EARLY DEVELOPMENT OF CRIBRELLA OCULATA. 417 



LIST OF ILLUSTRATIONS. 



Plate I. 



Figs. 1-3. Three segmenting eggs, external view. ( x 28.) 



Fig. 4. An egg at completion of segmentation. ( x 32.) The egression tracts are commencing to 

 appear. 



Fig. 5. A later embryo, with numerous ingression tracts in the form of grooves covering the surface. 

 (x32.) 



Fig. 6. A gastrula stage, with gaping blastopore and vestiges of four egression tracts. ( x 32.) 



Fig. 7. Median section through a stage with two blastomeres. Notice the many nuclei. ( x 32.) 



Fig. 8. Median section through the unequal total type, with small archiccele. ( x 32.) 



Figs. 9 and 10. Median sections of similar types at various stages. ( x 32.) 



Fig. 11. Median section through another type of segmentation, partially superficial, but indefinite. 

 (x32.) 



Figs. 12, 13. Median sections through a very common type of segmentation, with slight inequality 

 in fig. 12, which disappears in fig. 13. ( x 47.) 



Fig. 14. Median section through the morula, to which all the types of segmentation eventually lead. 

 (x78.) 



Fig. 15. A single cell of the morula, showing the radiate yoke-granules. ( x 370.) 



Fig. 16. A later morula, in which the cell-walls are becoming indistinct, and the cells are commencing 

 to arrange themselves into aggregates producing egression tracts. ( x 47.) 



Fig. 17. The transition stage of a syncytial mass, with egression tracts ; a central blastoccele and 

 peripheral blastocceles are becoming evident. ( x 47.) 



Fig. 18. The gastrula in median section. ( x 47.) 



Plate II. 



Fig. 19. The embryo immediately before hatching. ( x 28.) 



Fig. 20. The free larva, stage A. ( x 28.) 



Fig. 21. The free larva, stage B. ( x 28.) 



Fig. 22. The same as fig. 21, anterior view. 



Fig. 23. Stage C, from right side. ( x 28.) 



Fig. 24. The same, but slightly later. ( x 28.) 



Fig. 25. The same, anterior view. ( x 28.) 



Fig. 26. Median section through the embryo in fig. 19. ( x 34.) 



Fig. 27. The epiblast of the embryo. ( x 370.) 



Fig. 28. The hypoblast and mesenchyme of ditto. ( x 370.) 



Fig. 29. A median coronal section of stage A. ( x 57.) 



Figs. 30-34. A coronal selected series through stage A/B. ( x 28.) 



Fig. 35. A median coronal section through stage B. ( x 28.) 



Figs. 36 and 37. A dorsal and ventral view of stage C, seen as a transparent object. (Oil of cloves.) 

 (x40.) 



Figs. 38-45. A transverse series through stage C. ( x 28.) The numbers of the sections are 3, 25, 

 30, 37, 47, 53, 57. 



Plate III. 



Figs. 46-49. A sinistral, ventral, dextral and anterior view, respectively, of stage D. Fig. 48 shows 

 the method of temporary fixation. 



Figs. 50-52. A dorsal, ventral and sinistral view, respectively, of stage D, viewed as transparent 

 objects. (Oil of cloves,) ( x 40.) 



TRANS. ROY. SOC. EDIN., VOL. XL. PART II. (NO. 19). 3 r 



