170 Papers from the Marine Biological Laboratory at Tortugas. 



Plate 5. 



Figs. 25-27. Successive stages in the cleavage of the same egg; fig. 25, 8 cells; fig. 26, 

 16 cells ; fig. 27, 32 cells ; the daughter cells are connected b}- arrows. 



Fig. 28. Stage transitional between 32 and 64 cells ; every cell shows a nucleus in 

 some stage of mitotic division. 



Fig. 29. 64-cell stage, showing (in dotted outline) the enlarging cleavage cavity. 



Fig. 30. 128-cell stage, showing the cleavage cavity much enlarged. 



Plate 6. 



Fig. 31. Stage of about 500 cells, showing the rounded endoderm cells at the vegetal 

 pole ; the cleavage cavity is eccentric toward this pole ; several of the 

 cells are dividing by mitosis. 



Fig. 32. Stage of about 1,000 cells, showing the appearance of pseudopod-like proc- 

 esses from the ectoplasm, which later become cilia. 



Figs. 33, 34. Optical sections of gastrulae, showing gastrula invagination, rounded endo- 

 derm, and mesogloea cells, and the high columnar ectoderm cells. 



Fig. 35. Optical section of a stage after the closure of the blastopore. 



Fig. 36. Optical section of an elongated planula. 



Plate 7. 



Fig. 37. Photomicrograph of egg after the formation of the polar bodies, showing the 



thin peripheral layer of protoplasm, the dense layer of yolk, and the 



central more fluid area. 

 Fig. 38. Egg centrifugalized for l minute at 12,000 revolutions per minute, and fixed 



in Flemming's fluid 6 hours later. The clear protoplasm is aggregated 



chiefly at the animal pole : the germ nuclei are large clear vesicles which 



have not undergone division ; the central area is still less dense than the 



peripheral yolk layer. 

 Fig. 39. Normal egg, showing the germ nuclei in contact near the animal pole, and 



the substances of the egg arranged as in fig. 37. 

 Figs. 40-42. Successive stages in the first cleavage of the egg, showing the furrow 



cutting into the central area and the formation of the cleavage cavity; 



one or both of the nuclei of the two cells show in all the figures. 

 Fig. 43. Oblique section through an egg at the close of the first cleavage, showing 



the cleavage cavity filled with the central matrix of the egg. 

 Fig. 44. Vertical section of an 8-cell stage, showing the contents of the cleavage 



cavity escaping at the vegetal pole. 



Plate 8. 



Fig. 45. Horizontal section of an 8-cell stage, showing the mitotic spindles for the 

 fourth cleavage; the blastomeres are partially torn apart. 



Fig. 46. Section through a 32-cell stage, showing mitotic figures in some of the cells ; 

 these figures are surrounded by small, deeply-stained spherules. 



Figs. 47, 48. Vertical sections through blastulse at the beginning of invagination ; 

 nuclei are shown in many of the cells ; the cleavage cavity is greatly 

 enlarged and its contents stain less deeply than in previous stages. 



Fig. 49. Invaginate gastrula, showing the thin-walled endoderm and the thick-walled 

 ectoderm, together with blastocrel, gastroccel, and blastopore ; in the 

 ectoderm the cells are faintly indicated by the vertical rows of yolk 

 spherules. 



Fig. 50. Longitudinal section through a planula, showing ectoderm, endoderm, and 

 three invaginations of ectoderm into the blastocoel ; the cells of these 

 ectodermal invaginations are quite unlike the endoderm cells, while 

 thev closelv resemble those of the ectoderm. 



