300 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



EXPLANATION OF KEFERENCE LETTERS USED IN THE PLATES. 



a. Venous eucl or sinus of heart. 



b. Uriuary vesicle or bladder. 

 ha. Bulbus aorlaj of heart. 



c. Bliud capillarj' prolougatious from the larger blood-vessels on the surface of the 

 yelk. 



e. Head end of developing embryo. 

 /. Rudiment of breast tin. 

 g. Germinal disk. 

 k. Kuptfer's vesicle. 



0. Optic vesicles ; rudiments of the eye-balls. 

 p. Pericardiac or heart space. 

 r. Thickened rim of blastoderm. 



8. Muscular or elfistic baud binding the ventricle to the floor of the heart space. 

 »c. Segmentation caviiy. 

 V. Median vitelline blood-vessel. 

 v' v". Right and left vitelline blood-vessels. 

 ve. Ventricle of the heart. 



«'. Wreath of cells around the germinal disk which enter into the formation of i)art 

 of the blastodermic rim r. 

 X. Vent or anus. 

 z. Point where the caudal vein pr sses into the median vitelline blood-vessel. 



EXPLANATION OF PLATE XIX. 



S^'All of the figures except 13 and 15 are enlarged twenty-one and a third times 

 the natural size. 



Fig. 1. — Egg of the silver gar in its membrane, with the tentacular filaments at- 

 tached to its surface, S hours and 23 minutes after impregnation. The germinal disk 

 g at its upper pole has been segmented into 8 cells. 



Fig. 2. — Germinal disk, 4f hours after impregnation, divided into 16 cells. 



Fig. 3. — Germinal disk, 10 "hours after impregnation, showing the formation of a 

 wreath of cell, w, round its margin. 



Fig. 4. — Blastoderm of silver gar, viewed from above, 24 hours after impregnation 

 to show the form and extent of the segmentation cavity. 



Fig. 5. — Blastoderm of silver gar, viewed from above and obliquely, 31 hours and 

 20 minutes after impregnation, showing the body of the embryo budding out from the 

 edge of the blastodermic rim. 



Fig. 6. — Blastoderm nearly inclosing the vitellus, 43 hours and 40 minutes after im- 

 pregnation, eyes o, muscular segments so and Kupfl:er's vesicle k are developed. 



Figs. 7 and 8. — Head and tail ends of embryos, 51 hours after impregnation. 



EXPLANATION OF PLATE XX. 



Fig. 9. — Embryo silver gar with the head seen in optic section, the tail end and 

 the conjoined oval blastodermic rim seen through the transparent vitellus, which is 

 not represtnted, 51 hours after impregnation. 



Fig. 10. — Embryo one hour later, represented without the vitellus, the number of 

 muscular stigments has greatly increased in number, and the blastoderm has closed 

 over the yelk. 



Fig. 11. — Embryo silver gar, seen from the side as a transparent object, 70 hours 

 after impregnation. The tail is about to begin to bud out behind, the heart is formed 

 but is still tubular, and a vessel passes forward around the yelk back to the tail and 

 on forward through the body to the hind end of the heart. The direction of the blood 



