblastic Cleavage or Ova''' See under Cleavage or Egg.  
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
'''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.)
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.  


'''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  
'''Mesendoderm''' — newly formed layer of (urodele) gastmla before there has been any separation of endoderm and mesoderm. (Syn., mesentoderm, mesentoblast, ento-mesoblast.)  
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).  
'''Mesentery''' — sheet of (mesodermal) tissue generally supporting organ systems (e.g., mesorchium, mesocardium).  


Mesial — median, medial, middle.  
'''Mesial''' — median, medial, middle.  


Mesoblast, Gastral — See Gastral Mesoderm.  
'''Mesoblast, Gastral''' — See Gastral Mesoderm.  


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


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


Mesoderm — the third primary germ layer developed in point of time, may  
Mesoderm''' — the third primary germ layer developed in point of time, may  
be derived from endoderm in some forms and from ectoderm in  
be derived from endoderm in some forms and from ectoderm in  
others. (See other terms such as Mesoblast, Mesenchyme, Lateral  
others. (See other terms such as Mesoblast, Mesenchyme, Lateral  
Plate Mesoderm, Epimere, Mesomere, Hypomere, Gastral Mesoderm, Peristomial Mesoderm, Axial Mesoderm, etc.)  
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:  
'''Mesomere''' — cell of intermediate size where there are conspicuous size differences in an early embryo; also refers to intermediate cell mass:  
intermediate mesoderm.  
intermediate mesoderm.  


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


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


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


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


Mesothelium — epithelial layers or membranes of mesodermal origin.  
'''Mesothelium''' — epithelial layers or membranes of mesodermal origin.  


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


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


'''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.


habitat from one that is aquatic to one that is terrestrial; change in
'''Metaphase''' — stage in mitosis when paired chromosomes are lined up on  
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  
equatorial plate midway between amphiasters, supported by mitotic  
spindle, prior to any anaphase movement.  
spindle, prior to any anaphase movement.  


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


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


Midbrain — See Mesencephalon.  
'''Midbrain''' — See Mesencephalon.  


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


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


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


Mitosis — cytoplasmic division involving a nucleus and spindle apparatus.  
'''Mitosis''' — cytoplasmic division involving a nucleus and spindle apparatus.  


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


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


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


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


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


Morula — spherical mass of cells, as yet without segmentation cavity.  
'''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  
'''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  
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.  
excisions generally result in loss of specific organs in developing embryo. Opposed to regulative development.  


Miillerian Duct — See Oviducts.  
'''Miillerian Duct''' — See Oviducts.  




Muscle Plate — See Myotome.  
'''Muscle Plate''' — See Myotome.  


Myeloblasts — muscle-forming (embryonic) cells.  
'''Myeloblasts''' — muscle-forming (embryonic) cells.  


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


Myocardium — muscular part of heart arising from splanchnic mesoblast.  
'''Myocardium''' — muscular part of heart arising from splanchnic mesoblast.  


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


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


==N==
==N==


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


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


Nasal Choanae — openings of olfactory chambers into mouth.  
'''Nasal Choanae''' — openings of olfactory chambers into mouth.  


Nasal Pit — See Olfactory Pit.  
'''Nasal Pit''' — See Olfactory Pit.  


Nebenkern — cytological structure near nucleus of early spermatid.  
'''Nebenkern''' — cytological structure near nucleus of early spermatid.  


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


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


Nephrogenic Cord — continuous band of intermediate mesoderm (mesomere) without apparent segmentation, prior to budding off of mesonephric tubules.  
'''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  
'''Nephrogenic Tissue''' — intermediate cell mass, mesomere, or nephrotome  
which will give rise to excretory system.  
which will give rise to excretory system.  


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


Nephrotome — intermediate cell mass.  
'''Nephrotome''' — intermediate cell mass.  


Nephrotomic Plate — intermediate mesoderm, mesomere.  
'''Nephrotomic Plate''' — intermediate mesoderm, mesomere.  


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


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


Nerve, Facial — seventh (VII) cranial nerve, both sensory and motor, related to taste buds and facial muscles.  
'''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  
'''Nerve, Glossopharyngeal''' — ninth (IX) cranial nerve, mixed, associated  
with superior and petrosal ganglia.  
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, 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.  
'''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  
'''Nervous Layer''' — innermost of two layers found in roof of segmentation  
cavity of amphibian blastula, from which bulk of central nervous system is developed.  
cavity of amphibian blastula, from which bulk of central nervous system is developed.  


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


Neural Canal — See Neurocoel and Neural Tube.  
'''Neural Canal''' — See Neurocoel and Neural Tube.  


Neural Crest — continuous cord of ectodermally derived cells lying on each  
'''Neural Crest''' — continuous cord of ectodermally derived cells lying on each  
side in angle between neural tube and body ectoderm, separated from  
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  
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.  
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  
'''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.)  
tube. (Syn., medullary folds.)  


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


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


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


Neurenteric Canal — posterior neurocoel where it is connected with closing blastopore and posterior enteron of amphibian; the large com
'''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  
292 GLOSSARY OF EMBRYOLOGICAL TERMS
 
mon 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  
with epithelial and glia cells) from germinal cells of embryonic neural  
tube.  
tube.  


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


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


Neuroglia — see Glia Cells.  
'''Neuroglia''' — see Glia Cells.  


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


Neurula — stage in embryonic development which follows gastrulation and  
'''Neurula''' — stage in embryonic development which follows gastrulation and  
during which neural axis is formed and histogenesis proceeds rapidly.  
during which neural axis is formed and histogenesis proceeds rapidly.  
Notochord and neural plate are already differentiated, and basic vertebrate pattern is indicated,  
'''Notochord and neural plate are already differentiated, and basic vertebrate pattern is indicated,  


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


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


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


==O==
==O==


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


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


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


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


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




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


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


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


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


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


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


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


Opercular Chamber — See Branchial Chamber.  
'''Opercular Chamber''' — See Branchial Chamber.  


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


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


Optic Cup — invagination of outer wall of primary optic vesicle to form  
'''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  
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  
retinal layer continuous at pupil and choroid fissure, and a thin external  
Line 1,381: Line 1,347:
chamber of eye.  
chamber of eye.  


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


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


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


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


Opticoel — cavity of primary optic cup.  
'''Opticoel''' — cavity of primary optic cup.  




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


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


Organization — indicated by interdependence of parts and the whole. "When  
'''Organization''' — indicated by interdependence of parts and the whole. "When  
elements of a certain degree of complexity become organized into an  
elements of a certain degree of complexity become organized into an  
entity belonging to a higher level of organization," says Waddington,  
entity belonging to a higher level of organization," says Waddington,  
Line 1,413: Line 1,379:
Gestalten.  
Gestalten.  


Organizer — chorda-mesodermal field of amphibian embryo; a tissue area  
'''Organizer''' — chorda-mesodermal field of amphibian embryo; a tissue area  
which has power of organizing indifferent tissue into a neural axis;  
which has power of organizing indifferent tissue into a neural axis;  
possibly comparable to Henson's node of chick embryo.  
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.  
'''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  
'''Osteoclasts''' — bone-destroying cells; cells which appear in and tend to  
destroy formed bone; constantly active, even in embryo.  
destroy formed bone; constantly active, even in embryo.  


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


Otic Vesicle — auditory vesicle, otocyst.  
'''Otic Vesicle''' — auditory vesicle, otocyst.  


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


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


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


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


Oviposition — process of laying eggs.  
'''Oviposition''' — process of laying eggs.  


Ovocyte — See Oocyte.  
'''Ovocyte''' — See Oocyte.  


Ovogenesis — See Oogenesis.  
'''Ovogenesis''' — See Oogenesis.  


Ovogonia — See Oogonia.  
'''Ovogonia''' — See Oogonia.  


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


Ovum — Latin for egg.  
'''Ovum''' — Latin for egg.  


==P==
==P==


Pachytene — stage in maturation when allelomorphic pairs of chromosomes are fused (telosynapsis or parasynapsis) so as to appear haploid, during  
'''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.)  
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.  
'''Paedogenesis''' — reproduction during larval stage; precocious sex development.  


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


Parthenogenesis — development of an egg without benefit of spermatozoon.  
'''Parthenogenesis''' — development of an egg without benefit of spermatozoon.  


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


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


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


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


Perforatorium — See A crosome.  
'''Perforatorium''' — See A crosome.  


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


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


Perichondrium — mesenchymal layer immediately around forming cartilage.  
'''Perichondrium''' — mesenchymal layer immediately around forming cartilage.  


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


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




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


Peritoneal cavity — body cavity (coelom).  
'''Peritoneal cavity''' — body cavity (coelom).  


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


Perivitelline Membrane — See Vitelline Membrane.  
'''Perivitelline Membrane''' — See Vitelline Membrane.  


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


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


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


Pituitary — See Hypophysis.  
'''Pituitary''' — See Hypophysis.  


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


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


Plasmosome — a true nucleolus. (See Nucleolus.)  
'''Plasmosome''' — a true nucleolus. (See Nucleolus.)  


Plectrum — See Columella.  
'''Plectrum''' — See Columella.  


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


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


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


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


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


Pole, Animal — region of egg where polar bodies are eliminated; ectoderm  
'''Pole, Animal''' — region of egg where polar bodies are eliminated; ectoderm  
forming portion of pre-cleaved egg. (Syn., apical or animal hemisphere.)  
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.  
'''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  
'''Polyembryony''' — production of several separate individuals from one egg  
by an early separation of its blastomeres; possible origin of some  
by an early separation of its blastomeres; possible origin of some  
identical twins.  
identical twins.  


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


Polyspermy — insemination of an egg with more than a single sperm,  
'''Polyspermy''' — insemination of an egg with more than a single sperm,  
occurring generally in chick egg, although but a single sperm nucleus  
occurring generally in chick egg, although but a single sperm nucleus  
is functional, in syngamy.  
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-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  
'''Post-Reduction''' — maturation in which equational and reductional divisions  
occur in that order.  
occur in that order.  


Posterior Tubercle — See Tuberculum posterius.
'''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
'''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.  
or sperm and that development is simply enlargement.  


Pre-migratory Germ Cell yolk-laden cells of splanchnopleuric origin
'''Preformation''' theory that adult is represented in miniature within egg or sperm and that development is simply enlargement.  
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
'''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.  
occur in that order.  


Presumptive expected or predicted outcome of development of a given
'''Pre-Reduction''' maturation in which reductional and equational divisions occur in that order.  
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
'''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.  
oogonial stage, prior to any maturational divisions.  


Primary Spermatocyte stage in spermatogenesis in which division results
'''Primary Oocyte''' termination of growth phase in maturation of ovum from oogonial stage, prior to any maturational divisions.  
in secondary spermatocytes; stage beginning with growth of spermatogonia.  


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


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


Primordium — See Anlage.  
'''Primordium''' — See Anlage.  


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


Pronephric Capsule — mesodermal connective tissue covering of pronephric  
'''Pronephric Capsule''' — mesodermal connective tissue covering of pronephric masses derived from adjacent myotomes and somatic mesoderm.  
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 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  
'''Pronephric Duct''' — outer portion of pronephric nephrotomes which develops a lumen connected posteriorly with mesonephric or Wolffian duct. (Syn., segmental duct.)  
duct. (Syn., segmental duct.)  


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


Pronephros — embryonic kidney of all vertebrates, extending from second  
'''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.)  
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  
'''Pronucleus''' — egg nucleus after polar body formation and sperm nucleus  
after entrance of spermatozoon into egg.  
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.  
'''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.  
'''Prosencephalon''' — See Forebrain.  


Prosocoel — cavity of prosencephalon.  
'''Prosocoel''' — cavity of prosencephalon.  


Proximal — nearer the point of reference, toward main body mass.  
'''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.  
'''Pupil''' — opening into secondary optic vesicle, occluded in part by lens, and regulated in diameter by ciliary muscles of iris.  


==R==
==R==


Ramus Communicans — connection between sympathetic ganglion and  
'''Ramus Communicans''' — connection between sympathetic ganglion and spinal nerve, as numerous as ganglia in any vertebrate; probably originating from crest cells. Ramus means branch.  
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  
'''Recapitulation Theory''' — theory that embryonic development reviews major steps in evolutionary history. (See qualifications under Biogenetic Law.)  
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.  
'''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  
'''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.)  
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.  
'''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.  
'''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  
'''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.  
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  
'''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.  
striking evidences of recapitulation.  


Rete Cords — strands of epithelial cells containing many primordial germ  
'''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.)  
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  
'''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.  
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.  
'''Rhombencephalon''' — See Hindbrain.  


==S==
==S==


Saccule — -outer and ventral portion of inner ear from which are derived cochlea associated with eighth or auditory nerve. (Syn., sacculus.)  
'''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.  
'''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.  
'''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.  
'''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.)  
'''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.  
'''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 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.)  
'''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.  
'''Secretory Tubule''' — portion of kidney tubule actually involved in excretory process.  


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


Segmental Plate — See Axial Mesoderm.  
'''Segmental Plate''' — See Axial Mesoderm.  


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


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


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


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


Semination — act 'of fertilizing by discharge of spermatozoa.  
'''Semination''' — act 'of fertilizing by discharge of spermatozoa.  


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


Sense Plate — narrow band of elevated ectodermal tissue which passes  
'''Sense Plate''' — narrow band of elevated ectodermal tissue which passes  
transversely across anterior end of amphibian embryo, ventral to level  
transversely across anterior end of amphibian embryo, ventral to level  
of fused neural folds, with ends of band bending dorsally to merge  
of fused neural folds, with ends of band bending dorsally to merge  
Line 1,723: Line 1,657:
ear.  
ear.  


Septum — partition.  
'''Septum''' — partition.  


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


Sertoli Cell — derivative of sexual cords of testis, found within seminiferous  
'''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  
tubule and functionally similar to follicle cell in ovary in that it is the  
nutritive, supporting, or nurse cell of the maturing spermatozoa. The  
nutritive, supporting, or nurse cell of the maturing spermatozoa. The  
Line 1,735: Line 1,669:
Sertoli cells.  
Sertoli cells.  


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


Sex Determination — See Determination of Sex.  
'''Sex Determination''' — See Determination of Sex.  


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


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


Sexual Cords of the Testis — sex cords of the originally indifferent gonad  
'''Sexual Cords of the Testis''' — sex cords of the originally indifferent gonad  
primordium which give rise to seminiferous tubules of testis, forming  
primordium which give rise to seminiferous tubules of testis, forming  
a rather solid mesenchymatous reticulum when cavities begin to appear  
a rather solid mesenchymatous reticulum when cavities begin to appear  
Line 1,753: Line 1,687:
the whole constituting seminiferous tubules.  
the whole constituting seminiferous tubules.  


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


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


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


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


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


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




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


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


Spawning — act of expelling eggs from uteri of anamniota (e.g., amphibia).  
'''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.)  
'''Sperm''' — germ cell characteristically produced by the male. (Syn., spermatozoon, sperm cell, male gamete, spermatosome.)  


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


Spermatogenesis — entire process which results in maturation of spermatozoon.  
'''Spermatogenesis''' — entire process which results in maturation of spermatozoon.  


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


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


Spermatosphere — See Idiosome.  
'''Spermatosphere''' — See Idiosome.  


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


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


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


Spindle — group of fibers between centrosomes during mitosis, to which  
'''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.  
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  
'''Spinous Process''' — prolongation of neural processes fused dorsally to neural  
canal; becomes dorsal spine of vertebra.  
canal; becomes dorsal spine of vertebra.  


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


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


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


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


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


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


Stomodeum ectodermal invagination (pit) which fuses with pharyngeal
'''Spongioblasts-''' cells of mantle layer of developing spinal cord destined to form merely supporting tissue.  
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.  
'''Stereoblastula''' solid blastula as found in Crepidula.  


Sub-Germinal Cavity See Blastocoel, Segmentation Cavity.  
'''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.  


Sub-Notochordal Rod or Bar hypochordal rod of amphibian embryo,
'''Stroma''' mesodermally derived, medullary, supporting tissues of an organ.  
found dorsal to midgut. Transitory.  


Sucker adhesive, connecting organ of oral region (larval stage).  
'''Sub-Germinal Cavity''' See Blastocoel, Segmentation Cavity.  


Sustentacular Cell cell which provides nourishment for another, such as
'''Sub-Notochordal Rod or Bar''' hypochordal rod of amphibian embryo, found dorsal to midgut. Transitory.  
Sertoli or follicle cells of gonads.  


Sylvius, Aqueduct of — See Aqueduct of Sylvius.  
'''Sucker''' — adhesive, connecting organ of oral region (larval stage).  


Sympathetic System originating either from mesenchymal element arising
'''Sustentacular Cell''' cell which provides nourishment for another, such as Sertoli or follicle cells of gonads.
in situ or, more probably, from ectodermal elements emanating from


'''Sylvius, Aqueduct of''' — See Aqueduct of Sylvius.


neural crests, to organize as a chain of ganglia near dorsal aorta and  
'''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.  
controlling involuntary (visceral) musculature.  


Synapsis — union, such as the lateral (parasynapsis) or terminal (telosynapsis) union of embryos; or pairing of homologous chromosomes.  
'''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.)  
'''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.  
'''Syncytium''' — nuclei and cytoplasm without cellular boundaries; multinucleate protoplasm without cell boundaries.  


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


Synizesis — stage in maturation between synaptene and pachytene when  
'''Synizesis''' — stage in maturation between synaptene and pachytene when  
chromatin threads are short and thick and ends away from centrosome  
chromatin threads are short and thick and ends away from centrosome  
are tangled.  
are tangled.  
Line 1,887: Line 1,806:
==T==
==T==


Telencephalon — portion of forebrain (ventricle) anterior to a plane which  
'''Telencephalon''' — portion of forebrain (ventricle) anterior to a plane which  
includes posterior side of choroid plexus and anterior side of optic  
includes posterior side of choroid plexus and anterior side of optic  
recess of 5 mm. frog embryo. Gives rise to torus transversus (anterior  
recess of 5 mm. frog embryo. Gives rise to torus transversus (anterior  
Line 1,894: Line 1,813:
Monro.  
Monro.  


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


Telocoel — cavity of telencephalon.  
'''Telocoel''' — cavity of telencephalon.  


Telolecithal — See Egg, telolecithal.  
'''Telolecithal''' — See Egg, telolecithal.  


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


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


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


Theca externa- — outermost of coverings of ovarian follicle, rather loose connective tissue with abundant blood supply. Continuous with peritoneum.  
'''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  
'''Theca interna''' — layer of connective tissue consisting of closely packed  
fibers, possibly some of smooth muscle, immediately external to egg.  
fibers, possibly some of smooth muscle, immediately external to egg.  
Consists of cyst wall.  
Consists of cyst wall.  


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


Thyroid (Body or Gland) — originates as an endodermal thickening in floor  
'''Thyroid (Body or Gland)''' — originates as an endodermal thickening in floor  
of pharynx between second pair of visceral arches; evaginates to form a  
of pharynx between second pair of visceral arches; evaginates to form a  
vesicle temporarily connected with gut by a duct; separates from gut;  
vesicle temporarily connected with gut by a duct; separates from gut;  
Line 1,931: Line 1,850:
Somewhat similar history in all vertebrate embryos. Endocrine function.  
Somewhat similar history in all vertebrate embryos. Endocrine function.  


Tissue Culture — in vitro culturing of isolated tissues; excision of tissues or  
'''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.  
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  
'''Tongue''' — solid mesodermal mass, covered with endoderm, derived by cell  
proliferation from floor of pharynx beginning in the 9 mm. frog tadpole.  
proliferation from floor of pharynx beginning in the 9 mm. frog tadpole.  


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


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


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


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


Tracheal Groove — Syn., laryngotracheal groove.  
'''Tracheal Groove''' — Syn., laryngotracheal groove.  


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


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




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


Trigeminal Ganglion — cranial (V) ganglia which consist of motor and  
'''Trigeminal Ganglion''' — cranial (V) ganglia which consist of motor and  
sensory portions and arise from segments of the most anterior crest  
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  
in conjunction with cells from inner (ganglionic) portion of corresponding placode. Give rise to ophthalmic, mandibular, and maxillary  
Line 1,969: Line 1,888:
of fourth ventricle.  
of fourth ventricle.  


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


Truncus Arteriosus — anterior continuation of buibus arteriosus beneath  
'''Truncus Arteriosus''' — anterior continuation of buibus arteriosus beneath  
foregut, divided in antero-posterior direction by a septum which is  
foregut, divided in antero-posterior direction by a septum which is  
continuous through buibus to ventricle; gives off external carotids to  
continuous through buibus to ventricle; gives off external carotids to  
Line 1,978: Line 1,897:
join dorsal aorta. (Syn., ventral aorta.)  
join dorsal aorta. (Syn., ventral aorta.)  


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


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


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


Tunica Albuginea — See Albuginea of Testis.  
'''Tunica Albuginea''' — See Albuginea of Testis.  


Tympanic Cavity — cavity of middle ear, a vestige of hyomandibular pouch. (See Tubo-tympanic Cavity.)  
'''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.)  
'''Tympanic Membrane''' — membrane made up of ectoderm, mesenchyme, and endoderm which separates tympanic cavity from exterior. (Syn., ear drum.)  


==U==
==U==


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


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


Urogenital Duct — ducts which open into cloaca of male amphibia and  
'''Urogenital Duct''' — ducts which open into cloaca of male amphibia and  
convey both excretory and genital products, derived from mesonephric (Wolffian) ducts.  
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  
'''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  
degrees of common origin and ultimate function. (See specific excretory  
and reproductive components.)  
and reproductive components.)  


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


Utricle— a vesicle, generally referring to superior portion of otocyst which  
'''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  
gives rise to the various semi-circular canals of the ear, and into which  
these canals open. Lined with ectoderm.  
these canals open. Lined with ectoderm.  
Line 2,019: Line 1,938:
==V==
==V==


Vasa Deferentia — mesonephric or Wolffian ducts of frog, which persist  
'''Vasa Deferentia''' — mesonephric or Wolffian ducts of frog, which persist  
as male gonoducts of bird and mammal, connecting with testes through  
as male gonoducts of bird and mammal, connecting with testes through  
vasa efferentia and epididymis and functioning as sperm ducts after  
vasa efferentia and epididymis and functioning as sperm ducts after  
Line 2,025: Line 1,944:
(Sing., vas deferens.)  
(Sing., vas deferens.)  


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


Vein — See under specific names.  
'''Vein''' — See under specific names.  


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


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


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


Ventral — belly surface. Ventrad means toward belly surface.  
'''Ventral''' — belly surface. Ventrad means toward belly surface.  


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


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


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


Ventricle, Lateral — See Lateral Ventricles of the Brain.  
'''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;  
'''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.  
connected with bulbus arteriosus anteriorly.  


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


Vertebral Arch — See Neural Arch.  
'''Vertebral Arch''' — See Neural Arch.  


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


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


Visceral — pertaining to viscera.  
'''Visceral''' — pertaining to viscera.  


Visceral Arches — mesodermal masses (usually six pairs) between visceral  
'''Visceral Arches''' — mesodermal masses (usually six pairs) between visceral  
pouches and lateral to pharynx of all vertebrate embryos, including  
pouches and lateral to pharynx of all vertebrate embryos, including  
mandibular, hyoid, and four branchial arches. Each arch is bounded  
mandibular, hyoid, and four branchial arches. Each arch is bounded  
Line 2,088: Line 2,007:
respectively; pharyngeal arch.)  
respectively; pharyngeal arch.)  


Visceral Clefts — slit-like openings between pharynx and outside, found in  
'''Visceral Clefts''' — slit-like openings between pharynx and outside, found in  
vertebrate embryos on either side of visceral arches II to V, or less,  
vertebrate embryos on either side of visceral arches II to V, or less,  
consisting of peripheral lining of ectoderm and mesial lining of  
consisting of peripheral lining of ectoderm and mesial lining of  
Line 2,094: Line 2,013:
clefts.)  
clefts.)  


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


Visceral Groove — See Visceral Furrow.  
'''Visceral Groove''' — See Visceral Furrow.  




Visceral Mesoderm — See Splanchnic Mesoderm, Splanchnopleure.  
'''Visceral Mesoderm''' — See Splanchnic Mesoderm, Splanchnopleure.  


Visceral Plexus — aggregation of sympathetic neurons which control viscera,  
'''Visceral Plexus''' — aggregation of sympathetic neurons which control viscera,  
having migrated posteriorly from tenth (vagus) cranial ganglia.  
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.  
'''Visceral Pouch''' — endodermal evagination of pharynx which, if it meets corresponding visceral furrow, often breaks through to form visceral cleft.  
(Syn., pharyngeal pouch.)  
(Syn., pharyngeal pouch.)  


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


Vitalism — a philosophical approach to biological phenomena which bases  
'''Vitalism''' — a philosophical approach to biological phenomena which bases  
its proof on present inability of scientists to explain all phenomena of  
its proof on present inability of scientists to explain all phenomena of  
development. Idea that biological activities are directed by forces  
development. Idea that biological activities are directed by forces  
Line 2,117: Line 2,036:
agency. (See Mechanism.)  
agency. (See Mechanism.)  


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


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


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


Vitelline Substance — yolk.  
'''Vitelline Substance''' — yolk.  


Vitelline Vein — paired veins, first to be formed in embryo, found in ventrolateral splanchnopleure, carrying nutritious blood from yolk region  
'''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  
to their junction with sinus venosus prior to the full development and  
function of heart.  
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,  
'''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.)  
hence ectodermal and probably also mesenchymal in origin. (See Aqueous Humor.)  


==W==
==W==
Wolffian Body — See Mesonephros.  
'''Wolffian Body''' — See Mesonephros.  


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


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


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






{{Rugh1951 footer}}
{{Rugh1951 footer}}

Revision as of 18:33, 5 April 2013

Embryology - 28 Mar 2024    Facebook link Pinterest link Twitter link  Expand to Translate  
Google Translate - select your language from the list shown below (this will open a new external page)

العربية | català | 中文 | 中國傳統的 | français | Deutsche | עִברִית | हिंदी | bahasa Indonesia | italiano | 日本語 | 한국어 | မြန်မာ | Pilipino | Polskie | português | ਪੰਜਾਬੀ ਦੇ | Română | русский | Español | Swahili | Svensk | ไทย | Türkçe | اردو | ייִדיש | Tiếng Việt    These external translations are automated and may not be accurate. (More? About Translations)

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

Frog Development (1951): 1 Introduction | 2 Rana pipiens | 3 Reproductive System | 4 Fertilization | 5 Cleavage | 6 Blastulation | 7 Gastrulation | 8 Neurulation | 9 Early Embryo Changes | 10 Later Embryo or Larva | 11 Ectodermal Derivatives | 12 Endodermal Derivatives | 13 Mesodermal Derivatives | 14 Summary of Organ Appearance | 15 Glossary | 16 Bibliography | Figures
Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
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 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 
Mark Hill.jpg
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)
Frog Development (1951): 1 Introduction | 2 Rana pipiens | 3 Reproductive System | 4 Fertilization | 5 Cleavage | 6 Blastulation | 7 Gastrulation | 8 Neurulation | 9 Early Embryo Changes | 10 Later Embryo or Larva | 11 Ectodermal Derivatives | 12 Endodermal Derivatives | 13 Mesodermal Derivatives | 14 Summary of Organ Appearance | 15 Glossary | 16 Bibliography | Figures

Reference

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


Cite this page: Hill, M.A. (2024, March 28) 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

What Links Here?
© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G