166 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



EXPLANATION OF PLATE L 



Fig. 1. — Micropyle and micropylar area, in two positions, of an unimpregnated egg 

 of the Spanish mackerel. X 64.* 



Fig. 2. — Unimpregnated egg of the Spanish mackerel, showing the germinal layer or 

 pellicle gp which envelopes the yelk or vitellus. X 24. 



Fig. 3. — Morula stage of cleavage of the germinal disk of the egg of the Spanish 

 mackerel one hour and forty minutes after impregnation. X 25. 



Fig. 4. — Germinal disk of a Spanish mackerel egg shown with its germinal pole 

 slightly inclined towards the observer. The disk gd is elevated and bis- 

 cuit-shaped, and is surrounded at its base by a circle of two rows of flat- 

 tened cells lying immediately upon the yelk. The cells in the disk are 

 four or more deep at this stage, three hours after impregnation, x 70. 



Fig. 5. — Incipient blastoderm of the egg of the Spanish mackerel, viewed from the 

 side somewhat obliquely. The segmentation of the marginal cells has ad- 

 vanced somewhat as compared with Fig. 4, and it is clear that they will 

 become a part of the rim of the blastoderm. X 25. 



Fig. 6. — Developing blastoderm of an egg three hours and thirty minutes after im- 

 pregnation, viewed from above, showing the blastodermic rim r in its 

 incipiency, while the darker space bounded by it corresponds with the 

 area beneath the blastoderm shortly to become the segmentation cavity. 

 X25. 



Fig. 7. — Blastoderm of the Spanish mackerel egg seven hours after impregnation, in- 

 closing nearly half of the yelk, and showing the lumen of the segmenta- 

 tion cavity on one side at sg, which extends beneath the fore part of the 

 head at crs to form the rudiment of the cardiac sinus just in front of the 

 carina cr. X 48. 



Fig. 8. — Embryo of the Spanish mackerel eleven hours after impregnation, showing 

 the rudiments of the optic vesicles opv, the chorda ch, and the cardiac 

 sinus crs. The segmentation cavity sg is shown between the epiblast and 

 hypoblast extending nearly all round the yelk. The blastoderm has not 

 yet closed behind the tail. X 50. 



*iii every case the number of diameters to which the figure is enlarged is indicated 

 in Arabic numerals preceded by the sign X . 



