WALSH and LUND: EARLY DEVELOPMENT OF LONGHORN SCR.PIN 



dicular to the first. Four blastomeres were about 

 equal in size. This stage was first observed at 10.0 h; 

 most eggs had reached this stage by 10.5 h (Fig. 

 1C). 



Eight-cell stage: Third cleavage was vertical. Blas- 

 tomeres were about equal in size. First observation of 

 eggs at this stage was at 12.0 h, and all observed eggs 

 had reached this stage by 14.0 h (Fig. ID). 



Sixteen-cell stage: Another vertical cleavage 

 resulted in 16 cells in a single layer. In some eggs, the 

 shape of the cells and the lines of cleavage appeared 

 irregular. This stage was first observed at 18.0 h and 

 all eggs were in this stage by 20.0 h (Fig. IE). 



Thirty-two- cell stage: A horizontal cleavage pro- 

 duced a central double layer of cells, with peripheral 

 cells in a single layer. Cells were irregular in shape. 

 This cleavage was first noted at 23.0 h; all observed 

 specimens had attained this stage by 24.0 h (Fig. IF). 



Sixty- four-cell to multicell stage: Eggs classified as 

 this stage were first seen at 27.0 h. By 52 h, all eggs 

 were considered to be at this stage. The blastodisc 

 became raised, but did not yet begin moving over the 

 yolk. This long stage was characterized by continued 

 increase of cell number and decrease of cell size (Fig. 

 2A, B). 



Blastula stage: This stage was initially observed at 

 86.0 h, and all observed eggs had reached this stage 

 by 110.0 h. Blastoderm began to expand over the 

 yolk. Periblast was visible at the periphery of the 

 blastoderm. Blastocoel was flattened out over the 

 yolk (Fig. 2 C). 



Gastrula stage: E piboly proceeded. Germ ring was 

 first noted at 132.0 h (5.5 d). Embryonic shield was 

 most easily viewed on the horizon of the egg (Fig. 

 2D). 



Early embryogenesis: Eggs with clearly discern- 

 ible embryos were first observed at 168.0 h (7.0 d). 

 Blastopore was reduced to a small opening. Oil 

 droplets were aggregated near the opening. Neural 

 groove was apparent and extended less than one-half 

 way around the yolk. From a dorsal view, rudiments 

 of the optic vesicles were visible (Fig. 2E). 



At 196.0 h (7.75 d), closure of the blastopore was 

 first seen. Optic vesicles were larger, and visible to 

 direct view. Slight cephalic swelling was apparent. 

 Oil droplets were coalescing (Fig. 2F). 



Middle embryogenesis: Differentiation of the main 

 divisions of the brain had begun by 250.0 h (10.4 d). 

 Cephalic and caudal swelling were present. Optic cup 

 and lens of the eye were best seen in dorsal view; the 



FlCiURK 2. — Development of the eggs of the longhorn sculpin, Myoxacephalus nctodecemspinosus, artificially propagated in the laboratory: A) 

 64- cell stage, 2. 19 mm (27 h); Bl multicell stage. 2.19 mm (52 h);C) blastula stage, 2.22 mm (86 hi; D) gastrula stage. 2.15 mm (132 h); E-F) early 

 embryonic stages. 2.19 mm (168 and 186 h); scale = 0.5 mm. Measurements refer to mean egg capsule diameters. 



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