Report U. S. F. C. 1 897. (To face page 340.) 



Plate VII. 



Fig. 1. Unfertilized egg shortly 



after mixture of spawn and milt; 



spermatozoa are adhering to the 



surface. 

 Fig. 2. Egg after fertUization. 

 Fig. 3. Same egg --i minutes later. 



Polar body at broad end. 

 Fig. 4. Same egg G minutes later. 

 Fig. 5. About 6i hours later. 

 Fig. 6. Another egg at about the 



same stage. Mass of small cells 



growing over large cell or mac- 



romere a. 

 Fig. 7. Egg .55 minutes later. Mac- 



romere almost covered by small 



cells of ectoderm. 



Fig. 8. Optical section of egg 37' 

 hours after impregnation, show- 

 ing two large cells, derived from 

 o in fig 6, covered by a layer of 

 small ectodermal cells. 



Fig. 9. Egg a few hours older,show- 

 ing large cells viewed from below. 



Fig. 10. An egg somewhat older 

 viewed from above, showing fur 

 ther subdivision of large cells as 

 seen through cells of upper layer. 



Fig. 11. An older egg, now become 

 flattened from above downward. 

 Viewed in optical section. 



Fig. It*. Surface view of an embryo 

 just beginning to swim. 



After W. K. Brooks. 



Fig. 13. Optical section of same. 



Fig. 14. Surf ace view of same from 

 another position. 



Fig. 15. Surf ace view of same from 

 another position. 



Fig. 16. An older embryo in same 

 position as in flg 12 



Fig. 17. A still older embryo show- 

 ing spherical ciliated digestive 

 cavity opening by mouth, vi. 



Fig. 18. An embryo with well-de- 

 veloped larval shells, older than 

 flg. 1 , Plate VIII. fs, right shell ; 

 Is, left shell; vl, velum; m, 

 mouth. 



