THh] LIFE-HISTORY OF NUCULA DELPHINODONTA. 389 



PLATE 21. 



Fig. 14. — Lateral view of an embryo in wliich the test is fully formed, 

 represented as a sliglitly transparent object. Tlie lines numbered 16, 17, 18, 

 and 19 indicate the planes of sections represented in corresponding figures. 

 X 150. 



Fig. 15. — Sagittal section of an embryo in the stage represented by Fig. 

 14. The anterior ends point in opposite directions in the two figures, x 

 275. 



Figs. 16, 17, 18, and 19. — Transverse sections of an embryo in the stage 

 represented by Fig. 14. The lines 16, 17, 18, and 19 on Fig. 14 indicate the 

 planes of the sections represented by these figures. X 275. 



Figs. 20, 21, and 22. — Transverse sections of an embryo older than the one 

 represented by Fig. 14, and a little younger than the embryo represented by 

 Fig. 23. The lines 20, 21, and 22 on Fig. 23 represent planes that correspond 

 to these sections. X 275. 



Fig. 23. — Lateral view of an embryo in which the mantle is beginning to 

 form, represented as a slightly transparent object. The lines 20, 21, and 22 

 indicate the planes of sections represented in corresponding figures, but the 

 embryo represented in Fig. 23 is slightly older than the one from which these 

 sections were obtained, x 150. 



Fig. 24. — Sagittal section of an embryo in the stage represented by Fig. 23. 

 X 275; 



Fig. 25. — Lateral view of an embryo that would soon shed its test. The 

 test cells, indicated in outline, are very indistinct, and are not accurately 

 drawn. Cilia have been indicated along the margins only. They cover the 

 whole of the surface. The organs are more clearly shown than in preceding 

 figures of embryos. Tliey are not visible in whole mounts, but have been 

 reconstructed from sections. The lines 27, 28, 29, 30, 31, 32, and 33 indicate 

 the planes of sections represented by these figures. (See Plate 22.) X 150. 



PLATE 22. 



Fig. 26. — Sagittal section of an embryo in the stage represented by Fig. 25, 

 PI. 21. X 275. 



Fjgs. 27, 28, and 29. — Transverse section of an embryo in the stage repre- 

 sented by Fig. 25, PI. 21. The lines numbered 27, 28, and 29 on Fig. 25 

 indicate the planes of sections represented by these figures, x 275. 



Figs. 30, 31, 32, and 33. — Horizontal sections of an embryo in the stage 

 represented by Fig. 25, PI. 21. The lines numbered 30, 31, 32, and 33 on Fig. 

 25 indicate the planes of the sections represented by these figures, x 275. 



PLATE 23. 



Fig. 34. — Lateral view of a reconstruction of an embryo that has just com^ 



