168 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



EXPLANATION OF PLATE H. 



Fig. 9. — Spanish mackerel egg viewed so as to show ouly the caudal end of the em- 

 bryo twelve hours after impregnation, showing the himen of the segmen- 

 tation cavity sg, the rudimentary chorda, and the somites or muscle iilates, 

 while the blastoderm has not yet quite closed over the yelk at y. The oil 

 sphere is not represente'd. X 50. 



Fig. 10.' — Embryo Spanish mackerel fourteen hours after impreguatiou. x 50. 



Fig. 11 — Transverse section through the opening y of the embryo represented in 

 Fig. 9. 



Fig. 12. — Spanish mackerel embryo eighteen hours after impregnation. The rudi- 

 mentary heart h occupies an asymmetrical position in its sinus beneath the 

 head and is a fusiform tubular oi-gan ojieu at the anterior venous end. 

 The oil sphere is attached to the ventral wall of the yelk sack or hypo- 

 blast and is partly covered by flattened cells which have budded from it. 

 X.50. 



Fig. 13. — Young Spanish mackerel just hatched twenty-four hours after impregnation. 

 The rudiments of the breast fins have made their appearance on each side 

 of the body at^, and the oil sphere is having pigment cells developed on 

 its surface, while those of the body are already aggregated in patches on 

 the tail. X 25. 



