Blood et al.: Embryonic development of Theragra chalcogramma 



221 



opment of walleye pollock embryos incubated at 6.0— 

 7.0°C (Yusa, 1954) for comparison with morphologi- 

 cal characteristics of eggs reared at 5.7°C in this 

 study We used hallmarks of each stage (e.g. num- 

 ber of cells, germ ring advancement, number of 

 myomeres, tail growth around yolk) to distribute 

 Yusa's (1954) descriptions into 21 stages. Yusa's 

 (1954) descriptions were similar to ours up to stage 

 11. Beginning with stage 11, Yusa (1954) described 

 the development of some structures occurring one 

 or more stages earlier than this study: myomeres 

 and nares were sighted one stage earlier; brain dif- 

 ferentiation and eye lenses, two stages earlier; the 

 heart, three stages earlier; and the otic capsules, five 

 stages earlier (Table 5). Otoliths sighted by Yusa 

 ( 1954) were not visible in our specimens. Conversely, 

 eye pigment was observed in our study one stage 

 earlier than that observed by Yusa (1954). Other 

 structures appeared at the same stage in each study: 

 optic vesicles, Kupffer's vesicle, liver, gut, and pec- 

 toral-fin anlagen. Also, after stage 13, similar num- 

 bers of myomeres were visible at like stages in both 

 studies as was the beating of the heart. 



Differences between the two studies may be the 

 result of egg condition when examined: Yusa (1954) 

 described live eggs, whereas most of our descriptions 

 were of preserved eggs. Formalin preservation may 

 obscure myomeres or destroy structures such as 

 embryonic otoliths (McMahon and Tash, 1979). 

 Stockard's solution darkens embryonic tissue and 

 obscures fine details. Also, morphological develop- 

 ment may differ between western and eastern North 



Pacific walleye pollock, further emphasizing the 

 need to restrict data collection to specific areas of 

 interest to increase accuracy of interpretation. 



Acknowledgments 



We thank the following people whose combined ef- 

 forts helped us accomplish our research and produce 

 this paper: William Rugen assisted with shipboard 

 experiments; Kevin Bailey, Gail Theilacker, and 

 Steve Porter helped us interpret the morphology of 

 late-stage eggs and yolk-sac larvae; Trish Brown 

 provided statistical analyses; Morgan Busby photo- 

 graphed the eggs; and Beverly Vinter illustrated 

 eggs and helped interpret many morphological struc- 

 tures. We thank Art Kendall, A. J. Paul, Kevin 

 Bailey, Susan Picquelle, and Bori 011a for prelimi- 

 nary reviews of the manuscript. Gail Theilacker and 

 Richard Brodeur helped refine later versions. We 

 also thank the members of FOCI who assisted with 

 field collections. 



Literature cited 



Alderdice, D. F. 



1988. Osmotic and ionic regulation in teleost eggs 

 and larvae. In W. S. Hoar and D. J. Randall 

 (eds.), Fish physiology, Vol. XI, Part A, p. 163-251. 

 Academic Press, Inc., San Diego. 



