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==Glossary of Embryological Terms==
=Glossary of Frog Embryological Terms=


* A supplementary list of some 350 specialized terms may be found in the author's "Experimental Embryology, a Manual of Techniques and Procedures," Minneapolis, Minn., Burgess Publishing Co., 1948.
A 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.  
 
{{Rugh1951 header}}
 
==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==
==A==
 
; Acidophil
'''Acidophil''' — oxyphil: cell constituents which stain with acid dyes, often used to designate an entire cell type. (See Basophil.)  
: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.
 
;Adnexa
'''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.  
: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.
'''Adnexa''' — extra-embryonic structures (e.g., yolk sac) discarded before adult condition is attained.  
;Agglutination
 
:cluster formation; a spontaneously reversible reaction of spermatozoa to certain chemical situations (e.g., egg water).
 
;Aggregation
'''Aestivation''' — reduced activity of some animals during heat of summer. Opposed to hibernation.  
: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.
'''Agglutination''' — cluster formation; a spontaneously reversible reaction of spermatozoa to certain chemical situations (e.g., egg water).  
;Albumen
 
:protein substance secreted by walls of oviducts around egg of reptiles and birds.
 
;Albumen Sac
'''Aggregation''' — coming together of cells, such as spermatozoa, without sticking; a non-reversible response comparable to chemotropism.  
: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.
'''Albuginea of Testis''' — stroma of primitive testis which forms a layer between germinal epithelium and seminiferous tubules.  
;Amphiblastula
 
:double-structured blastula as in Porifera (sponges).
 
;Amphimixis
'''Albumen''' — protein substance secreted by walls of oviducts around egg of reptiles and birds.  
:mixing of germinal substances accomplished during fertilization.
 
;Amphitene
 
:one end of chromosome is thick, one end is thin, moving toward full pachytene during maturation.
'''Albumen Sac''' — 2-layered ectodermal sac enclosing albumen of chick egg during early development of embryo, separated for a time from yolk  
;Amplexus
by vitelline membrane; later to release some of its contents into amniotic cavity through ruptured sero-amniotic connection.  
: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.)
'''Amitosis''' — direct nuclear division without chromosomal rearrangements; generally thought to be a sign of decadence or of high specialization, if it occurs at all.  
;Analogy
 
:similarity of parts in respect to function rather than to structure.
 
;Anamniota
'''Amphiblastula''' — double-structured blastula as in Porifera (sponges).  
: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.
'''Amphimixis''' — mixing of germinal substances accomplished during fertilization.  
;Anastomosis
 
:joining together, as of blood vessels and nerves, generally forming a network.
 
;Androgen
'''Amphitene''' — one end of chromosome is thick, one end is thin, moving toward full pachytene during maturation.  
: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.
'''Amplexus''' — sexual embrace of female amphibian by male, a process which may (frogs and toads) or may not (urodeles) occur at time of oviposition.  
;Angioblast
 
:migratory mesenchyme cell associated with formation of vascular endothelium.
 
;Animal Pole
'''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.)  
: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.)
'''Analogy''' — similarity of parts in respect to function rather than to structure.  
;Anterior
 
:toward head; head end. (Syn., cephalic, cranial, rostral.)
 
;Anura
'''Anamniota''' — forms which never develop an amnion, e.g., cyclostomes, fishes, amphibia.  
:tailless amphibia (e.g., frogs and toads). (Syn., Salientia.)
 
;Aortic Arch
 
:blood vessel which connects dorsal and ventral aortae by way of visceral arch.
'''Anaphase''' — phase of mitosis when paired chromosomes are separating at equatorial plate and begin to move toward ends of spindle.  
;Aqueduct of Sylvius
 
:ventricle of mesencephalon (mesocoel) becomes aqueduct of Sylvius, connecting with cavities of optic lobes. (Syn., iter.)
 
;Aqueous Humor
'''Anastomosis''' — joining together, as of blood vessels and nerves, generally forming a network.  
: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.
'''Androgen''' — hormonal secretion of interstitial tissue of testis.  
;Archenteric Pouch
 
:See Enterocoel.
 
;Archenteron
'''Androgenesis''' — development of an egg with paternal (sperm) chromosomes only, accomplished by removing or destroying egg nucleus before syngamy.  
: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.
'''Angioblast''' — migratory mesenchyme cell associated with formation of vascular endothelium.  
;Asexual Reproduction
 
:reproduction without union of gametes; generally with no maturation divisions.
 
;Aster
'''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.)  
:"star-shaped structure" surrounding centrosome (Fol, 1877); lines radiating in all directions from centrosome during mitosis.
 
;Astral Rays
 
:lines which make up aster.
'''Anlage''' — rudiment; group of cells which indicate a prospective development into a part or an organ. (Syn., ebauche or primordium.)  
;Astrocytes
 
:stellate-shaped cells arising from spongioblasts of mantle layer, classified under the more general term of neuroglia.
 
;Atrium
'''Anterior''' — toward head; head end. (Syn., cephalic, cranial, rostral.)  
: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.)
'''Anura''' — tailless amphibia (e.g., frogs and toads). (Syn., Salientia.)  
;Attraction Sphere
 
:See Centrosphere.
 
;Auricles
'''Aortic '''Arch''' — blood vessel which connects dorsal and ventral aortae by way of visceral arch.  
:two upper chambers of adult frog's heart, derived from embryonic atria.
 
;Autogamy
 
:self-fertilization.
'''Aqueduct of Sylvius''' — ventricle of mesencephalon (mesocoel) becomes aqueduct of Sylvius, connecting with cavities of optic lobes. (Syn., iter.)  
;Autosome
 
:any chromosome except so-called sex (X or Y) chromosomes.
 
;Auxocyte
 
:pre-meiotic germ cell. (Syn., primary cyte, meiocyte.)
'''Aqueous Humor''' — fluid which fills anterior and posterior chambers of eye between lens, probably derived from mesoderm.  
;Axial Filament
 
:central fiber in tail of a spermatozoon.
 
;Axial Mesoderm
'''Archencephalon''' — pre-chordal brain, e.g., forebrain. Brain anterior to anterior end of notochord.  
:that portion of epimeric mesoderm nearest notochord. (Syn., vertebral plate.)
 
;Axis
 
:imaginary central or median line, generally correlated with a gradient.
'''Archenteric Pouch''' — See Enterocoel.  
;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
'''Archenteron''' — primitive gut found in gastrula and communicating with outside by blastopore; precursor of embryonic gut. (Syn., gastrocoel, enteron.)  
:imaginary line representing antero-posterior axis of the future embryo.
 
 
'''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==
==B==
 
; Balancers
'''Balancers''' — cylindrical and paired projections of ectoderm with mesenchymatous cores, used as adhesive organs in place of (anuran) suckers by many urodele amphibia.  
: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.
 
;Basophil
'''Basal Plate''' — ventro-lateral wall of myelencephalon, separated from dorsolateral alar plate by sulcus limitans.  
: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.
'''Basophil''' — cell constituents having an affinity for basic dyes, often used as an adjective for an entire cell. (See Acidophil.)  
;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
'''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.  
: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.)
'''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.  
;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
'''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.  
: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.
'''Blastocoel''' — cavity of blastula. (Syn., segmentation or subgerminal cavity.)  
;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
'''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).  
: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.)
'''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.  
;Blood Islands
 
:pre-vascular groups of mesodermal cells found in splanchnopleure, from which will arise blood vessels and corpuscles.
 
;Bowman's Capsule
'''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.  
:double-walled glomerular cup associated with uriniferous tubule.
 
;Branchial
 
:having to do with respiration. (Syn., gill.)
'''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.)  
;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
'''Blastopore, Ventral Lip of''' — region of blastopore opposite dorsal lip; region which gives rise to peristomial mesoderm of frog. (Syn., germ ring.)  
: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.)
'''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.)  
;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
'''Blood Islands''' — pre-vascular groups of mesodermal cells found in splanchnopleure, from which will arise blood vessels and corpuscles.  
: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.
'''Bowman's Capsule''' — double-walled glomerular cup associated with uriniferous tubule.  
;Bud
 
:undeveloped branch, generally an anlage of an appendage (e.g., limb or wing bud).
 
;Budding
'''Branchial''' — having to do with respiration. (Syn., gill.)  
: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.  
'''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==
==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 Lineage
'''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.  
: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 Lineage''' — study of origin and fate of specific blastomeres in embryonic development. (Syn., cytogeny.)  
;Central Canal
 
:See Neurocoel.
 
;Centriole
'''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).  
: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.
'''Central Canal''' — See Neurocoel.  
;Centrosphere
 
:rayed portion of centrosome; structure in spermatid which gives rise to acrosome. (Syn., spermatosphere, idiosome, attraction sphere. )
 
;Cephalic Flexure
'''Centriole''' — granular core of centrosome.  
: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.
'''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.  
;Choana
 
:openings of olfactory organ into pharynx, internal nares. Sometimes also used in connection with external olfactory opening.
 
;Chondrification
'''Centrosphere''' — rayed portion of centrosome; structure in spermatid which gives rise to acrosome. (Syn., spermatosphere, idiosome, attraction sphere. )  
: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.
'''Cephalic Flexure''' — ventral bending of embryonic head at level of midbrain and hindbrain.  
;Chondrocranium
 
:that portion of skull which is originally cartilaginous.
 
;Chorda Dorsalis
'''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.  
: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.
'''Choana''' — openings of olfactory organ into pharynx, internal nares. Sometimes also used in connection with external olfactory opening.  
;Choroid Coat
 
:mesenchymatous and sometimes pigmented coat within sclerotic coat but surrounding pigmented layer of eye in vertebrate embryos.
 
;Choroid Fissure
'''Chondrification''' — process of forming cartilage, by secretion of a homogeneous matrix between the more primitive cells.  
: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.
'''Chondrin''' — chemical substance in cartilage which makes it increasingly susceptible to basic stains.  
;Chromatid
 
:one of the parts of a tetrad (McClung, 1900); really a longitudinal half of a chromosome.
 
;Chromatin
'''Chondrocranium''' — that portion of skull which is originally cartilaginous.  
: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.
'''Chorda Dorsalis''' — Syn., notochord.  
;Chromidia
 
:granules within cytoplasm which stain like chromatin and which may actually be extruded chromatin granules.
 
;Chromomere
'''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.  
:unit of chromosome recognized as a chromatin granule.
 
;Chromonema
 
:slender thread of chromatin which is core of chromosome during mitosis.
'''Choroid Coat''' — mesenchymatous and sometimes pigmented coat within sclerotic coat but surrounding pigmented layer of eye in vertebrate  
;Chromophil
embryos.  
:cells which have an affinity for dyes.
 
;Chromophobe
 
:cells whose constituents are non-stainable; have no affinity for dyes.
'''Choroid Fissure''' — inverted groove in optic stalk whose lips later close around blood vessels and nerves that enter eyeball.  
;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
'''Choroid Knot''' — thickened region of fused lips of choroid fissure, near pupil, from which arise cells of iris.  
: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.
'''Chromatid''' — one of the parts of a tetrad (McClung, 1900); really a longitudinal half of a chromosome.  
;Cleavage, Asymmetrical
 
:extremely unequal divisions of egg as in Ctenophore.
 
;Cleavage, Bilateral
'''Chromatin''' — deeply staining substance of nuclear network and chromosomes, consisting of nuclein; gives Feulgen reaction and stains with basic dyes.  
: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.)
'''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.  
;Cleavage, Dexiotropic
 
:cleavage resulting in a right-handed production of daughter blastomere(s), as in spiral cleavage.
 
;Cleavage, Discoidal
'''Chromidia''' — granules within cytoplasm which stain like chromatin and which may actually be extruded chromatin granules.  
: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.)
'''Chromomere''' — unit of chromosome recognized as a chromatin granule.  
;Cleavage, Holoblastic
 
:complete division of egg into blastomeres, generally equal in size although not necessarily so (e.g., Amphioxus). (Syn., total cleavage.)
 
;Cleavage, Horizontal
'''Chromonema''' — slender thread of chromatin which is core of chromosome during mitosis.  
: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.)
'''Chromophil''' — cells which have an affinity for dyes.  
;Cleavage, Latitudinal
 
:See Cleavage, Equatorial.
 
;Cleavage Laws
'''Chromophobe''' — cells whose constituents are non-stainable; have no affinity for dyes.  
: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 spiralcleavage.
'''Chromosome''' — chromatic or deeply staining bodies derived from nuclear  
;Cleavage, Meridional
network and containing a matrix and one or more chromonemata during process of mitosis; bodies found in all somatic cells of normal  
:cleavage along egg axis, opposed to equatorial; generally the first two cleavages on any egg. (Syn., vertical cleavage.)
organism in a number characteristic of the species; bearers of gene.  
;Cleavage, Meroblastic
 
:cleavage restricted to peripherally located protoplasm, as in chick egg. (Syn., discoidal cleavage.)
 
;Cleavage Nucleus
'''Cleavage''' — mitotic division of egg resulting in blastomeres. (Syn., segmentation. )  
: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, Accessory''' — cleavage in peripheral or deeper portions of (chick)  
;Cleavage, Radial
germinal disc caused by supernumerary sperm nuclei following (normal) polyspermy, sometimes occurring in urodeles.  
: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, Asymmetrical''' — extremely unequal divisions of egg as in Ctenophore.  
;Cleavage, Superficial
 
:cleavage around periphery of centrolecithal eggs. (Syn., peripheral cleavage.)
 
;Cochlea
'''Cleavage, Bilateral''' — cleavage in which egg substances are distributed symmetrically with respect to median plane of future embryo.  
: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).
'''Cleavage, Determinate''' — cleavage in which certain parts of future embryo  
;Coelom
may be circumscribed in certain specific (early) blastomeres; cleavage  
: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.)
which produces blastomeres that are not qualitatively equipotential,  
;Coitus
i.e., when such blastomeres are isolated they will not give rise to  
:copulation of male and female, term generally used in connection with mammals. Comparable situation in amphibia is called amplexus.
entire embryos. (Syn., mosaic development.)  
;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
'''Cleavage, Dexiotropic''' — cleavage resulting in a right-handed production  
:dispersed substance whose particles are not smaller than 1 ^ and not larger than 100 jx, approximately. Physical state of protoplasm.
of daughter blastomere(s), as in spiral cleavage.  
;Columella
 
:bone in tubo-tympanic cavity of frog which aids in auditory sensations. (Syn., plectrum, malleus.)
 
;Competence
'''Cleavage, Discoidal''' — See Cleavage, Meroblastic.  
: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.)
'''Cleavage, Equatorial''' — cleavage at right angles to egg axis, opposed to  
;Cone, Fertilization
vertical or meridional; often the typical third cleavage plane. (Syn.,  
: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.)
latitudinal or horizontal cleavage.)  
;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
'''Cleavage, Holoblastic''' — complete division of egg into blastomeres, generally equal in size although not necessarily so (e.g., Amphioxus).  
:similar to concrescence except that this term refers specifically to "flow" of cells (or areas) together, whether or not they are piled up.
(Syn., total cleavage.)  
;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
'''Cleavage, Horizontal''' — See Cleavage, Equatorial.  
: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.
'''Cleavage, Indeterminate''' — cleavage resulting in qualitatively equipotential  
;Cords, Medullary
blastomeres in early stages of development. When such blastomeres  
:structures which give rise to urogenital connections and take part in formation of seminiferous tubules, and are derived from blastema of mesonephric cords.
are isolated from each other they give rise to complete embryos. Opposed to mosaic development. (Syn., regulatory 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
'''Cleavage, Latitudinal''' — See Cleavage, Equatorial.  
: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.)
'''Cleavage Laws''' — See specific laws under names of Balfour, Hertwig, and  
;Cranial
Sachs.  
: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.)
'''Cleavage, Levotropic''' — cleavage resulting in left-handed or counterclockwise production of daughter blastomere(s) as in some cases of spiral
;Crescent, Gray
cleavage.  
: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.
'''Cleavage, Meridional''' — cleavage along egg axis, opposed to equatorial;  
;Crest Segment
generally the first two cleavages on any egg. (Syn., vertical cleavage.)  
: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.
'''Cleavage, Meroblastic''' — cleavage restricted to peripherally located protoplasm, as in chick egg. (Syn., discoidal cleavage.)  
;Crossing Over
 
:mutual exchange of portions of allelomorphic pairs of chromosomes during process of synapsis in maturation.
 
;Cyclopia
'''Cleavage Nucleus''' — nucleus which controls cleavage. This may be syngamic nucleus of normal fertilization, egg nucleus of parthenogenetic  
: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.
or gynogenetic eggs, or sperm nucleus of androgenetic eggs.  
;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
'''Cleavage Path''' — path taken by syngamic nuclei to position awaiting first  
:narrow, proximal portion of embryonic bile duct leading from gallbladder to common bile duct.
division.  
;Cytasters
 
:asters arising apart from nucleus in cytoplasm.
 
;Cyte
'''Cleavage, Radial''' — holoblastic cleavage which results in tiers of cells.  
:suffix meaning cell (e.g., osteocyte for bone cell, oocyte for egg cell). (See specific definitions.)
 
;Cytology
 
:study 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).  
;Cytolysis
 
:breakdown of cell indicated by dispersal of formed components.
 
;Cytoplasm
'''Cleavage, Superficial''' — cleavage around periphery of centrolecithal eggs. (Syn., peripheral cleavage.)  
:material of cell exclusive of nucleus; protoplasm apart from nucleoplasm.
 
 
'''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==
==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.


'''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==
==E==
 
; Ecdysis
 
:process of molting a cuticular layer, shedding of epithelium.
'''Ecdysis''' — process of molting a cuticular layer, shedding of epithelium.  
;Ectoblast
 
:See Epiblast.
 
;Ectoderm
'''Ectoblast''' — See Epiblast.  
:outermost layer of didermic gastrula. (Syn., epiblast.)
 
;Ectoplasm
 
:external layer of protoplasm of egg cell; layer immediately beneath cell membrane. (Syn., egg cortex.)
'''Ectoderm''' — outermost layer of didermic gastrula. (Syn., epiblast.)  
;Edema
 
:condition in which tissues hold an excess of water, common in parthenogenetic tadpoles. (Older spelling: oedema.)
 
;Egg, Alecithal
'''Ectoplasm''' — external layer of protoplasm of egg cell; layer immediately  
:eggs with little or no yolk. Literally means "without yolk."
beneath cell membrane. (Syn., egg cortex.)  
;Egg, Cleidoic
 
:eggs, such as those of reptiles, birds, and oviparous mammals, which are covered by a protective shell.
 
;Egg, Ectolecithal
'''Edema''' — condition in which tissues hold an excess of water, common in  
:egg having yolk around formative protoplasm. Opposed to centrolecithal.
parthenogenetic tadpoles. (Older spelling: oedema.)  
;Egg Envelope
 
:material enveloping egg but not necessarily a part of the egg, such as vitelline membrane, chorion, jelly, albumen.
 
;Egg, Giant
'''Egg, Alecithal''' — eggs with little or no yolk. Literally means "without yolk."  
: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, Cleidoic''' — eggs, such as those of reptiles, birds, and oviparous mammals, which are covered by a protective shell.  
;Egg, Isolecithal
 
:eggs with homogeneous distribution of yolk; may be isolecithal, alecithal, or homolecithal.
 
;Egg Jelly
'''Egg, Ectolecithal''' — egg having yolk around formative protoplasm. Opposed to centrolecithal.  
:mucin covering deposited on amphibian egg as it passes through oviduct.
 
;Egg, Macrolecithal
 
:egg with large amount of yolk, generally telolecithal.
'''Egg Envelope''' — material enveloping egg but not necessarily a part of the  
;Egg Membranes
egg, such as vitelline membrane, chorion, jelly, albumen.  
: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
'''Egg, Giant''' — abnormal polyploid condition where chromosome complexes  
:part of Lillie's scheme picturing parts that go into the fertilization reaction involving fertilizin. Egg receptor plus amboceptor plus sperm receptor gives fertilization.
are multiplied, resulting in giant cells and embryos.  
;Egg, Telolecithal
 
:egg with large amount of yolk concentrated at one pole.
 
;Egg Water
'''Egg, Homolecithal''' — egg (e.g., mammal) in which but little yolk is scattered throughout cytoplasm.  
: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.
'''Egg, Isolecithal''' — eggs with homogeneous distribution of yolk; may be isolecithal, alecithal, or homolecithal.  
;Ejaculatory Duct
 
:short portion of mesonephric duct (mammal) between seminal vesicles and urethra.
 
;Emboitement
'''Egg Jelly''' — mucin covering deposited on amphibian egg as it passes  
: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.)
through oviduct.  
;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
'''Egg, Macrolecithal''' — egg with large amount of yolk, generally telolecithal.  
:delicate endothelial tissue forming lining of heart.
 
;Endochondral Bone
 
:bone preformed in cartilage. (Syn., cartilage bone.)
'''Egg Membranes''' — include all egg coverings such as vitelline membrane,  
;Endoderm
chorion, and tertiary membranes.  
:innermost layer of didermic gastmla. (Syn., entoderm.)
 
;Endolymphatic Duct
 
:See Ductus Endolymphaticus.
'''Egg, Microlecithal''' — egg with small amount of yolk. (Syn., meiolecithal  
;Endolymphatic Sac
egg, oligolecithal egg.)  
:See Saccus Endolymphaticus.
 
;Endoplasm
 
:inner medullary substance of (egg) cell which is generally granular, soft, watery, and less refractive than ectoplasm.
'''Egg Receptor''' — part of Lillie's scheme picturing parts that go into the  
;Entelchy
fertilization reaction involving fertilizin. Egg receptor plus amboceptor plus sperm receptor gives fertilization.  
: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.)
'''Egg, Telolecithal''' — egg with large amount of yolk concentrated at one pole.  
;Enteron
 
:definitive gut of embryo, always lined with endoderm.
 
;Ento-mesoderm
'''Egg Water''' — watery extract of materials diffusing from living eggs, presumably the "fertilizin" of Lillie. (Syn., egg water extract.)  
: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.
'''Ejaculation''' — forcible emission of mature spermatozoa from body of male.  
;Entrance Path
 
:See Path, penetration.
 
;Ependymal Cells
'''Ejaculatory Duct''' — short portion of mesonephric duct (mammal) between  
: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.
seminal vesicles and urethra.  
;Epiblast
 
:outermost layer of early embryo from which the various germ layers may be derived.
 
;Epiboly
'''Emboitement''' — preformationist theory of Bonnet and others based on  
: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.
idea that ovary of first female (Eve?) contained the miniatures of all  
;Epibranchial Placode
subsequently existing human beings. (Syn., encasement theory.)  
: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.
'''Embryo''' — any stage in ontogeny of fertilized egg, generally limited to period prior to independent food-getting. Stage between second week  
;Epigenesis
and second month of human embryo.  
: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.)
'''Endocardium''' — delicate endothelial tissue forming lining of heart.  
;Epiphysis
 
:evagination of anterior diencephalon of vertebrates which becomes separated from brain as pineal (endocrine) gland of adult.
 
;Epithelioid Bodies
'''Endochondral Bone''' — bone preformed in cartilage. (Syn., cartilage bone.)  
:endodermal masses arising from second and third visceral pouches of amphibia.
 
;Epithelium
 
:thin covering layer of cells; may be ectodermal, endodermal, or mesodermal.
'''Endoderm''' — innermost layer of didermic gastmla. (Syn., entoderm.)  
;Equational Maturation Division
 
:maturational divisions in which there is no (qualitative) reduction in chromosomal complex, similar in results to mitosis.
 
;Equatorial Plate
'''Endolymphatic Duct''' — See Ductus Endolymphaticus.  
: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.
'''Endolymphatic Sac''' — See Saccus Endolymphaticus.  
;Evagination
 
:growth from any surface outward.
 
;"Ex Ovo Omnia"
'''Endoplasm''' — inner medullary substance of (egg) cell which is generally  
:all life comes from the egg (Harvey, 1657).
granular, soft, watery, and less refractive than ectoplasm.  
;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.
'''Entelchy''' — Driesch's theory of an (intangible) agent controlling development. (Syn., elan vital.)  
;External Gills
 
:outgrowths of (amphibian) branchial arches which function as temporary (anura) or permanent neotonic (urodela) respiratory organs.
 
;Extra-Embryonic
'''Enterocoel''' — cavity or pouch within mesoderm just formed by evagination  
:refers to structures apart from embryonic body, such as membranes.
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==
==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 Map''' — map of blastula or early gastrula stage which indicates prospective significance of various surface areas, based upon previously  
;Fate, Prospective
established studies of normal development aided by means of vital  
: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.
dye markings.  
;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).
'''Fate, Prospective''' — destination toward which we know, from previous experience, that a given part would develop under normal conditions;  
;Feulgen Reaction
lineage of each part of egg through its cell descendants into a definite  
:chemical test for thymo-nucleic acid, used as a specific staining test for chromatin.
region or portion of adult organism.  
;Field
 
:mosaic of spatio-temporal activities within developing organism.
 
;Field, Morphogenetic
'''Fertilization''' — activation of egg by sperm and syngamy of pronuclei; union  
:embryonic field out of which will normally develop certain specific structures.
of male and female gamete nuclei.  
;Flexure
 
:refers to a bending such as cranial, cervical, and pontine flexures. Also dorsal and lumbo-sacral flexures of the pig.
 
;Follicle
'''Fertilization, Anti "eggs contain within their interior a substance capable of combining with the agglutinating group of the fertilizin, but  
:cellular sac within which egg generally goes through early maturation stages.
which is separate from it as long as the egg is inactive" (Lillie).  
;Forebrain
 
:most anterior of first three primary brain vesicles, associated with lateral opticoels. (Syn., prosencephalon.)
 
;Foregut
'''Feulgen Reaction''' — chemical test for thymo-nucleic acid, used as a specific  
:more anterior portion of enteric canal, first to appear, aided by development of pharyngeal derivatives.
staining test for chromatin.  
;Fovea Germinativa
 
:pigment-free spot of animal hemisphere where amphibian germinal vesicle gives off its polar bodies.
 
;Frontal
'''Field''' — mosaic of spatio-temporal activities within developing organism.  
:plane at right angles to both transverse and sagittal, dividing dorsal from ventral. (Syn., coronal.)  
 
 
 
'''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==
==G==
 
; Gamete
'''Gamete''' — differentiated (matured) germ cell, capable of functioning in  
: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.
 
;Ganglion
'''Gametogenesis''' — process of developing and maturing germ cells.  
: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''' — aggregation of neurons, generally derived from a neural crest  
;Ganglion, Acustico-facialis
(e.g., cranial and spinal ganglia).  
: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, Acoustic''' — eighth cranial ganglion from which fibers of eighth  
;Ganglion, Geniculate
cranial nerve arise, purely sensory.  
: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, Acustico-facialis''' — early undifferentiated association of seventh  
;Ganglion, Petrosal
and eighth cranial ganglia.  
: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.
'''Ganglion, Gasserian''' — fifth cranial ganglion, carrying both sensory and  
;Gasserian Ganglion
motor fibers. (Syn., trigeminal ganglion, semilunar 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.
'''Ganglion, Geniculate''' — ganglion at root of facial (VII) cranial nerve,  
;Gastral Mesoderm
carrying both sensory and motor fibers.  
: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.)
'''Ganglion, Nodosal''' — ganglion associated with vagus (X) cranial nerve  
;Gastrula
which carries afferent fibers to pharynx, larynx, trachea, oesophagus,  
:didermic embryo, possessing a newly formed cavity, gastrocoel or archenteron. The two layers are ectoderm and endoderm.
and thoracic and abdominal viscera.  
;Gastrular Cleavage
 
:separation of ectoderm and endoderm, during gastrulation, by a slit-like crevice, actually compressed blastocoel.
 
;Gastrulation
'''Ganglion, Petrosal''' — ganglion associated with glossopharyngeal (IX) cranial nerve, more peripheral than superior ganglion carrying sensory  
: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.
fibers from pharynx and root of tongue.  
;Genital
 
:refers to reproductive organs or processes, or both.
 
;Genital Ducts
'''Ganglion, Superior''' — ganglion associated with glossopharyngeal (X) cranial nerve, mesial to petrosal ganglion.  
:any ducts which convey gametes from their point of origin to region of insemination (e.g., collecting tubules, vas deferens, vas efferens, seminal vesicle, oviduct, uterus, etc.).
 
;Genital Ridge
 
:initial elevation or thickening for development of external genitalia.
 
;Germ
'''Gasserian Ganglion— fifth cranial or trigeminal ganglion, derived from  
:egg throughout its development, or at any stage.
midbrain.  
;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
'''Gastraea Theory''' — theory of Haeckel that since all higher forms have gastrula stages there may have existed a common ancestor built on the  
: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.
plan of a permanent gastrula, as are the recent Coeloenterata.  
;Germ Plasm
 
:hereditary material, generally referring specifically to the genotype. Opposed to somatoplasm.
 
;Germ Ring
'''Gastral Mesoderm''' — mesoderm derived from dorso-lateral bands (enterocoelic) in Amphioxus or from dorsal lip in frog. Opposed to peristomial mesoderm.  
: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.)
'''Gastrocoel''' — cavity formed during process of gastrulation. (Syn., archenteron.)  
;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
'''Gastrula''' — didermic embryo, possessing a newly formed cavity, gastrocoel  
:nucleolus of ovum.
or archenteron. The two layers are ectoderm and endoderm.  
;Germinal Vesicle
 
:pre-maturation nucleus of egg.
 
;Gestalten
'''Gastrular Cleavage''' — separation of ectoderm and endoderm, during gastrulation, by a slit-like crevice, actually compressed blastocoel.  
: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.
'''Gastrulation''' — dynamic process involving cell movements which change  
;Gill Plate
embryo from a monodermic to either a di- or tridermic form. Generally involves inward movement of cells to form enteric endoderm.  
:elevated and thickened areas of ectoderm posterior to sense plate of embryo where visceral grooves will subsequently form.
Description includes epiboly, concrescence, confluence, involution,  
;Gill Rakers
invagination, extension, and convergence.  
: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.
'''Genital''' — refers to reproductive organs or processes, or both.  
;Glomerulus
 
:aggregation of capillaries associated with branches of dorsal aorta but lying within substance of functional kidney; function is excretory.
 
;Glomus
'''Genital Ducts''' — any ducts which convey gametes from their point of origin  
:vascular aggregations within head kidney or pronephros, never to become a glomerulus.
to region of insemination (e.g., collecting tubules, vas deferens, vas  
;Glottis
efTerens, seminal vesicle, oviduct, uterus, etc.).  
: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.)
'''Genital Ridge''' — initial elevation or thickening for development of external  
;Gonadromorph
genitalia.  
: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).
'''Germ''' — egg throughout its development, or at any stage.  
;Gonoduct
 
:See Genital Ducts.
 
;Gradient
'''Germ Cell''' — cell capable of sharing in reproductive process, in contrast  
:gradual variation of developmental forces along an axis; scaled regions of preference. (See Axis.)
with a somatic cell (e.g., sperm or egg cell). (Syn., gamete.)  
;Gray Crescent
 
:See Crescent, Gray.
 
;Growth
'''Germ Layer''' — more or less artificial spatial and histogenic distinction of  
:developmental increase in total mass of protoplasm at expense of raw materials; an embryonic process, generally differentiation; cell proliferation.
cell groups beginning in gastrula stage, consisting of ectodermal,  
;Gynogenesis
endodermal, and mesodermal layers. No permanent and clear-cut distinction, as shown by transplantation experiments.  
:development of sperm activated egg but without benefit of sperm nucleus.
 
 
'''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==
==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 are equally ready to respond to organism as a whole. Isolated blastomeres of such a system may give rise to complete embryos.
 
;Hatching
'''Harmonious-Equipotential System''' — embryonic system in which all parts  
:beginning of larval life of amphibian, accomplished by temporarily secreted hatching enzymes which aid embryo to escape from its gelatinous capsule.
are equally ready to respond to organism as a whole. Isolated blastomeres of such a system may give rise to complete embryos.  
;Hepatic Sinusoids
 
:maze of dilated and irregular capillaries between loosely packed framework of hepatic tubules.
 
;Hepatic Veins
'''Hatching''' — beginning of larval life of amphibian, accomplished by temporarily secreted hatching enzymes which aid embryo to escape from  
:veins from liver to heart, originating as anterior portions of vitelline veins of amphibia.
its gelatinous capsule.  
;Hepatic Veins, Portal
 
:remnants of posterior portions of left vitelline vein.
 
;Hermaphrodite
'''Hepatic Sinusoids''' — maze of dilated and irregular capillaries between loosely  
:individual capable of producing both spermatozoa and ova.
packed framework of hepatic tubules.  
;Hermaphrodite, Protandrous
 
:male elements mature prior to female elements in hermaphrodite.
 
;Hermaphrodite, Protogynous
'''Hepatic Veins''' — veins from liver to heart, originating as anterior portions  
:female elements mature prior to male elements in hermaphrodite.
of vitelline veins of amphibia.  
;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
'''Hepatic Veins, Portal''' — remnants of posterior portions of left vitelline vein.  
: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.
'''Hermaphrodite''' — individual capable of producing both spermatozoa and  
;Hibernation
ova.  
:spending the cold (winter) period in a state of reduced activity.
 
;Hindbrain
 
:most posterior of the three original brain divisions. (Syn., rhombencephalon. )
'''Hermaphrodite, Protandrous''' — male elements mature prior to female elements in hermaphrodite.  
;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
'''Hermaphrodite, Protogynous''' — female elements mature prior to male elements in hermaphrodite.  
: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.
'''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  
;Homology
axis of yolk-free cytoplasm of cell.  
: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.)
'''Heteroagglutinin''' — agglutinin (fertilizin) of eggs which acts on sperm of  
;Homozygous
different species, substance extractable from egg water which causes  
:condition in which zygote is composed of gametes bearing identical rather than allelomorphic genes.
irreversible agglutination of foreign species.  
;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.
'''Heterozygous''' — condition in which zygote is composed of gametes bearing  
;Hyaloplasm
allelomorphic genes. Opposed to homozygous.  
:viscid liquid regarded as essential living protoplasm.
 
;Hybrid
 
:successful cross between different species (e.g., horse and ass give a mule, which is sterile).
'''Hibernation''' — spending the cold (winter) period in a state of reduced activity.  
;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
'''Hindbrain''' — most posterior of the three original brain divisions. (Syn.,  
:pertaining to pouch, cleft, or slit between mandibular and hyoid arches.
rhombencephalon. )  
;Hyperplasia
 
:overgrowth; abnormal or unusual increase in elements composing a part.
 
;Hypertrophy
'''Hindgut''' — portion of amphibian embryonic gut just anterior to neurenteric  
:increase in size due to increase in demands upon part concerned.
canal. Level of origin of rectum, cloaca, post-anal gut, and caudal  
;Hypochordal Rod
portions of urogenital systems.  
: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., sub-notochordal rod.)
 
;Hypomere
 
:most ventral segment of mesoderm out of which develop somatopleure, splanchnopleure, and coelom. (Syn., lateral plate mesoderm.)
'''Histogenesis''' — development of tissues.  
;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
'''Homoiothermal''' — pertaining to a condition in which temperature of body  
:undergrowth or deficiency in elements composing a part.
of organism is under control of an internal mechanism; body temperature regulated. Opposed to poikilothermal.  
;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.
 
'''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==
==I==
 
; Idiosome
'''Idiosome''' — material out of which acrosome is formed during metamorphosis  
: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 morphogenetic changes within embryo.
 
;Inductor
'''Induction''' — successive and purposeful influences which bring about  
: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.
morphogenetic changes within embryo.  
;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
'''Inductor''' — a loose word which includes both organizer and evocator (Needham). Generally means a piece of living tissue which brings about  
:See Ostium Abdominale Tubae.
differentiation within otherwise indifferent tissue.  
;Ingression
 
:inward movement of yolk endoderm of amphibian blastula (Nicholas, 1945).
 
;Insemination
'''Infundibulum of the Brain''' — funnel-like evagination of floor of diencephalon  
:process of impregnation; fertilization.
which, along with hypophysis, will give rise to pituitary gland of adult.  
;Interauricular Septum
 
:longitudinal sheet of mesodermal tissue which grows ventrally from roof of atrial chamber to divide it into right and left halves.
 
;Interkinesis
'''Infundibulum of the Oviduct''' — See Ostium Abdominale Tubae.  
: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.)
'''Ingression''' — inward movement of yolk endoderm of amphibian blastula  
;Internal Gills
(Nicholas, 1945).  
: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.
'''Insemination''' — process of impregnation; fertilization.  
;Intersex
 
:individual without typical sexual differentiation.
 
;Interstitial Cells
'''Interauricular Septum''' — longitudinal sheet of mesodermal tissue which  
:specialized cells between seminiferous tubules of testis which produce hormones.
grows ventrally from roof of atrial chamber to divide it into right and  
;Interstitial Tissue of Testis
left halves.  
: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.
'''Interkinesis''' — resting stage between mitotic divisions.  
;Involution
 
:rolling inward or turning in of cells over a rim, as in gastrulation of chick embryo.
 
;Iris
'''Intermediate Cell Mass''' — narrow strip of mesoderm which, for a time, joins  
: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.
dorsal epimere with ventral hypomere, being made up of a dorsal  
;Isogamy
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.,  
:similar gametes, without differentiations into spermatozoa and ova.
nephrotome or middle plate.)  
;Isolation Culture
 
:removal of a part of an organism and its maintenance in a suitable medium in living condition.
 
;Isthmus of the Brain
'''Internal Gills''' — filamentous outgrowths on posterior side of first three pairs  
:depression in dorsal wall of embryonic brain which partially separates mesencephalon from metencephalon.
of branchial arches and a single row on anterior side of fourth pair  
;Isthmus of the Oviduct
of branchial arches of frog tadpole, which have a respiratory function  
:short, tubular, posterior end of oviduct (e.g., chick) in which fluid albumen and shell membranes are applied to egg.
concurrent with and following absorption of external gills.  
;Iter
 
:See Aqueduct of Sylvius.
 
'''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==
==J==
 
; Jacobson's Organ
'''Jacobson's Organ''' — ventro-medial evaginations from olfactory pits (amphibia and reptilia) which later become glandular and sensitive olfactory epithelia.  
: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.
'''Jelly''' — mucin covering of amphibian egg derived from oviduct and applied  
;Jugular Veins
outside vitelline membrane.  
: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.  
 
 
 
'''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==
==K==
 
; Karyoplasm
'''Karyoplasm''' — protoplasm within confines of nucleus.  
: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 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.


 
;
'''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==
==M==
 
; Macromere
'''Macromere''' — ^larger of the blastomeres where there is a conspicuous size  
:larger of the blastomeres where there is a conspicuous size difference.
difference.  
;Malpighian Body
 
:unit of functional kidney including Bowman's capsule and glomerulus. (Syn., renal corpuscle, Malpighian corpuscle.)
 
;Mandibular Arch
'''Malpighian Body''' — unit of functional kidney including Bowman's capsule  
:rudiment of lower jaw, mesodermal, and anterior to first or hyomandibular pouch.
and glomerulus. (Syn., renal corpuscle, Malpighian corpuscle.)  
;Mantle Fibers
 
:those fibers of mitotic spindle which attach chromosomes to centrosomes.
 
;Mantle Layer of the Cord
'''Mandibular Arch''' — rudiment of lower jaw, mesodermal, and anterior to  
:zone of developing spinal cord with densely packed nuclei slightly peripheral to germinal cells from which they are derived. Includes elongated cells of ependyma.
first or hyomandibular pouch.  
;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
'''Mantle Fibers''' — those fibers of mitotic spindle which attach chromosomes  
: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.)
to centrosomes.  
;Meckel's Cartilage
 
:core of lower jaw derived from ventral part of cartilaginous mandibular arch.
 
;Median plane
'''Mantle Layer of the Cord''' — zone of developing spinal cord with densely  
:"middle" plane, as of an embryo. May be median sagittal or median frontal.
packed nuclei slightly peripheral to germinal cells from which they  
;Medulla Oblongata
are derived. Includes elongated cells of ependyma.  
: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.)
'''Maturation''' — process of transformation of a primordial germ cell (spermatogonium or oogonium) into a functionally mature germ cell, the  
;Medullary
process involving two special divisions, one of which is always meiotic.  
:See terms under Neural, such as canal, fold, groove, plate, tube.
Divisions known as equational and reductional.  
;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
'''Mechanism''' — assumption that biological processes do not violate physical  
:process of nuclear division found in maturation of germ cells, involving a separation of members of pairs of chromosomes. (Syn., reductional division.)
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  
;Melanophore
scientific attitude.) (See Vitalism.)  
: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.
'''Meckel's Cartilage''' — core of lower jaw derived from ventral part of cartilaginous mandibular arch.  
;Membrane, Vitelline
 
:See Vitelline Membrane.
 
;Membranes
'''Median plane''' — "middle" plane, as of an embryo. May be median sagittal  
:See Egg Membranes.
or median frontal.  
;Meroblastic Cleavage or Ova
 
:See under Cleavage or Egg.
 
;Mesencephalon
'''Medulla Oblongata''' — that portion of adult brain derived from rhombencephalon.  
: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.
'''Medullarin''' — sex differentiating substance spread in some amphibia by blood stream as a hormone and in other forms by diffusion. (See Corticin.)  
;Mesendoderm
 
:newly formed layer of (urodele) gastmla before there has been any separation of endoderm and mesoderm. (Syn., mesentoderm, mesentoblast, ento-mesoblast.)
 
;Mesentery
'''Medullary''' — See terms under Neural, such as canal, fold, groove, plate, tube.  
:sheet of (mesodermal) tissue generally supporting organ systems (e.g., mesorchium, mesocardium).
 
;Mesial
 
:median, medial, middle.
'''Medullary Cords''' — that portion of suprarenal glands derived from sympathetic nervous system; central cords. Also that portion of embryonic  
;Mesoblast, Gastral
gonad presumably derived from pre-migratory germ cells upon reaching genital ridge.  
:See Gastral Mesoderm.
 
;Mesoblast, Peristomial
 
:involuted, ventral lip mesoderm, continuous with gastral mesoderm from dorsal lip.
'''Meiosis''' — process of nuclear division found in maturation of germ cells, involving a separation of members of pairs of chromosomes. (Syn.,  
;Mesocardium
reductional division.)  
: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
'''Melanophore''' — well with black or brown pigment (melanin), derived from neural crests and migrating throughout body.  
: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.)
'''Membrane Bone''' — bone developed in regions occupied by connective tissue, not cartilage.  
;Mesonephric Tubules
 
:primary, secondary, and sometimes tertiary tubules developing in Wolffian body, functioning in adult amphibia.
 
;Mesonephros
'''Membrane, Vitelline''' — See Vitelline Membrane.  
:Wolffian body, or intermediate kidney, functional as kidney in adult fish and amphibian.
 
;Mesorchium
 
:mesentery (mesodermal) which surrounds and supports testis to body wall.
'''Membranes''' — See Egg Membranes.  
;Mesothelium
 
:epithelial layers or membranes of mesodermal origin.
 
;Mesovarium
'''Meroblastic Cleavage or Ova''' — See under Cleavage or Egg.  
:mesentery (mesodermal) which suspends ovary from dorsal body wall.
 
;Metamerism
 
:serial segmentation, as seen in nervous, muscular, and circulatory systems.
'''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.)  
;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
'''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.  
: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.
'''Mesendoderm''' — newly formed layer of (urodele) gastmla before there has been any separation of endoderm and mesoderm. (Syn., mesentoderm, mesentoblast, ento-mesoblast.)  
;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
'''Mesentery''' — sheet of (mesodermal) tissue generally supporting organ systems (e.g., mesorchium, mesocardium).  
:See Mesencephalon.
 
;Midgut
 
:that portion of archenteron which will give rise to intestines.
'''Mesial''' — median, medial, middle.  
;Milieu
 
:Term used to include all of the physico-chemical and biological factors surrounding a living system (e.g., external or internal milieu).
 
;Mitochondria
'''Mesoblast, Gastral''' — See Gastral Mesoderm.  
: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.
'''Mesoblast, Peristomial''' — involuted, ventral lip mesoderm, continuous with gastral mesoderm from dorsal lip.  
;Mitotic Index
 
:proportion in any tissue and at any specified time of the dividing cells; percentage of actively dividing cells.
 
;Monospermy
'''Mesocardium''' — mesentery of heart; may be dorsal, ventral, or lateral. (See  
:fertilization accomplished by only one sperm. Opposed to polyspermy.
under Lateral Mesocardium.)  
;Monro, Foramina of
 
:tubular connections between single third and paired lateral ventricles of forebrain.
Mesoderm''' — the third primary germ layer developed in point of time, may  
;Morphogenesis
be derived from endoderm in some forms and from ectoderm in  
: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.
others. (See other terms such as Mesoblast, Mesenchyme, Lateral  
;Morphogenetic Movements
Plate Mesoderm, Epimere, Mesomere, Hypomere, Gastral Mesoderm, Peristomial Mesoderm, Axial Mesoderm, etc.)  
: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.
'''Mesomere''' — cell of intermediate size where there are conspicuous size differences in an early embryo; also refers to intermediate cell mass:  
;Mosaic
intermediate mesoderm.  
: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.
'''Mesonephric Duct''' — duct which grows posteriorly from mesonephros to  
;Muscle Plate
cloaca and functions also as vas deferens in male. (Syn., Wolffian  
:See Myotome.
duct.)  
;Myeloblasts
 
:muscle-forming (embryonic) cells.
 
;Myoblasts— formative cells within myotome or muscle plate which will give rise to true striated muscles of adult.
'''Mesonephric Tubules''' — primary, secondary, and sometimes tertiary tubules  
;Myocardium
developing in Wolffian body, functioning in adult amphibia.  
: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.
'''Mesonephros''' — Wolffian body, or intermediate kidney, functional as kidney  
;Myotome
in adult fish and amphibian.  
:thickened primordium of muscle found in each somite, (Syn., muscle plate.)  
 
 
'''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==
==N==
 
; Nares, External
 
:external openings of tubes which are connected with olfactory vesicles.
'''Nares, External''' — external openings of tubes which are connected with  
;Nares, Internal
olfactory vesicles.  
: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.
'''Nares, Internal''' — openings of tubular organ from olfactory placodes into  
;Nasal Pit
anterior part of pharynx of 12 mm. frog tadpole. (Syn., choanae.)  
:See Olfactory Pit.
 
;Nebenkern
 
:cytological structure near nucleus of early spermatid.
'''Nasal Choanae''' — openings of olfactory chambers into mouth.  
;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
'''Nasal Pit''' — See Olfactory Pit.  
: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.
'''Nebenkern''' — cytological structure near nucleus of early spermatid.  
;Nephrogenic Tissue
 
:intermediate cell mass, mesomere, or nephrotome which will give rise to excretory system.
 
;Nephrostome
'''Neoteny''' — condition of many urodeles and of experimentally produced  
: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.
(thyroidless) anuran embryos in which larval period is extended or  
;Nephrotome
retained, i.e., larvae fail to go through normal metamorphosis. Sexual  
:intermediate cell mass.
maturity in larval stage (e.g., axolotl, Necturus).  
;Nephrotomic Plate
 
:intermediate mesoderm, mesomere.
 
;Nerve, Abducens
'''Nephrocoel''' — cavity, found in nephrotome or intermediate cell mass, which  
:sixth (VI) cranial nerve arising from basal plate of rhombencephalon which controls external rectus muscles of eye.
temporarily joins myocoel and coelom.  
;Nerve, Auditory
 
:eighth ( VllI ) cranial nerve, purely sensory, arising from acoustic ganghon and associated with geniculate ganglion of seventh nerve.
 
;Nerve, Facial
'''Nephrogenic Cord''' — continuous band of intermediate mesoderm (mesomere) without apparent segmentation, prior to budding off of mesonephric tubules.  
: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.
'''Nephrogenic Tissue''' — intermediate cell mass, mesomere, or nephrotome  
;Nerve, Oculomotor
which will give rise to excretory system.  
: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.
'''Nephrostome''' — funnel-shaped opening of kidney tubules into coelom; outer  
;Nervous Layer
tubules of amphibian mesonephric kidney acquire ciliated nephrostomal openings from coelom and shift their connections to renal  
:innermost of two layers found in roof of segmentation cavity of amphibian blastula, from which bulk of central nervous system is developed.
portal sinus.  
;Neural Arch
 
:ossified cartilages which extend dorsally from centrum around nerve cord.
 
;Neural Canal
'''Nephrotome''' — intermediate cell mass.  
: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.
'''Nephrotomic Plate''' — intermediate mesoderm, mesomere.  
;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
'''Nerve, Abducens''' — sixth (VI) cranial nerve arising from basal plate of  
: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.)
rhombencephalon which controls external rectus muscles of eye.  
;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
'''Nerve, Auditory''' — eighth ( VllI ) cranial nerve, purely sensory, arising from  
:tube formed by dorsal fusion of neural folds, rudiment of nerve or spinal cord.
acoustic ganghon and associated with geniculate ganglion of seventh  
;Neurenteric Canal
nerve.  
: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.
'''Nerve, Facial''' — seventh (VII) cranial nerve, both sensory and motor, related to taste buds and facial muscles.  
;Neurocoel
 
:cavity of neural tube, formed simultaneously with closure of neural folds. (Syn., central canal, neural canal.)
 
;Neurocranium
'''Nerve, Glossopharyngeal''' — ninth (IX) cranial nerve, mixed, associated  
:dorsal portion of skull associated with brain and sense organs.
with superior and petrosal ganglia.  
;Neuroglia
 
:see Glia Cells.
 
;Neuropore
'''Nerve, Oculomotor''' — third (III) cranial nerve which arises from neuroblasts in ventral zone of midbrain near median line just before hatching in frog tadpole.  
: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,
'''Nerve, Vagus''' — tenth (X) cranial nerve, mixed, arising from rhombencephalon and associated with jugular ganglion.  
;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
'''Nervous Layer''' — innermost of two layers found in roof of segmentation  
: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.
cavity of amphibian blastula, from which bulk of central nervous system is developed.  
;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.)
 
'''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==
==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.
'''Oesophagus''' — elongated portion of foregut between future glottis and  
;Olfactory Lobes
opening of bile duct of frog embryo; temporarily occluded just behind  
:anterior extremities of telencephalic cerebral lobes, partially constricted, associated with first pair of cranial nerves.
glottis but opens again.  
;Olfactory Pit
 
:depressions within olfactory placodes of 6 mm. frog embryo which will become olfactory organs (external nares).
 
;Olfactory Placode
'''Olfactory Lobes''' — anterior extremities of telencephalic cerebral lobes,  
:thickened ectoderm lateral to stomodeal region found in 5 mm. frog embryo, primordia of olfactory pits.
partially constricted, associated with first pair of cranial nerves.  
;''Omne Vivum e Vivo''
 
:all life is derived from preexisting life (Pasteur).
 
;Omnipotent
'''Olfactory Pit''' — depressions within olfactory placodes of 6 mm. frog embryo  
: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.
which will become olfactory organs (external nares).  
;''Omnis Cellula e Cellula''
 
:all cells from preexisting cells (Virchow).
 
;Ontogeny
'''Olfactory Placode''' — thickened ectoderm lateral to stomodeal region found  
:developmental history of an organism; sequence of stages in early development.
in 5 mm. frog embryo, primordia of olfactory pits.  
;Oocyte
 
:presumptive egg cell after initiation of growth phase of maturation. (Syn., ovocyte.)
 
;Oogenesis
'''"Omne Vivum e Vivo"''' — all life is derived from preexisting life (Pasteur).  
: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
'''Omnipotent''' — term used in connection with a cell which could, under  
:cytoplasmic substances connected with building rather than reserve materials utilized in developmental processes.
various conditions, assume every cytological differentiation known to  
;Opercular Chamber
the species or which, by division, could give rise to such varied differentiations.  
:See Branchial Chamber.
 
;Operculum
"'''Omnis Cellula e Cellula"''' — all cells from preexisting cells (Virchow).  
: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.
'''Ontogeny''' — developmental history of an organism; sequence of stages in  
;Optic Cup
early development.  
: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.
'''Oocyte''' — presumptive egg cell after initiation of growth phase of maturation.  
;Optic Recess
(Syn., ovocyte.)  
: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.
'''Oogenesis— process of maturation of ovum; transformation of oogonium  
;Optic Vesicle
to mature ovum. (Syn., ovogenesis.)  
: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.
'''Oogonia''' — multiplication (mitotic) stage prior to maturation of presumptive  
;Oral Plate
egg cell (ovum), found most frequently in peripheral germinal epithelium.  
: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.
'''Ooplasm''' — cytoplasmic substances connected with building rather than reserve materials utilized in developmental processes.  
;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
'''Opercular Chamber''' — See Branchial Chamber.  
: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.
'''Operculum''' — integumentary growth posteriorly from each of the hyoid  
;Osteoclasts
arches of frog embryo, which covers and encloses gills.  
: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.)
'''Optic Chiasma''' — thickening in forebrain ventral to infundibulum, found  
;Otic Vesicle
as a bunch of optic nerve fibers in future diencephalon.  
: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.)
'''Optic Cup''' — invagination of outer wall of primary optic vesicle to form  
;Oviducal Membranes of Ovum
a secondary optic vesicle made up of two layers; a thick internal or  
:tertiary membranes applied over egg as it passes through oviduct.
retinal layer continuous at pupil and choroid fissure, and a thin external  
;Oviducts
layer which is pigmented. Cavity of cup becomes future posterior  
:paired MUllerian ducts in both males and females, which generally persist in males.
chamber of eye.  
;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
'''Optic Lobes''' — thickened, evaginated, dorso-lateral walls of mesencephalon.  
:process of laying eggs.
 
;Ovocyte
 
:See Oocyte.
'''Optic Recess''' — depression in forebrain anterior to optic chiasma which  
;Ovogenesis
leads to optic stalks.  
:See Oogenesis.
 
;Ovogonia
 
:See Oogonia.
'''Optic Stalk''' — attachment of optic vesicle to forebrain, at first a tubular  
;Ovulation
connection between optic vesicle and diencephalon. Lumen is later  
:release of egg from ovary, not necessarily from body.
obliterated by development of optic nerve fibers.  
;Ovum
 
:Latin for egg.
 
'''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==
==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.)
'''Pachytene''' — stage in maturation when allelomorphic pairs of chromosomes are fused (telosynapsis or parasynapsis) so as to appear haploid, during  
;Paedogenesis
which process crossing over may occur; stage just prior to diplotene. Term means thick or condensed. (Syn., diplonema.)  
:reproduction during larval stage; precocious sex development.
 
;Pancreas
 
:digestive and endocrine glands arising as single posterior and single anterior primordia in vicinity of liver.
'''Paedogenesis''' — reproduction during larval stage; precocious sex development.  
;Parthenogenesis
 
:development of an egg without benefit of spermatozoon.
 
;Parthenogenesis, Artificial
'''Pancreas''' — digestive and endocrine glands arising as single posterior and  
:initiation of development of an egg by artificial means.
single anterior primordia in vicinity of liver.  
;Parthenogenesis, Natural
 
:maturation of eggs of some forms leads directly to development without aid of spermatozoa.
 
;Parthenogenetic Cleavage
'''Parthenogenesis''' — development of an egg without benefit of spermatozoon.  
:fragmentation of protoplasm of old and unfertilized chick eggs, originally thought to be true cleavage.
 
;Path, Copulation
 
:See Copulation Path.
'''Parthenogenesis, Artificial''' — initiation of development of an egg by artificial  
;Path, Penetration
means.  
:initial direction of sperm entrance into egg, often shifting toward egg nucleus along a new copulation path. (Syn., entrance path.)
 
;Perforatorium
 
:See A crosome.
'''Parthenogenesis, Natural''' — maturation of eggs of some forms leads directly  
;Pericardial Cavity
to development without aid of spermatozoa.  
: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.
'''Parthenogenetic Cleavage''' — fragmentation of protoplasm of old and unfertilized chick eggs, originally thought to be true cleavage.  
;Perichondrium
 
:mesenchymal layer immediately around forming cartilage.
 
;Perichordal Sheath
'''Path, Copulation''' — See Copulation Path.  
:thin, mesodermal (sclerotomal), continuous sheet of tissue immediately around notochord.
 
;Periosteum
 
:mesenchymal layer, often originally perichondrium, which will be found immediately around forming bone.
'''Path, Penetration''' — initial direction of sperm entrance into egg, often shifting toward egg nucleus along a new copulation path. (Syn., entrance  
;Peristomiai Mesoderm
path.)  
:mesoderm of amphibian gastrula derived from (ventral) lips of blastopore. Opposed to gastral mesoderm.
 
;Peritoneal cavity
 
:body cavity (coelom).
'''Perforatorium''' — See A crosome.
;Peritoneum
 
:coelomic mesothelium of abdominal region reinforced by connective tissue.
 
;Perivitelline Membrane
'''Pericardial Cavity''' — cavity or membrane sac which encloses heart, representing a cephalic portion of coelom within embryonic body. (Syn.,  
:See Vitelline Membrane.
parietal cavity.)  
;Perivitelline Space
 
:space between vitelline (fertilization) membrane and contained egg, generally filled with a fluid.
 
;Pfliiger's Law
'''Pericardium''' — thin mesodermal membrane which encloses pericardial cavity and heart.  
:dividing nucleus elongates in direction of least resistance.
 
;Phenotype
 
:outward appearance of an organism regardless of its genetic make-up. Opposed to genotype.
'''Perichondrium''' — mesenchymal layer immediately around forming cartilage.  
;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
'''Perichordal Sheath''' — thin, mesodermal (sclerotomal), continuous sheet of  
:See Epiphysis.
tissue immediately around notochord.  
;Pituitary
 
:See Hypophysis.
 
;Placode
'''Periosteum''' — mesenchymal layer, often originally perichondrium, which  
:Plate-like thickening of ectoderm from which arise sensory or nervous structures (e.g., olfactory placode).
will be found immediately around forming bone.  
;Plane
 
:imaginary two-dimensional surface; may be frontal, sagittal, transverse, median, or lateral.
 
;Plasmosome
 
:a true nucleolus. (See Nucleolus.)
'''Peristomiai Mesoderm''' — mesoderm of amphibian gastrula derived from  
;Plectrum
(ventral) lips of blastopore. Opposed to gastral mesoderm.  
:See Columella.
 
;Plexus Choroid
 
:Vascular folds in roof of prosencephalon, diencephalon, and rhombencephalon.
'''Peritoneal cavity''' — body cavity (coelom).  
;Poikilothermal
 
:cold-blooded; animals whose body temperatures are subject to environmental changes because they lack regulating mechanisms. Opposed to homoiothermal.
 
;Polar
'''Peritoneum''' — coelomic mesothelium of abdominal region reinforced by  
:pertaining, in most cases, to animal pole, although may refer to vegetal pole, or both.
connective tissue.  
;Polar Body
 
:relatively minute, discarded nucleus of maturing oocyte (generally three). (Syn., polocytes.)
 
;Polarity
'''Perivitelline Membrane''' — See Vitelline Membrane.  
: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.)
'''Perivitelline Space''' — space between vitelline (fertilization) membrane and  
;Pole, Vegetal
contained egg, generally filled with a fluid.  
: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.
'''Pfliiger's Law''' — dividing nucleus elongates in direction of least resistance.  
;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
'''Phenotype''' — outward appearance of an organism regardless of its genetic  
: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.
make-up. Opposed to genotype.  
;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
'''Pigment Layer of Optic Cup''' — thin outer wall of primary optic cup, posterior to retina, which never fuses with rods and cones of retina.  
:maturation in which equational and reductional divisions occur in that order.
 
;Posterior Tubercle
 
:See Tuberculum posterius.
'''Pineal''' — See Epiphysis.
;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
'''Pituitary''' — See Hypophysis.  
: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.
'''Placode''' — Plate-like thickening of ectoderm from which arise sensory  
;Pre-Reduction
or nervous structures (e.g., olfactory placode).  
: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.
'''Plane''' — imaginary two-dimensional surface; may be frontal, sagittal, transverse, median, or lateral.  
;Primary Oocyte
 
:termination of growth phase in maturation of ovum from oogonial stage, prior to any maturational divisions.
 
;Primary Spermatocyte
'''Plasmosome''' — a true nucleolus. (See Nucleolus.)  
: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.
'''Plectrum''' — See Columella.  
;Primordial Germ Cells
 
:diploid cells which are destined to become germ cells (e.g., oogonia and spermatogonia). (Syn., primitive germ cells.)
 
;Primordium
'''Plexus Choroid''' — Vascular folds in roof of prosencephalon, diencephalon,  
:See Anlage.
and rhombencephalon.  
;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
'''Poikilothermal''' — cold-blooded; animals whose body temperatures are subject to environmental changes because they lack regulating mechanisms.  
:mesodermal connective tissue covering of pronephric masses derived from adjacent myotomes and somatic mesoderm.
Opposed to homoiothermal.  
;Pronephric Chamber
 
:portion of amphibian coelomic cavity open anteriorly and posteriorly but closed ventrally by development of lungs.
 
;Pronephric Duct
'''Polar''' — pertaining, in most cases, to animal pole, although may refer to  
:outer portion of pronephric nephrotomes which develops a lumen connected posteriorly with mesonephric or Wolffian duct. (Syn., segmental duct.)
vegetal pole, or both.  
;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
'''Polar Body''' — relatively minute, discarded nucleus of maturing oocyte  
: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.)
(generally three). (Syn., polocytes.)  
;Pronucleus
 
:egg nucleus after polar body formation and sperm nucleus after entrance of spermatozoon into egg.
 
;Prophase
'''Polarity''' — axial distribution of component parts; animal and vegetal poles;  
:first stage in mitotic cycle when spireme is broken up into definite chromosomes, prior to lining up on metaphase (equatorial) plate.
stratification.  
;Prosencephalon
 
:See Forebrain.
 
;Prosocoel
'''Pole, Animal''' — region of egg where polar bodies are eliminated; ectoderm  
:cavity of prosencephalon.
forming portion of pre-cleaved egg. (Syn., apical or animal hemisphere.)  
;Proximal
 
:nearer the point of reference, toward main body mass.
 
;Pupil
'''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.  
:opening into secondary optic vesicle, occluded in part by lens, and regulated in diameter by ciliary muscles of iris.  
 
 
'''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==
==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.
'''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
'''Recapitulation Theory''' — theory that embryonic development reviews major steps in evolutionary history. (See qualifications under Biogenetic Law.)  
: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.)
'''Rectum''' — narrowed posterior portion of hindgut, lined with thickened endodermal epithelium, which opens directly into cloaca.  
;Regeneration
 
:repair or replacement of lost part or parts, a power gradually lost in the ontogeny of most animals.
 
;Regions, Presumptive
'''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.)  
: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.
'''Regeneration''' — repair or replacement of lost part or parts, a power gradually lost in the ontogeny of most animals.  
;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
'''Regions, Presumptive''' — regions of blastula which, by previous experimentation, have been demonstrated to develop in certain specific directions under normal ontogenetic influences.  
: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.
'''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.  
;Rhombencephalon
 
:See Hindbrain.  
 
'''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==
==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.
 
;Sachs' Law
'''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.  
: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.
'''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.  
;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.
'''Sagittal''' — mesial plane, or any plane parallel to it, dividing right parts of body from left. Right angles to both frontal and transverse planes.  
;Secondary Oocyte
 
:stage in oogenesis between primary oocyte and ovum; may be either haploid or diploid, depending upon species considered.
 
;Secondary Spermatocyte
'''Sclerotic Coat''' — tough mesenchymatous and partially cartilaginous coat outside of choroid coat of vertebrate eye. (Syn., sclera.)  
: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.
'''Sclerotome''' — loose mesenchymal cells proliferated off from inner and ventral edges of myotomes (5 mm. frog) which contribute to formation of axial skeleton.  
;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
'''Secondary Oocyte''' — stage in oogenesis between primary oocyte and ovum; may be either haploid or diploid, depending upon species considered.  
:See Axial Mesoderm.
 
;Segmentation
 
:repetition of structural pattern; used as synonym for cleavage as well as for metamerism.
'''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.)  
;Segmentation Cavity
 
:cavity of blastula. (Syn., subgerminal cavity, blastocoel.)
 
;Semi-Circular Canals
'''Secretory Tubule''' — portion of kidney tubule actually involved in excretory process.  
: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.
'''Section''' — generally a slice of an embryo, often of microscopic dimensions,  
;Semination
taken in any one of the various planes such as frontal, transverse, or  
:act 'of fertilizing by discharge of spermatozoa.
sagittal. (See Serial Sections.)  
;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
'''Segmental Plate''' — See Axial Mesoderm.  
: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.
'''Segmentation''' — repetition of structural pattern; used as synonym for cleavage as well as for metamerism.  
;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
'''Segmentation Cavity''' — cavity of blastula. (Syn., subgerminal cavity, blastocoel.)  
: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.
'''Semi-Circular Canals''' — tubular derivatives of utricle lined with ectoderm  
;Sex Determination
from otocyst, which constitute accessory balancing mechanisms of  
:See Determination of Sex.
vertebrates.  
;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
'''Seminal Vesicle''' — glandular dilation of distal end of ductus deferens  
:sex cords of the originally indifferent gonad primordium which form only cords of ovary, the functional follicles coming from germinal epithelium.
(Wolffian duct) where spermatozoa are temporarily collected prior to  
;Sexual Cords of the Testis
ejaculation.  
: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.
'''Semination''' — act 'of fertilizing by discharge of spermatozoa.  
;Sinus Venosus
 
:point of fusion of vitelline veins of amphibian embryo bilaterally symmetrical and related to ducts of Cuvieri and ductus venosus.
 
;Skeletogenous Sheath
'''Seminiferous Tubule''' — tubular divisions of testis derived from rete cords,  
:sclerotomal cells which first form a continuous layer around both notochord and nerve cord.
covered by a connective tissue theca and containing supporting (Sertoli)  
;Skin
cells and all stages of spermatogenesis.  
: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.
'''Sense Plate''' — narrow band of elevated ectodermal tissue which passes  
;Somatoblast
transversely across anterior end of amphibian embryo, ventral to level  
:blastomeres with specific germ layer predisposition, i.e., ectodermal somatoblasts.
of fused neural folds, with ends of band bending dorsally to merge  
;Somatopleure
with neural folds. Lower margins represent mandibular arch, the plate giving rise to mucous glands (oral suckers) of amphibia and to  
:layer of somatic mesoderm and closely associated ectoderm, extension of which (from body wall) gives rise to both amnion and chorion.
parts of olfactory organs, lens of eye, and possibly to part of inner  
;Somite
ear.  
: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).
'''Septum''' — partition.  
;Sperm
 
:germ cell characteristically produced by the male. (Syn., spermatozoon, sperm cell, male gamete, spermatosome.)
 
;Spermatid
'''Serial Sections''' — thin (often of microscopic dimensions) sections of embryos which are mounted on slides in order of their removal from the  
: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.
embryo, so that a study in sequence will provide an understanding  
;Spermatocyte
of all organ systems from one region of embryo to the other.  
: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.
'''Sertoli Cell''' — derivative of sexual cords of testis, found within seminiferous  
;Spermatogonium
tubule and functionally similar to follicle cell in ovary in that it is the  
: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.
nutritive, supporting, or nurse cell of the maturing spermatozoa. The  
;Spermatophore
heads of adult spermatozoa may be seen embedded in the cytoplasm of  
:sperm-bearing bundle, such as that which is shed by male urodele, the bundles later to be picked up by cloacal lips of female.
Sertoli cells.  
;Spermatosphere
 
:See Idiosome.
 
;Spermatozoon
'''Sex Cell Cord''' — division of sex cell ridge or gonad primordium, not to be  
:functionally mature male gamete. (Syn., sperm.)
confused with sexual (rete) cords.  
;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
'''Sex Determination''' — See Determination of Sex.  
: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.
'''Sexual Cords''' — derivatives of germinal epithelium from which they become  
;Spinous Process
separated and give rise to bulk of gonads of both sexes.  
: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.
'''Sexual Cords of the Ovary''' — sex cords of the originally indifferent gonad  
;Spireme
primordium which form only cords of ovary, the functional follicles  
:continuous chromatin thread characteristic of so-called resting cell nucleus. Existence questioned by current cytologists.
coming from germinal epithelium.  
;Splanchnic
 
:refers to viscera, opposed to somatic or body.
 
;Splanchnic Mesoderm
'''Sexual Cords of the Testis''' — sex cords of the originally indifferent gonad  
:visceral mesoderm, or that nearest embryonic axis in lateral plate.
primordium which give rise to seminiferous tubules of testis, forming  
;Splanchnocoel
a rather solid mesenchymatous reticulum when cavities begin to appear  
:that portion of enterocoel (of Amphioxus) which lies between somatic and splanchnic mesoderm within body. (Syn., coelom.)
lined with spermatogonia (from primordial germ cells) and Sertoli cells,  
;Splanchnocranium
the whole constituting seminiferous tubules.  
: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.
'''Sheath, Myelin''' — myelin covering of axons in so-called white matter of  
;Spongioblasts
spinal cord.  
:cells of mantle layer of developing spinal cord destined to form merely supporting tissue.
 
;Stereoblastula
 
:solid blastula as found in Crepidula.
'''Sinus Venosus''' — point of fusion of vitelline veins of amphibian embryo  
;Stomodeum
bilaterally symmetrical and related to ducts of Cuvieri and ductus  
: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.
venosus.  
;Stroma
 
:mesodermally derived, medullary, supporting tissues of an organ.
 
;Sub-Germinal Cavity
'''Skeletogenous Sheath''' — sclerotomal cells which first form a continuous layer  
:See Blastocoel, Segmentation Cavity.
around both notochord and nerve cord.  
;Sub-Notochordal Rod or Bar
 
:hypochordal rod of amphibian embryo, found dorsal to midgut. Transitory.
 
;Sucker
'''Skin''' — See Dermis and Epidermis. (Syn., integument.)  
:adhesive, connecting organ of oral region (larval stage).
 
;Sustentacular Cell
 
:cell which provides nourishment for another, such as Sertoli or follicle cells of gonads.
'''Somatic''' — relating to body in contrast to germinal cells; or relating to  
;Sylvius, Aqueduct of
outer body in contrast to inner splanchnic mesoderm.  
: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.
'''Somatoblast''' — blastomeres with specific germ layer predisposition, i.e., ectodermal somatoblasts.  
;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.)
'''Somatopleure''' — layer of somatic mesoderm and closely associated ectoderm,  
;Syncytium
extension of which (from body wall) gives rise to both amnion and  
:nuclei and cytoplasm without cellular boundaries; multinucleate protoplasm without cell boundaries.
chorion.  
;Syngamy
 
:specifically the fusion of the gamete pronuclei, but also the union of gametes at fertilization. (Syn., zygotogenesis, fertilization.)
 
;Synizesis
'''Somite''' — blocks of paraxial mesoblast, metamerically separated by transverse clefts, derived from enterocoelic or gastral mesoderm and  
:stage in maturation between synaptene and pachytene when chromatin threads are short and thick and ends away from centrosome are tangled.  
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==
==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.)


'''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==
==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.
 
;Urodele
'''Uriniferous Tubule''' — functional kidney tubule of mesonephros.  
: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.
'''Urodele''' — tailed amphibia (e.g., salamanders). (Syn., caudata.)  
;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.
'''Urogenital Duct''' — ducts which open into cloaca of male amphibia and  
;Utricle
convey both excretory and genital products, derived from mesonephric (Wolffian) ducts.  
: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.
 
 
'''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==
==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 Deferentia''' — mesonephric or Wolffian ducts of frog, which persist  
;Vasa Efferentia
as male gonoducts of bird and mammal, connecting with testes through  
: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.
vasa efferentia and epididymis and functioning as sperm ducts after  
;Vegetal Pole
degeneration of embryonic mesonephros and development of gonads.  
: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.)
(Sing., vas deferens.)  
;Vein
 
:See under specific names.
 
;Velar Plate
'''Vasa Efferentia''' — ducts which convey frog sperm from collecting tubules  
: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.
through mesorchium to Malpighian corpuscles of mesonephric kidney;  
;Velum Transversum
derived from rete cords and connected with mesonephric tubules of  
:depressed roof of telencephalon just anterior to lamina terminalis, which later becomes much folded and vascular as anterior roof of third ventricle.
anterior (sexual) half of the mesonephric or Wolffian body.  
;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
'''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  
: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.
antipolar hemisphere.) (See Animal Pole.)  
;Ventral
 
:belly surface. Ventrad means toward belly surface.
 
;Ventral Mesentery
'''Vein''' — See under specific names.  
: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.
'''Velar Plate''' — folds or flaps developing anterior and posterior to branchial  
;Ventricle IV
regions of frog (anuran) embryo derived from pharyngeal wall and  
: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.
serving as a gross sifting organ between pharynx and gill (branchial)  
;Ventricle, Lateral
chamber.  
: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.
'''Velum Transversum''' — depressed roof of telencephalon just anterior to  
;Vertebra
lamina terminalis, which later becomes much folded and vascular as  
:derivatives of sclerotome which surround nerve cord and notochord, and finally incorporate notochord by chondrification and ossification (centrum).
anterior roof of third ventricle.  
;Vertebral Arch
 
:See Neural Arch.
 
;Vertebral Plate
'''Vena Cava Anterior''' — junction of inferior jugular (anterior cardinal) and  
:See Axial Mesoderm. (Syn., segmental plate.)
subclavian and vertebral veins which empty into ductus Cuvieri, and  
;Vesicle, Germinal
later the right auricle. (Syn., superior vena cava, superior caval veins.)  
:nucleus of egg while it is a distinct entity and before elimination of either of the polar bodies.
 
;Visceral
 
:pertaining to viscera.
 
;Visceral Arches
'''Vena Cava Posterior''' — single median ventral vein which represents remnant of anterior right cardinal and which later receives hepatic vein  
: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.)
prior to joining ductus Cuvieri, and later joins right auricle directly.  
;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
'''Ventral''' — belly surface. Ventrad means toward belly surface.  
:ectodermal invaginations which may meet endodermal pharyngeal evaginations to form visceral clefts. (Syn., visceral groove.)
 
;Visceral Groove
 
:See Visceral Furrow.
'''Ventral Mesentery''' — double layer of mesoblast which connects alimentary  
;Visceral Mesoderm
canal with splanchnopleure in embryo.  
:See Splanchnic Mesoderm, Splanchnopleure.
 
;Visceral Plexus
 
:aggregation of sympathetic neurons which control viscera, having migrated posteriorly from tenth (vagus) cranial ganglia.
'''Ventricle III''' — main cavity (diocoel) of forebrain, related to paired lateral  
;Visceral Pouch
ventricles or telocoels, by way of foramina of Monro.  
: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.
'''Ventricle IV''' — main cavity of hindbrain (rhombencephalon) connected  
;Vitalism
anteriorly with aqueduct of Sylvius and posteriorly with neural canal,  
: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.)
having as a roof the vascular posterior choroid plexus.  
;Vitelline
 
:pertains to yolk (e.g., vitelline vein brings blood from yolk; vitelline membrane is that which covers yolked egg).
 
;Vitelline Artery
'''Ventricle, Lateral''' — See Lateral Ventricles of the Brain.  
: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.)
'''Ventricle of the Heart''' — chamber of the heart, single in frog and very muscular, developing from anterior myocardium and provided with valves;  
;Vitelline Substance
connected with bulbus arteriosus anteriorly.  
: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.
'''Vertebra''' — derivatives of sclerotome which surround nerve cord and notochord, and finally incorporate notochord by chondrification and  
;Vitreous Humor
ossification (centrum).  
: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.)  
 
 
'''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==
==W==
 
; Wolffian Body
'''Wolffian Body''' — See Mesonephros.  
:See Mesonephros.
 
;Wolffian Duct
 
:See Mesonephric Duct, Urogenital Duct, Vasa Deferentia.  
'''Wolffian Duct''' — See Mesonephric Duct, Urogenital Duct, Vasa Deferentia.  


==Y==
==Y==
 
; Yolk
'''Yolk''' — highly nutritious food (metaplasm) consisting of non-nucleated spheres and globules of fatty material found in all except alecithal eggs.  
: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 Nuclei''' — darkly staining chromatin-like substances within cytoplasm of young (immature) eggs around which yolk is accumulated during  
;Yolk Plug
growth phase of oogenesis. May be derived from nucleoli which escape from nucleus.  
: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}}


==B==
[[Category:Glossary]]
'''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.)
 
 
 
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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
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Glossary of Frog Embryological Terms

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

A

Acidophil
oxyphil: cell constituents which stain with acid dyes, often used to designate an entire cell type. (See Basophil.)
Acrosome
apical organ at tip of mature spermatozoon, derived from spermatosphere (idiosome or centrosome) and presumably functional in aiding penetration of egg cortex by spermatozoon during fertilization. (Syn., perforatorium.)
Activation
process of initiating development in egg, normally achieved by spermatozoon of same species but also accomplished artificially (parthenogenesis); term also used to refer to stimulation of spermatozoon to accelerated activity by chemical (fertilizin) means.
Adnexa
extra-embryonic structures (e.g., yolk sac) discarded before adult condition is attained.
Aestivation
reduced activity of some animals during heat of summer. Opposed to hibernation.
Agglutination
cluster formation; a spontaneously reversible reaction of spermatozoa to certain chemical situations (e.g., egg water).
Aggregation
coming together of cells, such as spermatozoa, without sticking; a non-reversible response comparable to chemotropism.
Albuginea of Testis
stroma of primitive testis which forms a layer between germinal epithelium and seminiferous tubules.
Albumen
protein substance secreted by walls of oviducts around egg of reptiles and birds.
Albumen Sac
2-layered ectodermal sac enclosing albumen of chick egg during early development of embryo, separated for a time from yolk by vitelline membrane; later to release some of its contents into amniotic cavity through ruptured sero-amniotic connection.
Amitosis
direct nuclear division without chromosomal rearrangements; generally thought to be a sign of decadence or of high specialization, if it occurs at all.
Amphiblastula
double-structured blastula as in Porifera (sponges).
Amphimixis
mixing of germinal substances accomplished during fertilization.
Amphitene
one end of chromosome is thick, one end is thin, moving toward full pachytene during maturation.
Amplexus
sexual embrace of female amphibian by male, a process which may (frogs and toads) or may not (urodeles) occur at time of oviposition.
Anal Plate
thickening and invagination of mid-ventral ectoderm which meets evaginating endoderm of hindgut, later to be perforated as proctodeum (anus). (Syn., cloacal membrane.)
Analogy
similarity of parts in respect to function rather than to structure.
Anamniota
forms which never develop an amnion, e.g., cyclostomes, fishes, amphibia.
Anaphase
phase of mitosis when paired chromosomes are separating at equatorial plate and begin to move toward ends of spindle.
Anastomosis
joining together, as of blood vessels and nerves, generally forming a network.
Androgen
hormonal secretion of interstitial tissue of testis.
Androgenesis
development of an egg with paternal (sperm) chromosomes only, accomplished by removing or destroying egg nucleus before syngamy.
Angioblast
migratory mesenchyme cell associated with formation of vascular endothelium.
Animal Pole
region of egg where polar bodies are formed; region of telolecithal egg containing nucleus and bulk of cytoplasm; gives rise largely to ectodermal derivatives. (Syn., apical pole or hemispheres.)
Anlage
rudiment; group of cells which indicate a prospective development into a part or an organ. (Syn., ebauche or primordium.)
Anterior
toward head; head end. (Syn., cephalic, cranial, rostral.)
Anura
tailless amphibia (e.g., frogs and toads). (Syn., Salientia.)
Aortic Arch
blood vessel which connects dorsal and ventral aortae by way of visceral arch.
Aqueduct of Sylvius
ventricle of mesencephalon (mesocoel) becomes aqueduct of Sylvius, connecting with cavities of optic lobes. (Syn., iter.)
Aqueous Humor
fluid which fills anterior and posterior chambers of eye between lens, probably derived from mesoderm.
Archencephalon
pre-chordal brain, e.g., forebrain. Brain anterior to anterior end of notochord.
Archenteric Pouch
See Enterocoel.
Archenteron
primitive gut found in gastrula and communicating with outside by blastopore; precursor of embryonic gut. (Syn., gastrocoel, enteron.)
Arcualia
small blocks of sclerotomal connective tissue involved in formation of vertebrae.
Asexual Reproduction
reproduction without union of gametes; generally with no maturation divisions.
Aster
"star-shaped structure" surrounding centrosome (Fol, 1877); lines radiating in all directions from centrosome during mitosis.
Astral Rays
lines which make up aster.
Astrocytes
stellate-shaped cells arising from spongioblasts of mantle layer, classified under the more general term of neuroglia.
Atrium
two upper chambers of frog's embryonic heart, later to be known as auricles.
Attachment Point
point of chromosome to which spindle fiber is attached and therefore portion of chromosome nearest centrosome in anaphase. (Syn., centrosome, chromocenter, kinetochore.)
Attraction Sphere
See Centrosphere.
Auricles
two upper chambers of adult frog's heart, derived from embryonic atria.
Autogamy
self-fertilization.
Autosome
any chromosome except so-called sex (X or Y) chromosomes.
Auxocyte
pre-meiotic germ cell. (Syn., primary cyte, meiocyte.)
Axial Filament
central fiber in tail of a spermatozoon.
Axial Mesoderm
that portion of epimeric mesoderm nearest notochord. (Syn., vertebral plate.)
Axis
imaginary central or median line, generally correlated with a gradient.
Axis of the Cell
imaginary line passing through centrosome and nucleus of a cell, generally also through geometrical center of cell. In an egg such an axis generally is also gradient axis of materials such as cytoplasm, yolk, pigment, etc.
Axis of the Embryo
imaginary line representing antero-posterior axis of the future embryo.

B

Balancers
cylindrical and paired projections of ectoderm with mesenchymatous cores, used as adhesive organs in place of (anuran) suckers by many urodele amphibia.
Balfour's Law
"The velocity of segmentation in any part of the ovum is, roughly speaking, proportional to the concentration of the protoplasm there; and the size of the segments is inversely proportional to the concentration of the protoplasm." The intervals between cleavages increase in proportion to the amount of yolk which a cell contains in its protoplasm.
Basal Plate
ventro-lateral wall of myelencephalon, separated from dorsolateral alar plate by sulcus limitans.
Basophil
cell constituents having an affinity for basic dyes, often used as an adjective for an entire cell. (See Acidophil.)
Bidder's Organ
anterior portion of anuran pro-gonad, somewhat ovarian in character, developing from part of gonad rudiment consisting wholly of cortex; its development indicates failure of medullary substance to diffuse to anterior extremity of gonad rudiment.
Biogenetic Law
embryos of higher species tend to resemble embryos of lower species in certain respects but are never like adults of lower species. Embryonic development is a gradual deviation from the more general (phylogenetic) to the more specific characters of the individual species. Not to be confused with recapitulation theory.
Blastema
indifferent group of cells about to be organized into definite tissue; nev/ly formed cells covering a cut surface, functional in regeneration of tissues.
Blastocoel
cavity of blastula. (Syn., segmentation or subgerminal cavity.)
Blastoderm
living portion of egg from which both embryo and all of its membranes are derived. The cellular blastodiscs. "; Because the embryo chooses this as its seat and its domicile, contributing much to its configuration out of its own substance, therefore, in the future we shall call it blastoderm" (Pander, 1817).
Blastomere
cellular unit of developing egg or early embryo, prior to time of gastrulation. Smaller blastomeres are micromeres; intermediate ones are mesomeres; larger ones are macromeres, where there is great disparity in size.
Blastopore
opening of archenteron (gastrocoel) to exterior, occluded by yoliv plug in amphibian embryos; consisting of a slit-like space between elevated margin of blastoderm and underlying yolk of chick egg; represented in amniota as primitive streak.
Blastopore, Dorsal Lip of
region of first involution of cells in amphibian gastrula; general area of the "organizer"; original gray crescent area; cells which turn in beneath potential central nervous system (Amphioxus) and form roof of archenteron. (Syn., germ ring or marginal zone.)
Blastopore, Ventral Lip of
region of blastopore opposite dorsal lip; region which gives rise to peristomial mesoderm of frog. (Syn., germ ring.)
Blastula
stage in embryonic development between appearance of distinct blastomeres and end of cleavage (i.e., beginning of gastrulation); a stage generally possessing a primary embryonic cavity or blastocoel; invariably monodermic. (See specific types under specific names.)
Blood Islands
pre-vascular groups of mesodermal cells found in splanchnopleure, from which will arise blood vessels and corpuscles.
Bowman's Capsule
double-walled glomerular cup associated with uriniferous tubule.
Branchial
having to do with respiration. (Syn., gill.)
Branchial Arch
visceral arches, beginning with third pair, which contain blood vessels which (phylogenetically) have respiratory function during embryonic development. Mesodermal components which support those blood vessels are branchial arches. (Syn,, gill arch.) (See Visceral Arches.)
Branchial Artery
blood vessel which actually passes through gills (external or internal) of frog embryo. (Syn., gill artery.)
Branchial Chamber
closed chamber (except for a single spiracular opening on left side) which encloses internal gills of frog embryo. (Syn., opercular or gill chamber.)
Branchial Cleft
opening between branchial arches formed by invaginating head ectoderm and evaginating pharyngeal endoderm (pouch) through which water passes from pharynx to outside of frog. (Syn., gill cleft or slit, some visceral clefts.)
Branchial Groove
ectodermal invagination anterior or posterior to visceral arch, which joins branchial pouch to form branchial cleft, in most instances.
Branchiomery
type of serial metamerism involving respiratory structures exemplified by visceral arches.
Bud
undeveloped branch, generally an anlage of an appendage (e.g., limb or wing bud).
Budding
reproductive process by which a small secondary part is produced from parent organism, and which gradually grows to independence.
Bulbus Arteriosus
most anterior division of early, tubular, embryonic heart which leads from ventricle to truncus arteriosus.

C

Cardinal Veins
anterior, posterior, and sub-cardinal veins; anterior veins receive blood from head, including first three segmental veins; posterior veins receive blood from all pairs of trunk segmental veins and from veins of Wolffian bodies; paired cardinals enlarge and fuse, left half degenerates, and balance fuses with developing inferior (posterior) vena cava.
Cell
protoplasmic territory under control of a single nucleus, whether or not territory is bounded by a discrete membrane. By this definition a syncytium is made up of many cells with physiological rather than morphological boundaries.
Cell Lineage
study of origin and fate of specific blastomeres in embryonic development. (Syn., cytogeny.)
Cell Theory
body of any living organism is composed of structural and functional units, the primary agents of organization called cells. Each cell consists of a nucleus and its sphere of influence, including the cytoplasm, generally circumscribed by a membrane. "Omnis cellula e cellula" (Virchow).
Central Canal
See Neurocoel.
Centriole
granular core of centrosome.
Centrosome
granule (centriole) and surrounding sphere of rays (centrosphere) which function as kinetic centers in mitosis. Center of aster which does not disappear when astral rays disappear. Dynamic center of mitosis.
Centrosphere
rayed portion of centrosome; structure in spermatid which gives rise to acrosome. (Syn., spermatosphere, idiosome, attraction sphere. )
Cephalic Flexure
ventral bending of embryonic head at level of midbrain and hindbrain.
Chimera
compound embryo generally derived by grafting major portions of two embryos, usually of different species; may be derived by abnormal chromosome distribution in cleavage after normal fertilization.
Choana
openings of olfactory organ into pharynx, internal nares. Sometimes also used in connection with external olfactory opening.
Chondrification
process of forming cartilage, by secretion of a homogeneous matrix between the more primitive cells.
Chondrin
chemical substance in cartilage which makes it increasingly susceptible to basic stains.
Chondrocranium
that portion of skull which is originally cartilaginous.
Chorda Dorsalis
Syn., notochord.
Chorda Mesoderm
region of the late (amphibian) blastula, arising from gray crescent area, which will give rise to notochord and mesoderm and will, if transplanted, induce formation of secondary medullary folds.
Choroid Coat
mesenchymatous and sometimes pigmented coat within sclerotic coat but surrounding pigmented layer of eye in vertebrate embryos.
Choroid Fissure
inverted groove in optic stalk whose lips later close around blood vessels and nerves that enter eyeball.
Choroid Knot
thickened region of fused lips of choroid fissure, near pupil, from which arise cells of iris.
Chromatid
one of the parts of a tetrad (McClung, 1900); really a longitudinal half of a chromosome.
Chromatin
deeply staining substance of nuclear network and chromosomes, consisting of nuclein; gives Feulgen reaction and stains with basic dyes.
Chromatophore
pigment-bearing cell frequently capable of changing size, shape, and color; cells responsible for superficial color changes in animals; behavior under control of sympathetic nervous system or neurohumors.
Chromidia
granules within cytoplasm which stain like chromatin and which may actually be extruded chromatin granules.
Chromomere
unit of chromosome recognized as a chromatin granule.
Chromonema
slender thread of chromatin which is core of chromosome during mitosis.
Chromophil
cells which have an affinity for dyes.
Chromophobe
cells whose constituents are non-stainable; have no affinity for dyes.
Chromosome
chromatic or deeply staining bodies derived from nuclear network and containing a matrix and one or more chromonemata during process of mitosis; bodies found in all somatic cells of normal organism in a number characteristic of the species; bearers of gene.
Cleavage
mitotic division of egg resulting in blastomeres. (Syn., segmentation. )
Cleavage, Accessory
cleavage in peripheral or deeper portions of (chick) germinal disc caused by supernumerary sperm nuclei following (normal) polyspermy, sometimes occurring in urodeles.
Cleavage, Asymmetrical
extremely unequal divisions of egg as in Ctenophore.
Cleavage, Bilateral
cleavage in which egg substances are distributed symmetrically with respect to median plane of future embryo.
Cleavage, Determinate
cleavage in which certain parts of future embryo may be circumscribed in certain specific (early) blastomeres; cleavage which produces blastomeres that are not qualitatively equipotential, i.e., when such blastomeres are isolated they will not give rise to entire embryos. (Syn., mosaic development.)
Cleavage, Dexiotropic
cleavage resulting in a right-handed production of daughter blastomere(s), as in spiral cleavage.
Cleavage, Discoidal
See Cleavage, Meroblastic.
Cleavage, Equatorial
cleavage at right angles to egg axis, opposed to vertical or meridional; often the typical third cleavage plane. (Syn., latitudinal or horizontal cleavage.)
Cleavage, Holoblastic
complete division of egg into blastomeres, generally equal in size although not necessarily so (e.g., Amphioxus). (Syn., total cleavage.)
Cleavage, Horizontal
See Cleavage, Equatorial.
Cleavage, Indeterminate
cleavage resulting in qualitatively equipotential blastomeres in early stages of development. When such blastomeres are isolated from each other they give rise to complete embryos. Opposed to mosaic development. (Syn., regulatory development.)
Cleavage, Latitudinal
See Cleavage, Equatorial.
Cleavage Laws
See specific laws under names of Balfour, Hertwig, and Sachs.
Cleavage, Levotropic
cleavage resulting in left-handed or counterclockwise production of daughter blastomere(s) as in some cases of spiralcleavage.
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 efferens, 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., sub-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 
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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 29) Embryology Book - The Frog Its Reproduction and Development 15. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Frog_Its_Reproduction_and_Development_15

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