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Vol. 42, 1c,..,
<br />? 1, 1 hour, l.ti
<br />)arent and
<br />„? l
<br />animal pole
<br />Water-harden'
<br />vity than w;;tl r
<br />,,e, ova adhe,,,_
<br />hours (Fiu. ,
<br />Stomeres
<br />cm total
<br />s distinguishal_
<br />cithal, clear
<br />adhesive. :1
<br />cells to
<br />becomes yel ,
<br />)arent.
<br />i hours; s,:,
<br />:imately 1.0 ,
<br />6 hours;
<br />1.2- by 0.
<br />7 hours; tc;" -
<br />9 hours:
<br />10 hours; .
<br />1 hours; seg.-
<br />ula (moruh:
<br />py 25 perce
<br />ula (morula
<br />;uishable; bl,
<br />round yolk I.
<br />c surface. B
<br />inner towarc
<br />.stocoel.
<br />s (Fig. 1C).
<br />ble except
<br />oved; blastoc.,
<br />mdersurface
<br />yolk sphere
<br />ted.
<br />v of blastode.
<br />tmeter (Fig. s
<br />yolk sphere
<br />oel present. ;
<br />along perip
<br />rmation of is
<br />er of yolk s.?:
<br />ived from bl«
<br />er bulging -• :l
<br />ent of sphere.
<br />December 1982 MINCKLEY, GUSTAFSON: RAZORBACK SUCKER DEVELOPMENT 555
<br />Stage 12: early gastrula, 34 hours (Fig. 1E).
<br />Underrim of blastodisc thickened to form.,
<br />"randwulst" or marginal ridge with inner
<br />layer termed the germ ring. Ring thickest
<br />posteriorly, recognized as embryonic shield.
<br />Presumptive endodermal cells at posterior
<br />edge of shield beginning to involute through
<br />blastopore and spread beneath blastoderm.
<br />Cells of prechordal plate and notochord mi-
<br />grating inward over dorsal lip of blastopore
<br />(establishment of embryonic axis). Presump-
<br />tive mesodermal cells also turning inward,
<br />` positioned either side of embryonic axis be-
<br />neath ectoderm and above endoderm. Peri-
<br />blast, randwulst cells, and germ ring cells not
<br />involved in involution spread over 50 percent
<br />of yolk sphere.
<br />Stage 13: middle gastrula, 35 hours. In-
<br />vagination lengthening embryonic shield;
<br />blastopore marks posterior axis of embryo.
<br />Stage 14: late gastrula, 36 hours (Fig. 1F).
<br />Embryonic shield nearly reaching animal
<br />pole of egg on dorsal meridian; shield ap-
<br />proximately 1.3 mm long; concentration of
<br />invaginated cells clearly visible at anterior
<br />end of shield; randwulst cells and germ ring
<br />cells, accompanied by presumptive ectoder-
<br />mal cells, forming outwardly as epiblast;
<br />marginal ridge shifted below equator of egg;
<br />uncovered portion of yolk protrudes as yolk
<br />plug.
<br />Stage 15: early neurula, 45 hours, 1.8 mm
<br />diameter. Blastopore closed; yolk plug no
<br />longer protruding. Embryonic shield approx-
<br />imately 2.0 mm long, circumscribing 75 per-
<br />cent of egg, overlying yolk sac. Neural plate
<br />formed, lateral and anterior margins not
<br />clearly delimited. Notochord in form of ridge
<br />pressing into yolk, lateral notochord rudi-
<br />ment not clearly separated from sheets of
<br />mesoderm.
<br />Stage 16: late neurula, 47 hours (Fig. 1G).
<br />Neural keel remains 2.0 mm long, circum-
<br />scribing 75 percent of egg. Neural plate con-
<br />tracted, more defined along edges. Cephalic
<br />region arrow shaped in dorsal view; eye rudi-
<br />ments forming; neural ridges as folds on ei-
<br />ther side of neural plate. Notochord sepa-
<br />rated from mesoderm.
<br />Stage 17: eye rudiments, day 3, "49 hours,
<br />embryo length 2.0 mm. Brain cavities and
<br />spinal cord formed by contraction of neural
<br />plate. Eye rudiments as lateral protrusions at
<br />2SeLL Suan B Lateral BI Lateral B?
<br />II ?$
<br />D X-section
<br />B, C.
<br />Pb
<br />ateral astula
<br />L
<br />Early Gastrula Blc X-section
<br />Fig. 1. Early embryological stages of razorback suck-
<br />er, Xyrauchen texanus; chorion removed in all but A.
<br />Legend: A1P = anterior intestinal portal, Ant. = ante-
<br />rior, AP = auditory placode, B1 = blastoderm, Ble =
<br />blastocoel, Blm = blastomere, Bp = blastopore, Ch =
<br />chorion, ES = embryonic shield, GR = germ ring, HF
<br />= head fold, LPM = lateral plate mesoderm, Mes =
<br />mesencephalon, MR = marginal ridge, NF = neural
<br />fold, NK = neural keel, NP = neural plate, OP = optic
<br />placode, Pb = periblast, PM = perivitelline membrane,
<br />PS = perivitelline space, Rh = rhombencephalon, som
<br />= somites, Y = yolk plug, and YS = yolk sac.
<br />anterior end of brain. Prosencephalon (fore-
<br />brain), mesencephalon (midbrain) and rhomb-
<br />encephalon (hindbrain) distinguishable. Audi-
<br />ESQ
<br />MR
<br />?R i
<br />at eral ??" Y
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