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554
<br />GREAT BASIN NATURALIST
<br />further explanation. By the time of hatching,
<br />both sets of embryos were of comparable
<br />length.
<br />Developing embryos were maintained in
<br />an indoor raceway at Willow Beach for 29
<br />hours at temperatures ranging from 13 to 17
<br />C. Eggs were then transported to aquaria at
<br />Arizona State University, where development
<br />continued at a constant temperature of 20 C.
<br />Observations and measurements were re-
<br />corded from living specimens by a stereo-
<br />microscope equipped with ocular microme-
<br />ter and camera lucida, and with a range of
<br />magnification of 2 to 2,000 X. All measure-
<br />ments are of total length.
<br />Observations on the 1975 cohort were
<br />made hourly through the first 30 hours, then
<br />every 2 hours until after hatching. Illustra-
<br />tions of selected stages through prolarval de-
<br />velopment were prepared through use of a
<br />camera lucida and from photographs. Ovum
<br />through prolarval development presented
<br />here is thus based on the 1975 fish. Speci-
<br />mens were preserved periodically in aceto-
<br />formalic acid (AFA; 9.0 parts ethanol, 0.4
<br />parts 40 percent formaldehyde, and 0.5 parts
<br />glacial acetic acid). When larvae began to
<br />swim and feed actively, samples were pre-
<br />served less frequently.
<br />Development of the razorback sucker was
<br />divided into four major phases as defined by
<br />Hubbs (1943): (1) embryological devel-
<br />opment, fertilization of the egg until hatch-
<br />ing; (2) prolarval development, hatching to
<br />absorption of yolk; (3) postlarval devel-
<br />opment, yolk absorption to ossification of
<br />pelvic fin-rays; and, (4) juvenile devel-
<br />opment, pelvic ray ossification to maturation
<br />of gonads. Development staging followed Ba-
<br />linsky's (1948) general pattern for cyprinid
<br />fishes. Descriptive terminology was derived
<br />from Ryder (1885), Stewart (1926), Tavolga
<br />(1949), Winn and Miller (1954), and Long
<br />and Ballard (1976).
<br />RESULTS
<br />Embryological Development
<br />Stage 1: unfertilized egg; day 0, 0 hour, 1.5
<br />mm diameter. Ova milky white and
<br />translucent.
<br />Vol. 42, No. 4
<br />Stage 2: fertilized egg; day 1, 1 hour, 1,8
<br />mm diameter. Chorion transparent and yolk
<br />milky-white and translucent; animal pole not
<br />yet visible to unaided eye. Water-hardened
<br />eggs with greater specific gravity than water,
<br />ova demersal, chorion adhesive, ova adhering
<br />to substrate and one another.
<br />Stage 3: 2 blastomeres, 3 hours (Fig. !A);
<br />beginning of cleavage. Blastomeres trans.
<br />parent, approximately 0.5 mm total diame_
<br />ter. Animal and vegetal poles distinguis =sable
<br />to unaided eye; ova telolecithal, clew-a`e
<br />meroblastic. Ova no longer adhesive.
<br />preservation causes animal cells to Nv!- rell
<br />and become opaque; yolk becomes Yz",In,,.
<br />white; chorion remains transparent.
<br />Stage 4: 4 blastomeres, 5 hours; sr cleavage. Blastomeres approximately
<br />total diameter.
<br />Stage 5: 8 blastomeres, 6 hours: ircl
<br />cleavage. Blastomeres occupy 1.2- by;,`:
<br />rectangle on animal pole.
<br />Stage 6: 16 blastomeres, 7 hours; rt{
<br />cleavage.
<br />Stage 7: 32 blastomeres, 9 hour: t{,
<br />cleavage (Fig. 1B).
<br />Stage 8: 64 blastomeres, 10 hours; .-.t},
<br />cleavage. 128 blastomeres, 11 hours;
<br />cleavage. Large-celled blastula (MOM. no
<br />blastocoel. Blastomeres occupy 25 per of
<br />yolk surface.
<br />Stage 9: small-celled blastula (more. 1-3
<br />hours. Individual cells distinguishable; ;o-
<br />meres bulging upward from round yol
<br />occupying 25 percent of yolk surface, -o-
<br />mere layers progressively thinner town - )e-
<br />riphery of blastoderm; no blastocoel.
<br />Stage 10: morula, 24 hours (Fig. 1C1. '
<br />vidual cells indistinguishable exce; ;t ah
<br />high power and chorion removed; blast - -rm
<br />with granular appearance; undersurfac'- .gat,
<br />lying on flattened surface of yolk sphc no
<br />blastocoel. Cleavage terminated.
<br />Stage 11: blastula (epiboly of blaste -' 'n),
<br />day 2, 28 hours; 1.8 mm diameter (F, J.
<br />Blastoderm spreading over yolk sphr nd
<br />thinning (blastodisc). Blastocoel presen' - - ri-
<br />blast visible as cellular rim along per'::erg
<br />of blastoderm, beginning formation c j:'
<br />layer of yolk sac. Outer layer of yolk ?.. to
<br />be formed from epiblast derived from ":?-Mo-
<br />derm. Blastoderm no longer bulging ,rom
<br />yolk, capping under 33 percent of sphet
<br />Deceml:
<br />Stage
<br />Underrir
<br />"randwu
<br />layer tei
<br />posterior
<br />Presump
<br />edge of
<br />blastopoi
<br />Cells of
<br />grating i
<br />(establish
<br />tive mes
<br />positione
<br />neath cc
<br />blast, ran
<br />involved
<br />of yolk sI
<br />Stage
<br />vaginatic
<br />blastopor
<br />Stage ]
<br />Embryor
<br />pole of e
<br />proximat(
<br />invaginat
<br />end of sh
<br />cells, ace
<br />mal cells
<br />marginal
<br />uncoverec
<br />plug.
<br />Stage 1
<br />diameter.
<br />longer pro
<br />imately 2.
<br />cent of eg
<br />formed, 1
<br />clearly de.
<br />pressing i
<br />ment not
<br />mesoderm
<br />r Stage 11
<br />Neural ke
<br />scribing T
<br />tracted, in
<br />region arro
<br />ments fon
<br />ther side
<br />rated from
<br />Stage F
<br />_ embryo le
<br />spinal core
<br />plate. Eye
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