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					                                    CHICK DEVELOPMENT
                               24-Hour and 33-Hour Chick Embryos

              24-HOUR CHICK                               2. Splanchnic Mesoderm [SP]:
                                                              mesodermal layer in contact with
I. Whole Mount                                                endoderm.
    A. Area Pellucida [APL]: relatively clear             3. Somatic Mesoderm [SO]: meso-
        area immediately surrounding embryo.                  dermal layer in contact with
    B. Area Opaca [AO]: outside area pellu-                   ectoderm.
        cida with somewhat mottle appear-
        ance.                                     Splanchnic and somatic mesoderms are best
    C. Neural Folds [NF]: two dense lateral       seen in embryo sections between subcephalic
        bands running parallel to each other      pocket and somites as part of lateral body
        throughout middle of embryo; space        folds.
        between these is neural groove; if
        neural folds have fused cranially, a          C. Endoderm [EN]: inner layer of embryo
        neural tube is present (may not have              (seen in later sections).
        occurred in some 24-hour embryos).            D. Neural Tube [NT]: formed by fusion or
    D. Somites: paired blocks of mesoderm                 meeting of neural folds.
        lateral to neural folds.                      E. Neural Plate [ONP]: thickened tissue
    E. Notochord [NO]: here, seen between                 seen on surface of embryo; found dor-
        neural folds by focusing up and down;             sal to notochord.
        a narrow mesodermal rod which sup-            F. Neural Groove [ONP]: space between
        ports the developing embryo.                      sides of neural plate (neural folds);
    F. Primitive Streak [PS]: continues caud-             becomes lumen of neural tube.
        ally from notochord.                          G. Notochord [NO]: seen throughout em-
                                                          bryo sections; usually a round mass of
II. Serial Transverse Sections                            tissue in very middle of embryo,
     A. Ectoderm [E]: outer (or upper) tissue             ventral to neural tube or plate.
          of embryo.                                  H. Foregut [FG]: flattened space (dorso-
          1. Epidermal Ectoderm [EE]: outer-              ventrally) ventral to neural tube and
              most layer of tissue covering               notochord
              embryo.                                 J. Anterior Intestinal Portal [AIP]: found
          2. Neural Ectoderm : tissue making              caudal to foregut; an opening through,
              up neural tube or neural folds.             what appears to be, ventral side of
     B. Mesoderm [M]: middle layer of tissue              embryo.
          1. Somite Mesoderm [SM]: tissue             K. Heart Primordium [H]: distinct thick-
              making up a somite (seen in later           ening of splanchnic mesoderm on
              sections).                                  either side of midline at about level of
                                                          anterior intestinal portal

              33-HOUR CHICK                       C.-G. Continuing caudally oval or pear shaped
                                                      midbrain [MB] appears, just posterior to
I. Whole Mount                                        infundibulum; moving further back hind-
    A. Area Opaca [AO]: two parts:                    brain [HB] can be seen (at about level of
        1. Area Vasculosa [VAS]: inner                heart ventricle); note change in shape of
             mottled area caused by                   neural tube; spinal cord [NT] is seen where
             developing blood islands [BI].           lateral walls become much thicker than
        2. Area Vitellina [VIT]: non-mottled          dorsal or ventral walls; as neural tube
             area covering the yolk.                  opens (and is no longer a tube) we are in
        Areas Vasculosa and Vitellina are             area of neural fold [NF] and neural groove
             separated from each other by             [ONP].
             sinus terminalis, a circular blood   H. Notochord [NO]: initially seen a few
             vessel.                                  sections back from infundibulum, seen
    B. Area Pellucida [APL]                           as small mass of cells beneath neural
    C. Anterior Neuropore [ANP]: opening at           tube.
        most cranial end of neural tube.          I. Foregut Lumen [FG]: beginning at level
    D. Forebrain (or prosencephalon) [FB]:            of optic vesicles, trace caudally to fol-
        cranial most enlargement of                   low a small endoderm lined foregut,
        embryonic brain; most characterized           found ventral to neural tube; in poster-
        by presence of optic vesicles,                ior sections foregut widens and it’s
        beginnings of eyes.                           floor makes contact with ventral skin
    E. Midbrain (or mesencephalon) [MB]:              ectoderm, forming oral membrane
        division of brain caudal to forebrain.        [OM].
    F. Hindbrain (rhombencephalon) [HB]:          J. Dorsal Aorta [DA]: two spaces seen at
        caudal most section of embryonic              level of midbrain between midbrain
        brain; may appear as two or three             and foregut.
        bulges of neural tube.                    K. Ventricle of the Heart [V]: approxi-
    G. Neural Tube or Spinal Cord [NT]:               mately at level of oral membrane will
        section of neural tube caudal to hind-        be seen an enlargement that bends
        brain; beyond this, tube diverges as          from midline somewhat towards em-
        neural folds and continues as open            bryo’s left; notice that endocardium
        neural plate; continuing yet further as       [END] is separated from epimyocard-
        primitive streak.                             ium [EMY].
    H. Notochord [NO]: best seen in region        L. Sinus Venosus [SV]: in more caudal
        of brain.                                     sections heart “moves” to midline and
                                                      endocardium and epimyocardium are
II. Serial Transverse Sections                        no longer as separated; region where
     A. Forebrain [FB]: broad anterior section        sinus venosus is found and where
          of brain containing lateral optic           atrium will form.
          vesicles [OP].                          M. Vitelline Vein [VV]: caudal to sinus
     B. Infundibulum [IN]: outpocketing of            venosus as two lateral vessels
          floor of forebrain seen as optic            consisting only of endothelium.
          vesicles fade out while moving

                          Legend of Structures for 24 Hour and 33 Hour Chicks

A1     first aortic arch             HB    hindbrain, rhomben-          PCV posterior cardinal vein
AC anterior cardinal vein                  cephalon                     PE  pericardial coelom
3+                                   H     heart primordium             PF  primitive fold
0AIP anterior intestinal portal      HM head mesoderm                   PG  primitive groove
AMM anterior margin of invagin-      HN Hensen’s node                   PP  primitive pit
       ated lateral mesoderm         IC    internal carotid artery      PS  primitive streak
ANP anterior neuropore               IN    infundibulum                 SCP subcephalic pocket
AO area opaca                        LBF lateral body fold              SI  sinus terminalis (circular
APL area pellucida                   LM lateral plate (hypomere)            vein of yolk sac)
AT     atrium of heart                     mesoderm                     SM somite (epimere) mesoderm
BI     blood islands                 M     mesoderm                     SO  somatic lateral plate (hypo-
CA conus arteriosus                  MB midbrain, mesencephalon             mere) mesoderm
CAP capillaries                      MFH margin of free head            SOP somatopleure
CC common cardinal vein              MG midgut open to yolk             SP  splanchnic lateral plate (hy-
CL     coelom                        MM mesomere, nephrotome,               Pomere) mesoderm
CNC cranial neural crest primor-           intermediate mesoderm        SPP splanchnopleure
       dium of nerves 5, 7, 8        MYC myocoele of somite             SV  sinus venosus
DA dorsal aorta                      NC neural crest                    TM tailbud mesoderm
DMC dorsal mesocardium               NCL neurocoele, neural canal       USM unsegmented somite meso-
E      ectoderm                      NF    neural fold                      Derm
EE     epidermal ectoderm            NM neuromeres of hindbrain         V   ventricle of heart
EMB ambryonal areal                  NO notochord                       VA ventral aorta
EMY epimyocardial layer of heart     NT    neural tube                  VAS area vasculosa
EN     endoderm                      OM oral membrane                   VIT area vitelline
END endoderm                         ONP open neural plate              VV vitelline vein
FB     forebrain, prosencephalon     OP    optic vesicle                Y   yolk
FG     foregut lumen                 OT    otic placode                 YS  yolk sac
FGE foregut endoderm                 PA    proamnion                    ZJ  zone of junction


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