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?a r? 560 GREAT BASIN NATURALIST keel evident to touch on predorsum. Except for further development of nuchal keel, evi- dent to the eye at a 250-300 days of age (Fig. 10), morphogenesis is completed. Individuals from the 1974 cohort achieved sexual matu- rity in their sixth year of life (Minckley 1983). SUMMARY Fertilized, water-hardened ova of Xyrau- chen texanus were 1.8 mm in diameter from females hormone-induced to mature and 2.9 mm from females that matured naturally. Eggs were adhesive for 3 to 4 hours after fertilization. Cleavage was completed in 24 hours at temperatures varying from 13 to 17 C; further development was at 20 C. Gastru- lation occurred at 34 hours. The notocord separated from the mesoderm at 47 hours, and eye rudiments and brain cavities were distinguishable at 49 hours. The tail process formed at 57 hours (2.5 mm). Heart beat be- gan at 83 hours, and blood circulation was es- tablished at 117 hours. Embryos began vio- Vol. 42, No. 4 lent flexing and some ruptured their chor e; .s at that time. All hatched between 125 a 131 hours after fertilization. Embryos were 6.8 to 7.3 mm TL at ha ing. The yolk sac is tubular, and the 1 flexes over it at a 45 degree angle to the .v axis. The pectoral fin buds, noted at ) hours (6.8 mm), became paddlelike at ? hours (8.0 mm), and first were movab a 238 hours (9.0 mm). The continuous, me. <!l fin fold first noted on caudal and ):..-;_- erodorsal areas at 103 hours (5.3 mm), b¢- ; 11 to develop on the venter (behind the pre:, ;- deum) at 142 hours (7.5 mm), and was cc.-, .= - uous at 162 hours (8.0 mm). The operclc: gan forming at 162 hours (8.0 mm). Thu ; bladder first appeared at 238 hours (9.0 The lower jaw became movable and larvae began directed swimming to the face to feed at that time; eye pigment c. plete and black. Melanophores develr, 7. over mid- and hindbrain and on paired sal, paired dorso-visceral, and unpaired ventral pigment lines between 162 and hours (8.0 and 9.0 mm). There was no la'i ? Fig. 10. Juvenile of razorback sucker, X yrauchen texanus, with essentially adult morphology. December 1982 pigmentation at 9.( the dorsum are larg corded by Winn aI hours (9.0+ mm) fir in any fin. Yolk was compl, hours (10.0 mm) an( to form the anus. turned between 263 4, ventral, caudal fi time period. Pect( three rays, but there sal and anal fins. Me ened and expanded dorsal and anal fii mm), and the operd Dorsal and anal fin- eral pigment line ap mm). The gas blade stricted toward thf tion. Pelvic fins app chyme and the emarginate at 115 pelvic fin buds w braneous paddles ai movement and pelv at 1536 hours (27.0 I Scale rudiments hours (43.0 mm) or faces. By 3000 hour! plete on all but the sum and ventrum. 1 about 5000 hours aft ACKNOI This research wa U.S. Fish and Wild 16-0002-3585 to A _ We thank personn( tional Fish Hatcher} Fig. 9. Juvenile of razorback sucker, X yrauchen texanus, shortly after initiation of nuchal keel.