PLATE I 



EARLY DEVELOPMENT OF THE EGG OF HETERODOHTUS JAPOHICUS 



Fig. 1. Egg taken at time of deposition. Polar view of upper hemisphere. The germinal disc ("orange 

 spot") is indicated by a tiny circle close to top of figure. 



Fig. 2. Upper hemisphere of an egg in a slightly later stage. The germinal disc is situated on the side of the 

 egg away from the observer. It is indicated, as if seen through the egg, by a tiny dotted circle 

 near the right hand margin of the figure. 



Fig. 3. Lateral view of an egg in a stage between Figures 1 and 2. The position of the germinal disc is not 



indicated. 

 Fig. 4. Lateral view of an egg similar to the one shown in Figure 3. The germinal disc is indicated by 



a small dark spot on the right of the figure. 



Fig. 5. An egg shown in lateral, sHghtly oblique, view. The germinal disc is indicated, as if seen through 

 the egg, by a dotted circle in the upper right quadrant of the figure. 



Fig. 6. An egg in a slightly later stage, oriented with ordinarily lower pole nearly uppermost. The 

 germinal disc is situated on the side of the egg away from the observer. It is indicated by a tiny 

 dotted circle in the upper left quadrant of the figure. 



. Figs. 1 to 6 have been published by Dean in the Annotationes Zoologicae Japonensis, 1901, vol. 4. They show furrows 

 interpreted by Dean as a reminiscence of holoblastic cleavage. Eggs in these stages vary from 40 to 50 milUmeters 

 in diameter. 



Fig. 7- The earliest observed stage of cleavage in the germinal disc. This region, constituting the early 

 blastoderm, was removed from the yolk mass. Viewed by transmitted Hght, it was drawn, 

 under magnification, in natural colors. 



Fig. 8. A later stage in the segmentation of the germinal disc. The blastoderm, removed from the egg 

 and viewed by transmitted light, was drawn under magnification in natural colors. 



Fig. 9. Slightly later stage of cleavage in a germinal disc viewed as an opaque object. 



Fig. 10. Blastoderm in an advanced cleavage stage, removed from the yolk and viewed by transmitted 



light. The crescentic hght area in lower part of figure is the blastocoele seen by focussing down- 



ward through its roof. 

 Fig. 11. Elongate blastoderm, perhaps ready for gastrulation, viewed as an opaque object and drawn in 



natural colors. This blastoderm is 5 mm. long. The pale area surrounding it is the marginal 



zone of the periblast. 

 Fig. 12. Surface view of a blastoderm in an early stage of gastrulation. Note, at the posterior (lower) end, 



the neural groove bordered by upraised neural folds. 



Fig. 13. Surface view of a blastoderm, with rudimentary embryo, slightly later than the preceding. 



Fig. 14. Optical section through embryonic region in a stage intermediate between Figures 12 and 13. This 

 figure, like the remaining ones of this plate, was drawn from a cleared preparation. 



Fig. 15. Optical section of an embryo slightly later than the one shown in Figure 13. 



Fig. 16. Optical section of an embryo with 4 pairs of complete mesoblastic somites. 



Fig. 17. Optical section of an embryo with 12 pairs of complete somites. 



Fig. 18. Optical section of an embryo with 14 or 15 pairs of somites. 



Fig. 19. Optical section of an embryo with 15 or 16 pairs of somites. 



